The Dalradian Rocks of the Sperrin Mountains and Adjacent Areas in Northern Ireland Author(s): J. J. Hartley Source: Proceedings of the Royal Irish Academy. Section B: Biological, Geological, and Chemical Science, Vol. 44 (1937/1938), pp. 141-171 Published by: Royal Irish Academy Stable URL: http://www.jstor.org/stable/20490442 . Accessed: 09/05/2011 09:23 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at . http://www.jstor.org/action/showPublisher?publisherCode=ria. . Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact
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141
VIII. THE DALRADIAN ROCKS OF THE SPERRIN MOUNTAINS AND ADJACENT AREAS IN NORTHERN IRELAND. BY J. J. HARTLEY, M.Sc., M.ENG., ASSOC.M.INST.C.E.,F.G.S. (PLATES VI-VIII.) [Read 31 MAY, 1937.
Published 8 MARCH, 1938.]
CONTNTS. PAGE 1.
...
INTRODUCTION
2. GENERAL
...
QUARTZITIC ...
...
GROUP 4. DUNGIVEN
LIMESTONE
5. EPIDIORITES
GROUP
(EXTRUSIVE
6. THE
SCHISTOSE ... DART
7. TOURMALINE
GRIT
SCHSTS
THE
143
11.
SUGGESTED
...
144
12.
INTRUSIVE
...
146
13. RELATION
9. THE MULLAGHCARN
OF BROUGH ...
...
...
TYPES
156
OF EPIDIORITES
BETWEEN EXTRUSIVE ... ...
149 151
15. STRUCTURE 16. RELATION
152
17.
TE
154
18.
SUMMARY AND CONCLUSION
154
TO WHCH 19. LIST OF WORKS ... ENCE IS MADE
BETWEEN
155
...
160
...
161
...
162
GEOLOGICAL
STRUCTURE AND PHYSICAL
...
158
AND
EPIDIORITES ... ...
FAULTS
156
WITH
CORRELATION
INTRUSIVE 14.
... ...
GROUP
Types
147
SCHISTS ...
Schists
SCHISTS ...
OTHER AREAS
TYPES)
...
(GREEN BEDS)
BLACK
DERG
GROUP OF ... ... OF GLENELLY
8. ALBITE-BIOTITE-EPIDOTE
(a) Albite (b) Bedded
10.
...
3. NEWTOWNSTEWART
PAGE
141
AND STRATI-
SUCCESSION
GRAPH.Y
...
FORM
167
...
168
...
169
REFER ...
170
METAMORPHISM
l.-INTRODUCTION. THESperrin Mountains with their associated foothills lie along the borders of the counties of Derry and Tyrone in Northern Ireland, and consist essentially of two ranges as shown in Fig. 1. The highest and most northerly of these, forming the Sperrins proper, trends almost east and west
and
includes
and Mullaghaneany,
the five summits all of which
foothills to the south, which
of Mullaclogha, exceed
Dart,
two thousand
feet
Sawel, Meenard, in height.
include the Munterloney Hills
The
and the
group, form a range broader, less continuous Mullaghearn-Carnanelly and in character more detached and broken in outline than that of the main This latter range is slightly lower than the Sperrin mountains. Sperrins. It culminates 1,800
feet
at its two ends high,
whilst
the
in Mullaghearn central part
PROC. R.I.A., VOL. XLIV, SECT. B.
and Carnanelly, by the
breached
both
about
two north [PI
Proceedings of the Royal Irish Academy.
142
westward flowing rivers, the Owenreagh and the Owenkillew, is cut up and discontinuous.
-Dot~~~~~~~~~~~~
Snic
Newtownsswuf
FIG. 1 Topographical Map of the Sperrin area. Districts lying between 700 and 800 feet elevation are shaded in black, and those below 300 feet are dotted.
The Mullaghearn-Carnanelly range is directed along north-east south west lines and coalesces with the northern range towards the north-east, so that the whole
district
here described
forms a triangle
with
the northern
edge running east-west and twenty miles in length and the western edge north-south and measuring about twelve miles, whilst on the east the depth tapers
away
to less than
four miles.
The
three
corners
of
the
triangle
are occupied respectively by the towns of Dungiven, Strabane, and Omagh. The western side defined by theMourne-Strule depression is low-lying It is from this direction that its length. throughout cut away by the complex river system, the headwaters
the country has been of the Foyle, which
includes amongst other rivers the Glenelly, Glenlark, Owenkillew, Owen reagh, and the Strule-Mourne,
HARTLEY-Dalradian Rocks of the Sperrin Mountains.
143
The northern and south-eastern portions of the district were geologically examined
J. E. Portlock in 1843, and subsequently by General the whole in somewhat fuller detail by Messrs. G. H. Kinahan, Egan, S. B. Wilkinson, and other members of the Survey.
area was'treated J. Nolan, P. W. Their
results
Ireland,
are
oni the one-inch
shown
sheets
of Geological
Survey
of
numbers
17, 18, 25, and 26, which together with the descriptive memoirs were published between the years 1884 and 1889. Since the area was one of glacial deposition,' the drift deposits reach locally a considerable thickness, but owing to the high relief and fairly
heavy rainfall post-glacial erosion has been sufficiently powerful for much of the solid floor to have become exposed. Practically all the streams flowing down the sides of the ranges have cut down to solid rock over their whole
length, makes
gorges main
rivers,
whilst the
the deep, rocky, and steep-sided character of of them in some cases almost impassable.
their
larger
though
their
are of a different
valleys
type
and
The
flat floored
with alluvial deposits, usually expose the solid rock at intervals. good continuous addition, such 'as Mullaghasturrakeen, channels
spillway
Dart cut
which
occur
exposures across
on some
of the summit
In
ridges
ald Sawel, and many of the glacial the ranges provide excellent sections.
With
to be the above exceptions, there is rarely anything however, in situ, and it was probably owing to the paucity of the exposures as of the lithological well as to the apparent monotony types that much of to the scale of one inch to the mile. the area was only mapped geologically seen
The
or datum
lack of subdivisions
lines
in the wide
expanse
of schists
has in consequence rendered the structure difficult to interpret from the maps,
published texture
and
enabled made
it
and
while
to remap
The
schists
detailed the
during with
the district
last
more
knowledge fifty
years
chance
of
on the six-inch
scale
of
schist
which
has
success,
and
throughout.
SUCCESSION AND STRATIGRAPHY.
2.-GENERAL
the main
the more
to be reattacked
the problem it worth
is only attained
composition
of of the Sperrins the south-eastern represent termination . In the Sperrin area they of the Dalradians of Ireland. to the south-east by an overthrust and rest upon Ordovician
outcrop
are bounded
they are cut into from the of Carboniferous tongues by shales, sand run radially which the lower and form stones, and conglomerates ground and into the area along the lines now followed by the rivers Roe, Strule, rocks
as previously
north-east,
west,
described
(19), whilst
and south-west
Owenreagh. 1
See
Roy.
J.
K.
Ir. Acad.,
Charlesworth, vol. xxxvi,
Glacial Sect.
B,
pp.
Geology 174-314.
of
the North-West
of
Ireland.
IF'2j
Proc.
Proceedings of the Royal Irish Academy.
144
The present paper deals merely with the schists, for which the proposed succession is given in the following table. The order is that found when the beds are traced across the strike fromnnorth-west to south-east, the north-westerly one being placed at the top: Quartzitic Group of Newtownstewart with associated Schists. Dungiven Limestone Group. Amphibolites or epidiorites of extrusive origin. Schistose Grit Group of Dart. Tourmaline Schists of Glenelly (north side). Albite Biotite Epidote Schists ("Green Beds"). Tourmaline Schists of Glenelly (south side). Mullaghcarn Schists; chiefly albite schists with siliceous bands. Black Schists of Broughderg. [Thrust Fault.] Palteozoic Series of Tyrone. The table would appear, from a preliminary examination of the map with its predominating north-westerly dips, to represent the Dalradian beds in their stratigraphically correct order, but it seems probable, though not certain, that the order is inverted, and that the Quartzitic Group of Newtownstewart is the lowest and not the highest member of the series. The rock groups will therefore be described, beginning with those lying towards the north-west. The various subdivisions are treated as groups rather than as individual bands in order to avoid the uncertainties and complications which would otherwise be introduced through the frequent difficulty and occasional impossibility of deciding whether, for example, two bands of limestone are stratigraphically equivalent or lie at different levels. That there is a and
gradual
general
change when
the beds
are
traced
across
the strike
is
certain, but local isoclinal folding introduces complexities in detail difficult to completely unravel. 3.-NEWTOWNSTEWART the area
Bounding
on
the north
in which quartzites predominate. sericite
and at times
somewhat
QUARTZITic GROUP. and
north-west
is a group
of beds
The quartzites, associated with quartz
phyllitic
schists,
or with
bands
of siliceous
grits indistinguishable from much of the material found further to the south-east,
gradually
diminish
in amount
in this direction,
only represented by thin isolated lenticular patches.
till
they
are
Their abundance
feature. appear to be a definite stratigraphical lies for the most part outside the area mapped this group Although in detail except in the west and north-west, it and has not been examined to the north-east
appears
to extend
would
more
or
less continuously
along
the northern
edge
of
HARTLEY-Dalradian Rocks of the Sperrin Mountains.
145
th.eSperrins till it is overlapped by the'Carboniferous a few miles west of Dungiven. The quartzites are yellowish or pinkish in colour and very finerin graln. Colour banding, probably indicating original bedding, was referred to by J. R. Kilroe to a small
(6, p. 9). extent
Both
felspars
in all the specimens
and
light and dark micas
examined,
and
are present
the quartzites
usually
pass by indistinguishable gradations into schistose grits, which they closely resemble, though the latter contain tiny elongated or rounded pebbles of both
clear
quartz
and
felspar,
and
are more
definitely
foliated
with
pronounced iron staining along the foliation planes. In many cases it is these felspathic grits which are in the earlier literature referred to as gneiss or gneissic schist. The mica schists associated with the quaitzites call for little remark. They are somewhat poor in mica, very siliceous and frequently felspathic. Though a few thin bands of a lead-grey or blue-grey coloured and more micaceous
and
pelitic
type
occur
on Bessy
Bell
as well
as amongst
the
quartzites to the south of Dunnamanagh, they are quite subordinate to the more psammitic varieties. Albite is rare and tourmaline practically absent from the schists, schistose grits, and quartzites of this group. In such an environment, however, albite might be easily overlooked. thin bands
The
of micaceous
are well
the quartzites
seen
and
black
schists
which
occur
amongst
in the roadside
quarry
south of Dunnamanagh
the quartzitic
group
as compared
illustrated in P1. VI, fig. 1. Taking
a general
view
of
discussed below, it would seem to have been less altered. biotite
is rare, and when
portion
of
the group
present
represents
forms a series
with
those
Secondary
The larger very small crystals. of sediments of an arkose type
only slightly affected by regional metamorphism. to be steadily series appears broadening the east, although the numerous quarries recently for road metal have shown that the outcrop of opened at this horizon than the existing maps would lead one quartzite is much more extensive of the whole series is diminishing both in to suppose, yet the breadth The
outcrop
westwards.
of
the siliceous
Towards
and in the persistence and breadth of the siliceous bands. general width with The northern edge of the series has not yet been determined to enable it to be shown on Plate VIII, but it seems to sufficient closeness the author
that
east of Dungiven. As to whether
the whole
series disappears
entirely
to the north
or north
or to is due to overlap, overthrust, this disappearance or synclinal fold is a question which of pitch on an anticlinal before it can be satisfactorily requires more extensive mapping answered; the last hypothesis is the most probable one but in the author's opinion the effect
of the three.
146
Proceedings of the Royal Irish Academy. GROUP. 4.-TmE DuNGivEN LIMESTONE
To
the south-east
the schists
of
and quartzites
the
of Newtownstewart
amount of siliceous material diminishes, and the quartzite is replaced first and
lenticles
by
then more
or
less
bands
continuous
of
limestones,
the
essential constituent of the Dungiven Limestone Group. The width of outcrop of the Limestone Group varies considerably. It is widest to the east of Dungiven, where, however, its continuity as shown on the published
maps
to the author
appears
to have
been
somewhat
a great
divergence.
over
estimated. The one
or
instances
layers
pelitic are only
to rapid
such
as has
lateral been
thick.
Lack
variation,
recorded
of
limestone
in other
of sufficient
exposures
cases
up
these banids
and difficulty
to say definitely
or to the splitting
In
without
is developed
(5, p. 16);
it impossible
in
this
whether
of a particular
The actual (13,, p. 25). is difficult to fix, as the limestones
from Argyll
of this division
boundary
feet
by J. Nolan
as noted
the seams make
due
eastern
to 250
100
a few inches
correlating
beds also shows
of the calcareous
thickness two
is
bed south again
diminish in this direction more or less gradually, the limestone outcrop changing from bands to discontinuous lenticles before it disappears entirely. Even when
the fullest
allowance
is made would
the thickness of the whole group than that found greater, for example, The
general
and
texture
colour
for close folding and repetition to be considerable and appear
in north-east of
the
Antrim.
limestones
are very
>ariable.
and somewhat schistose, but around Newtown they are dark-blue near and saccharoidal, while they are yellowish they are white black and presumably and occasionally graphitic, the Butterlope Pass, out It may be pointed near their contact with the epidiorites. especially
Usually stewart
that
the black massive
Co. Antrim
of dark calcite up to varieties with large crystals identical with those which occur at Torr Head,
in length,
ten millimetres
(cf. 1, p. 169), are only such as the one intrusions
found
near
to the
thickest
of
the
fig. 2, from epidiorite one in the locality valley. Dungiven and those Altnaheglish a feature show traces of dolomite, do the limestones (Tamnagh Bridge) to the north which seems more typical of those lying further (cf. 6, p. 10). Rounded
grains
illustrated
are almost
of quartz
invariably
in PI. VII, At only
present,
the proportion
being unusually high around Craig and Dungiven. Dark
is very
mica
in six-inch
and
Iron times
common.
It occurs
(presumably
Its abundance
elsewhere. Portlock
bands
as films parallel
bedding
in the
limestones
planes) has
(4, p. 20), Wilkinson (25, p. 181), Kinahan common is another constituent, pyrites
reach
10 mm.
in diameter
to the foliatior
at the Butterlope
cand
on by and others. p. 10), (6, and the crystals some
been
around Newtownstewart.
commented
HARTLEY-Dalradian Rocks of the Sperrin Mountains.
14'
The occasional presence of muscovite and absence of any recognisable orthoclase or microcline felspar would indicate, according to Prof. C. E. Tilley's note2 on the Loch Tay limestones in Argyllshire and Perthshire, that
the metamorphic
grade
is not
here
higher
than
that
the Biotite
of
Zone. The schists associated with and intercalated amongst the limestones are rarely felspathic or gritty, but more usually dark-grey or black, soft and very rich in biotite. To the south-east of the limestone group in the Glenelly valley and to the east of Gortin two exposures of limestone are marked on the one-inch maps. These outcrops, not now exposed, are referred to in sections 8 and 9. Since both these limestones, which were very thin and of such inferior quality that their working has been long since abandoned, are not associated with epidiorite, it is considered improbable that they represent the Dungiven
series
repeated
folding,
by
but
rather
that
they mark
a
separate and distinct calcareous horizon. TYPES.) 5.-EPIDIORITES. (EXTRUSIVE North of Plumb Bridge the limestones are succeeded by two bands of dark-green schists. The northerly band is narrow, but the southerly one is broader with
a maximum
on the western
side of the Butterlope
the general
of about half
the northerly
Pass. band
traced
up by folding as an inlier. in either direction along
extent
does not
south brought when
structure
width
lateral
amount
to more
a mile,
To
judge
is a repetition
they
include
Craig
which
of
is well
exposed
the dips and the one to the
outcrops become narrower strike, and their maximum
the
seven miles, while the feet. three or four hundred
than about
Many
and schistose
from
Both
would not be more than of these sehists, as, for example, the wide were not previously differentiated the Butterlope, thickness
greatest
and
outcrop at the head of schists from the mica
but as shown on P1. VIII grits by which they are surrounded, and a few of the exposures such as those of Stranagalwilly as epidiorites and were were shown on the Survey maps
to as such in the memoirs (cf. 5, p. 17). in the field they are all massive schists with rusty dark-green too uniform and fine in for any of the weathering. Though grain minerals to be identified in the show constituent field, they usually referred As
seen
effervescence with hydrochloric acid. shows them to be as a rule well foliated and examination Microscopic of a felted mass of very pale green, sometimes almost to consist essentially extinction from 18 to actinolitic colourless needles, with angles ranging 2'' Vesuvianite vol.
lxiv
(1927),
and Grossular p.
374.
as Products
of Regional
'' Metamorphism.
Geol.
Mag.,
Proceedings of the Royal Irish Academy.
148
20 degrees. Epidote and zoisite3 are usually present as scattered grains or granular aggregates, and they show in addition irregular patches of calcite and occasionally quartz. Small grains of albite have been noted, but this mineral rarely forms definite phenocrysts or porphyroblasts. In one or two cases, such as specimens from Stranagalwilly, the amphibole builds larger and more platy crystals. It is darker in colour and associated with crystals of ilmenite and leucoxene. The epidote and zoisite instead of occurring in scattered grains form granular aggregates about 2 mm. in length, suggestive of pseudomorphs after felspar. A similar type also occurs to the west of Dungiven. These particular types seem to approach in character the intrusive epidiorites which are discussed below in section 12. Other specimens, from the Dungiven area, represent varieties in which irregular phenocrysts of chlorite and felspar are present, while there is much white mica in the hornblendic matrix. With such variation in texture and degree of foliation the types range from hornblende schists to amphibolites, and since there was little in the way of igneous features to be seen under the microscope the use of the term epidiorite seemed hardly justifiable till field evidence showed that the rocks were, at least in part, not only of igneous but of effusive origin. The evidence for this statement consists of the pillow-structure, a feature
is so far as known confined to rocks of volcanic origin. lying about six miles niorth-west of Craig, a village
which
The
epidiorites
of
Plumb Bridge, supply one of the best of the few exposures showing definite pillow-structure. This section is illustrated in P1. VI, fig. 2. Here the pillow-like forms are rather small, averaging about eighteen inches across. or of concentric signs of chilled margins, do not show the usual to be expected when the generally but these are perhaps hardly amygdules, to which as well as the metamorphism fine grain of the material they have
They
undoubtedly been subjected are taken into account. Microscopically they differ little from the amphibolites of theButterlope described above. They are distinctly schistose, and small needles of tctinolite
felted
together
make
up
the bulk
of the rock.
Grains
of zoisite
are fairly plentiful, and a few small oval-shaped and quartz-filled lenticles may represent amygdules. The material green
between
in colour
and
It is grey is deeply weathered in. the pillows in texture as shown on P1. VI and consists
spongy
almost entirely of granular zoisite and quartz, with a little pale actinolite, muscovite, clinozoisite, calcite, and hematite. Though it may represent an altered sediment, such sediment must have been of approximately the '
Zoisite
(25, p. 209) of some of
to the epidiorites amongst The latter J. S. Hyland (4, p. 3-6). outside the area here these though rocks,
was
and
observed
north-west writer described.
by gives
both excellent
J. E.
Portlock
descriptions
149
HAxRTuY-Dalradian Rocks of the Sperrin Mountains. as the lavas same composition unusual. It is quite distinct lavas of the Tyrone beds,"
Igneous and
limestones
slates
themselves, and its general type is somewhat the pillow from the cherts found amongst Series (19, pp. 228 and 236), from the "green (23, p. 17), and
of Strabane
the quartz
from
hornblende, or quartz-mica oligoclase assemblages found by Tilley4 to compose the interstitial material of the pillow lavas of Cornwall. Further to the north-west Dr. McCallien has described (23, pp. 13-22) pillow lavas from two horizons at Strabane and Altigarvan. Although
do not
lavas of Craig
the pillow
those of Strabane
resemble
they show a closer similarity to some of the non-porphyritic types of Altigarvan, except for a scarcity of albite. It
is not
possible
to say with
certainty
that
Butterlope-Craig rocks lie on the same horizon. not directly
but
continuous,
are separated
the Altigarvan
and
The two exposures are
by some six miles
of a calcareo
siliceous assemblage not yet examined in detail, while, in addition, there is a certain
of
amount
faulting
between
them.
There
is, for example,
almost certainly a fault, running north - east south -west through Craignagapole, which cuts off the limestone on the east. Nevertheless the present author thinks it highly probable that the two sets of pillow-lavas
lie at the same horizon.
As previously mentioned the outcrop of the quartzites narrows towards the east and shows signs of thinning out altogether in that direction, while it is succeeded
to the north-west
by a band
limestone
of
a series
and
of
pebbly grits which are separated by lavas similar to those at the Butterlope and Craig. The threefold succession found on the two sides of the quartzite is so similar
repetition.
as to suggest
In both
cases we
find an association
with epidiorites, and although the two bands cannot actually be traced into each other owing to the overlap of the Carboniferous they are running eastwards in a manner which would if continued lead apparently to ultimate convergence. Though such evidence is perhaps not strong enough to be conclusive, it is at least of considerable presumptive value. It may
be noted
by McCallien succession,
incidentally
that of Altigarvan
6.-THE Succeeding
that while
the Strabane
(23, p. 16) reverses structurally repeats
section
as described
the general Sperrin
it in the same order
(23, p. 14).
SCHISTOSE GRIT GRouP OF DART.
the epidiorite
lavas towards
the south-west
is a rather
thick
succession of schists of somewhat varied type, but distinguished in the mnain by their gritty and siliceous character. *" p.
185.
Metasomatism
of Greenstone
Hornfelses,
Cornwall.
"
Min.
Mag.,
vol.
xxiv
(1935),
Proceedings of the Royal Irish Academy.
150
At their north-westem edge there occurs a pebbly band with pebbles to about
up
and vein
of an inch
three quarters This
quartz.
band
of quartzite east of the
in diameter
is seen north
composed of Plumb Bridge,
Butterlope Pass, on the slopes of Oughtnager and south of Craig. No compound pebbles have been detected in any of the exposures. The succeeding beds consist mainly of grits and highly siliceous granulites or sometimes quartzites, which formmassive and rugged features of the summit
(ef. 11, p. 11) along much
ridge
of the main
Sperrin
range.
They are especially well seen near the summits of Dart and Sawel. Another feature is the quartz phaeoids which though of sporadic occurrence in some of the other groups are most extensively developed the Schistose
amongst
Grit
of Dart.
Group
in various
occur
They
forns,
as knots, strings or lenticular seams usually conforming to the convolutions of the foliation but occasionally transgressing these in a sill-like manner.5 In some instances, especially in the neighbourhood of known fault planes,
they
modifications
show kataclastic
from
crystalline
these
varieties
to a more sometimes
granoblastic take
on
with type.
of
an alteration much
When
appearance
a breeciated
texture
broken
down
of
pseudo
sedimentaxy type. The
appears
quartz
to be
the product
of
the breaking
down
of
the
higher silicate compounds originally present in the sedimentary material the schists
from which
are produced,
and not derived
from external
so.
ces.
As pointed out by P. W. Clark,6 quartz is a bye-prduct of the sericitization of felspar either orthoclase or plagioclase, whilst the production of albite, together with epidote, zoisite, or actinolite from plagioclase, as well as tourmaline from more basic silicates would in every case lead to an excess of silica.8 There is of course a strong tend,ency for the silica so released to segregate in lenticles or inconstant bands, the process of segregation being one of continued local solution, diffusion and recrystallisation, and the transference being effected along lines perpendicular to the maximum that
pressure,9 with
is to say along
the intensity The
true
the planes
its extent
of schistosity,
varying
of the metamorphism.10
schistose
grits
are
somewhat
and
variable
impersistent
in
their distribution. They are, where mapped, indicated with circles on Their outcrops usually form lenticular areas which thini P1. VIII. and thicken rapidly when traced along the strike. Petrologically they are often
very
B Cf. Data 7
loe.
cit.,
p.
coarse
E.
in grain
as illustrated
in P1. VII,
of Anglesey, Mem. Geology Greenly, of Geochemistry, U.S. Sur. Bull., Geol. " 598. 8A. Harker, Metamorphism, 8 loa cit., pp. 203 and 207.
Geol. 695
fig. 1.
Surv.,
>> London 10
loc. cit,
p. 48. p.
(1920),
597.
(1932), p. 67.
p.
121.
HIARTLEY-DalradianRocks of the Sperrin Mowntains. The
quartz
is usually
white
or water
but
clear,
rounded
151 of a
grains
blue variety occur south of Tamnagh Bridge and in a few other localities. rather large, up to and contain Occasionally the grits are felspathic The localities of the more felspathic 10 mm., rounded crystals of felspar. types are
are
indicated
too cloudy
by
and
black
the heavy
decomposed
dots
for exact
on P1. VIII.
The
determination,
feilspars the optical
but
properties would appear to indicate albite or albite-oligoclase. The as an
general
gritty
indication
of
it is proposed
character its
to correlate
conglomerate
character
and of North
East
of the Glen
note
Sluan
is a feature
those
that
the
or Pitlochry to which
this gtoup,
of
to
similarity
it, one may
Antrim,
feature
is the special
lithological
and
beds with which coarse gritty and of Kintyre, has frequently
Schists
attention
been drawn
(cf. 3, p. 418, and 1, p. 170). Further and at structurally higher levels a more pelitic south-west type of schist is developed associated first with albite and then with albite
and tourmaline. SCOLISTSOr GLENELLY.
7.-TouRvALiNE The
Schistose
Grit
group
of Dart
to the south by a series
is succeeded
of quartz-muscovite-biotite schists with much tourmaline. They form a belt
nearly
through
four miles
the centre
in the Glenelly
wide
by a narrow
valley.
band of dark green
is broken
belt
This schists
(also tourmaline
bearing) which are discussed below in section 8. Although the tounmaline bearin,g schists on either side of the green schists are indistinguishable in the field the difference in the groups by which makes
to the north-west they are succeeded it probable that they are not repetitions
one another stratigraphically. as the Tourmaline
Schists
and south-west respectively of the same band but succeed
On Plate VIII
of Glenelly
they are thus distinguished
(north)
and Glenelly
(south),
but
since they are identical petrologically they will be here treated together. Though tourmaline does occur in one or two exposures outside themain t&urmaline
bearing
areas
as,
for
example,
west
of Dungiven
Bridgfe, south-east of Dreen and east of Altnaheglish,
and Eden
such occurrences
are small, rare and isolated. It is not possible to say whether inliers of the main band or a repetition of the same sedimentary
they mark conditions
at another horizon. These tourmaline schists of Glenelly differ from the Dart group in their higher percentage of mica and generally more pelitic character. Derived felspar is rare, though lentieles and bands of authigenie albite, which reach their maximum in the succeeding division, begin to show sporadic development. The tourmaline is most obvious in certain soft grey coloured varieties which
are very
rich
in mica.
up to one centimetre in length.
It occurs
in the
form of acicular
needles
Proceedings of the Royal Irish Academy.
152 As
noted many
ago by General along and across curved
as
sediment
196), also
especially
round
through
in the albite in a it may, if Tilley's
a particular marking by its abundance type of have been also inherent in the composition of the and hence be of which these schists were derived
to the albite be accepted,
original
a feature curve
they frequently
in Tyrone as inclusions from Cowal (9, p. 78)
oceurs Since the tourmaline similar manner to that recorded views
the
of the crystals,
character
common on the Glenedra Pass where an angle of ninety degrees.
(25, pp. 180 and foliation. This writer
Portlock
years
the needles run both refers to the frequently
mud
from
iefinite stratigraphicaJ value. Its relative
certainly
abundance
ship to the intrusive epidiorites The only change which may in size shown
to be expected, conditions and
since
feature
lknown
or to pegmatite be of metamorphic
the tourmaline
by
its
rapid 22,
of
from
17) and
recovery
relation
is some
increase
This
is only
under
accidents
kataelastic
such
any
areas.
for growth
tourmaline
recovery p.
veins. origin
in the gamnetiferous
the facility
(ef.
shows no signs of bearing
suitable is a well
be assisted
would
by
entrance into the garnet isograd. as described
Tourmaline,
from
the Dalradian
rocks
of other
localities,
in the Beinn Bheula or Ben Ledi to be most typically developed or or its equivalents, as well as the Green Beds and Glen Sluan
seems Group
Pitlochry Schists. It has been noted at the first horizon inArgyll (13, p. 16), Cowal (9, p. 40), (3, p. 414) and North-East Antrim (18, p. 131); in the Green Beds
Kintyre
(,f Argyll (3, p. 416); p. 307), etc.
(11, p. 13), Cowal (13, p,. 19), Dalmally (9, p. 35), and Kintyre and in the Pitlochry of the Ben Lwers area (24, Schists
It is these three particular by the tourmaline
schists
groups which
of Glenelly
are considered
and their
to be represented band of " Green
intercalated
Beds." In the Scottish
memoirs
reference
is frequently
one, it is not by any means exact. area both to the north-west and
made
to the association
45, 78, and 299, 3, p. 416, 13, p. 10 this correspondence is a very common
of tourmaline and albite (cf. 9, pp. 18, p. 131, etc.). In Tyrone, though
The
albite
south-east,
types overlap whilst
many
the tourmaline of
those
schists
richest in tourmaline are albite-free. SCHISTS OR " GREEN BEDS.
8.-ALB1TE-BioTITE-EPIDoTE For east
purposes
perhaps
of correlation
the most
albite-biotite-epidote of Scotland the Dalradians described
by Dr.
with
important schists which
E. B. Bailey
those
areas
stratigraphical are equivalent
as well
as
and Dr. W.
to those
the north lying towards is that of the horizon to the Green Beds of of North-East
J. MeCallien
(1, p 167).
Ant-rim
llARTLEY-Dalradian UIp to the present
Rocks
these Green
the Sperrin
Mountains.
have not been
separated
of
Beds
153
from
their
the published maps they are though occasionally given the same colour as the "mica schists" in as epidiorites, but their importance was not of course realised
surroundings usually marked
in Derry
and
On
Tyrone.
the seventies of the nineteenth century when Scottish Dalradian geology was but little advanced. These
particular
band which This main across.
band
This
in a thin but apparently
beds occur
can be traced
across
is always
is about
narrow
the same
has for the sake of clearness In addition
to the main
and most being with
are found
continuous
the folding likely
band
several
since
type of sedimentation
"Green
Beds"
have
been
from
the Dunoon
sixty
yards
(1, p. 173), but
of a mile the breadth
isolated
recorded (12, of the outerop
exposures
of
and south-east. in the section such a view
the same
The
longest
on P1. VIII
fits in most
as
readily
however, exposures, or later persistence
conditions. recorded
from various
(9, p. 89), through or Ben Ledi (2, p. 98, 12, p. 24) to the Ben Lui Schists is stated that some of these horizons cannot be referred ranging
about
exaggerated.
is interpreted
outcrop,
than
The other of the surrounding beds. to indicate either an earlier incoming
of the same special In Scotland
somewhat
conitinuous
to Draperstown.
in Antrim
to three-quarters On Plate VIII
to the north-west
an inlier of the main
are more
and not more
been
of these
fairly
from Omagh
as its outcrop
less than the half considerably p. 8, antd 3, p. 415) for Kintyre.
type of sediment
the area
Phyllites
horizons
the Beinn
Bheula
(2, p. 98), and it to a single group
reduplicated by folding (2, p. 100). In one or two localities, at Garvagh Bridge
and
such as the Oughtboy Burn north of Cranagh in the Glenelly are valley, the Green Beds
limestones with very caleareous lenticles or even thin impure This though of very poor quality have been at one time worked. the with nature of the Green Bed horizon is comparable calcareous associated
which
characters
described
The Green Beds
Albite
by McCallien
are in the main
of Glenelly
in South Knapdale.
(8, p. 161) as found
is an invariable constituent.
albite-biotite-epidote
schists.
It forms good porphyroblasts,
ranging from 0 5 mm. to 2 mm., crowded with inclusions of epidote, zoisite, etc. twinning is fairly common, but the lamellar type has not Simple
been noted. Dark mica occurs interstitially between the albite porphyroblasts in considerable
quantity,
distinguished and
chloritised,
from but
and
the colour
in the garnetiferous areas. sericitic
schists
by
the epidiorites
enables
these
beds
to be
It is as a rule greenish (cf. 1, p. 173). becomes darker and more definitely brown
The foxy red type found amongst the more
of the succeeding
amongst the Green Beds.
its abundance
Mullaghearn
Group
has not been observed
154
Proceedings of the Royal Irish Academy.
The quantity of epidote is very variable; though sometimes plentiful, in other
it may,
be almost
cases,
completely
lacking.
Quartz as microscopic lenticles or discontinuous bands of granulitic texture, suggestive of sheared pebbles, occasionally occurs, but the Green Beds of Tyrone are not characteristically gritty. White
mica
so plentiful
is occasionally
as in the beds
present
to north
in small
amounts,
but
it is never
and south.
The garnets, which are discussed more fully when dealing with the question of metamorphism in section 17, are always small, never more than 0 5 to 10 In spite of
mm.
in diameter. the occasional development
of garnet, hornblende has not in any of the slides of Green Beds examined been observed from this area. The absence may be due to the small amount of the, presumed potash effect bearing, white mica which has an inhibiting ment as pointed out by F. C. Phillips (10, p. 253). 9.-THE The Green group
Beds
are
of schists which
They
are on the whole
develop
MULLAGHCARN GROUP.
succeeded
have
on hornblende
to the south-east
a width
of outcrop
similar
very
by
another
of about
variable
four miles.
to those of the Dart
Group,
though
more micaceous ;1 but felspar, excluding albite, is distinctly subordinate in amount, and traced towards thoroughly
there
is a gradual
the
south-east
increase at
till,
in the percentage the boundary
of mica
fault,
we
when have
a
rock.
pelitic
rather silvery quartz Speaking they are characteristically generally, or quartz-muscovite-biotite muscovite schists and chiefly of a consist of quartz and sometimes granulitic admixture albite lying between foliae
of sericitic mica and biotite. to the above
In addition
there
are many
in the field. can be distinguished There are first a few thin bands similar
to
found
those
to
of very
the north-west,
intermediate siliceous though
varieties
which
quartz-mica
granulites
less
extensively
here
developed. Albite horizons
are exceedingly in addition while certain common, alternation of sediment type enables the original bedding seen and a few impure limestones deserve special mention.
schists at which
to be distinctly
9a.-THE is as common
Albite
Bheula
amongst
the Beinn
(3, p. 414), Cowal
In Kintyre
indistinguishable
the from
Glen each
as it is the Mullaghearn Group or Ben Ledi Schists of Antrim (1, p. 175), It (9, p. 39), or Arygll (13, pp. 13 and 15).
a constituent
Kintyre u
ALBITE SCHISTS.
Sluan other,
and cf.
of
Beinn 12,
Bheula p.
26.
Schists
are
at
times
lithologically
HARTLEY-Dalradian Rocks of the Sperrin Mountains. also extends
into the Green
to the north-east
Beds
and
155
into a few scattered
areas along the southern portion of the Dart Group. The albites possess the usual characters, being clear and colourless, fresh and undeformed, while they are usually crowded with inclusions. Their freshness renders them at times difficult to distinguish from the quartz without the use of convergent light. The size varies from about 2 to 10 mm. where
what
The
largest
appears
crystals
occur
to be a segregation
at the head vein
of the Finglen
of quartz
River, a
and albite, with
width of about four inches, runs parallel to the foliation. The nature of the felspar and the absence of any definite intrusions from this portion of the district would seem tomilitate against any suggestion of igneous action at this particular spot, while segregation has been suggested for similar veins
in Antrim
(1, p. 166), Knapdale
(12, p. 18), Oban
(11, p. 11), and
elsewhere. The albite of the Green Beds is frequently confined to definite bands of a width
of about
run parallel
1 cm. which
to the true
dip,
and
the
intervening layers equally well foliated are albite free. Such development which is similar to that recorded by Clough from Mid-Argyll (13, p. 15) to support the view of Professor C. E. Tilley (17, of the albite depends on the nature of the original
would appear certainly p. 108) that the presence
sediment. There are, however, other areas such as the Glenelly valley east of Cranagh, where the edge of the albite development follows a sharply marked irregular line having (apparently) no relation to strike or dip. Lines of incipient overthrusting, and the generally higher degree of shearing stress which developed towards the sout4i evidently favoured and
segregation,
an
therefore
increase
in the average
size
of
the albite
crystals.'2 Those found in theMullaghearn Group of Schists are usually larger
than
those which
sheared Dart
in the less highly
occur
Group.
9b.-BEDDED TYPES. There
are many
localities,
presence
especially
near
the northern
of
margin
the
the edge of the Dart Group, where and aqueous character of the beds are revealed by the Fine or in texture and composition. bands varying
schists Mullaghearn original stratification
and
of distinct
the southern
coarse grained grits alternate with softer micaceous layers representing altered The
sediments localities
of more
type. pelitic of these particular horizons,
of the true dips to be determined, detail, since many of them are shown on the map the schists. lie within of those arrows which and amounts
In the bedded inches, 12 A
though similar
types
usually relation
the thickness somewhat
apparently
occurs
enable
which need
not
(P1. VIII)
the directions be described
in
by the positions
of the bands may be as much as six it is as a rule difficult
less, and although in Cowal
(9, p. 43)
and
Argyll
(13,
p.
15).
Proceedings of the Royal Irish Academy.
156 to follow possible
for more
they can be traced
similar
than
folding,
alternation
of
a few yards
for a quarter
practically
the dip continues or intense isoclinal
changes A
sections
individual
occur where
of a mile,
unaltered except
schistose
at a time, and when
with
no
signs
on a very minute
grits
of
eases this is sharp
scale. and micaceous
or quartzites
schists of (or Beinn Bheula) Kintyre (3, p. 414), Argyll (11, p. 11), as well as (13, p. 15), and Oban in the Pitlochry and Ben Lui Schists of central Perthshire (24, p. 307). has been
bands
The by
a
noted
thin band small
calcareous
of
among
the Ben
impure
limestone
anticlinal
flexure
schist which
may
occur
Ledi
brought be
in the Beinn
to the east
up
compared
the
with
Bheula
schists
of Gortin lenticles
of Argyll
of (13,
p. 12) and elsewhere.
10.-THE BLACKSCHISTSOF BROUGHDERG. The petrology and general distribution of these beds have been discussed in a previous
communication
not be recapitulated. pelitic
of
schists
graphitic
dust,
Schists
a dull
black
be briefly or
above
with
referred
the Tyrone
has shown mapping limits of the schists
and need (19, pp. 225-226) as very fine grained described
lead-grey
in some cases a little
and
In the paper Black
to this Academy
They may
colour
finely
Series in smaller
that
they occur
of
the Sperrins,
quartz,
iron ore.
the to group was inclined (19, p. 226), but the present
to the writer Igneous
of
composed
divided
apparently
exposures well as infolded
within outliers
the or
inliers. These found
Black
amongst
Schists
also
the schists
share of
in the higher
the Sperrins,
and
degree of metamorphism are even more intensely
puckered and crumpled. It seems most the Sperrin
series.
probable, It may
that they should be included with therefore, be noted in this connection that Black Schists
to the above, have been described similar or phyllites, lithologically by of the Cowal area (9, p. 9), C. T. Clough from among the Dunoon Phyllites of Tyrone may this that the Black Schists and it is possible represenit
horizon. 11.-SUGGESTED The briefly
lithological discussed
variations together with
CORRELATION WVITHOTHER AREAS. of
the different
the close parallels
to the north-east in order to examine When and S.W. Scotland. Antrim with types
groups offered
in Tyrone
has been
by the Dalradians
the possibility of correlation with the relative geographical positions
of strike, and the order in which the various respect to the direction it is sucteed each other along the dip are also taken into account,
157
HARTLEY-Dalradian Rocks of the Sperrin Mountains.
thought that the following correlation may be put forward with some confidenee
as to its correctness:
South-west Scotland and Antrim.
Sperrins.
Stonefield or Ben Lui Schists. Loch Tay Limestone.
Newtownstewart Quartzitic Group. Dungiven Limestone Group. Epidiorites and Amphibolites (extrusive types). Schistose Grit Group of Dart and Tourmaline Schists of Glenelly (north). Schists of Albite-Biotite-Epidote Glenelly. Tourmaline Schists of Glenelly (south) andMullaghcarn Schists. Black Schists of Broughderg.
Glen Sluan or Pitlochry Schists.
Green Beds. Beinn Bheula or Ben Ledi Schists. Dunoon Phyllites (in part).
As to the correctness of these suggested correlations therewould appear to be, with the exception of the first and last, very little room for doubt. The evidence derived from the numerous lithological similarities shown by the rocks in Tyrone to those beds to the north-east with which they are correlated is considerably strengthened by the discovery of the "Green In addition, not only Bed" type of sediment in its expected position. very closely with agrees general succession whole the but groups individual Cowal. and Kintyre, that described from Antrim, The question of the position of the Black Schists has been discussed in section 10, but a few remarks may be made as to the suggested the greater part of it That Group. of the Newtownstewart correlation
already
represents the Stonefield or Ben Lui schists appears to be certain, but this correlation
may
not
to the whole,
apply
even
of
the mapped
portion,
of
this group, and some of it may possibly though not probably represent the Erins Quartzite. Although these two groups were considered'3 to be separated in S.W. by
Scotland
an
break
tectonic
important
are evidently very similar lithologically. in North
their occurrence between division p. 33).
"Recent Proc. PROC.
G. L. Elles
evidence, Soc. Geol. R.I.A.,
VOL.
3, p. 424),
they
Thus J. B. Hill, referring to
says (12, p. 29) that there Kintyre, drawn upon and that "the boundary
is no sharp is the map
(12, value it loses much of its stratigraphical (2, p. 96), to the difficulty of separation (16, pp. 628 and 637). and Prof. C. E. Tilley that
Dr. E. B. Bailey
as do Dr.
Abs.
them,
so indefinite
in places
(2, p. 96, and
also refers
however, (1936), XLIV,
No.
suggests 1304,
SECT.
B.
this view 37.
may
require
some
Cf.
modification.
p.
l1v]
Proceedings of the Royal Irish Academy.
158 The been
bands
of
observed
lead-grey
amongst
between
the Butterlope
common
to both
evidently bands
of
little.correlative
Schists
of
this horizon
have 145, which on Bessy Bell and
seem also to be a feature 12, p. 31, and 2, p. 96), so are the same time locally important
and Dunnamanagh
in Scotland
and
to on page
referred
the quartzites Pass
groups
of quartzite
Stonefield
schist,
(cf.
value.
At
schist
quartz
(12, p. 27, and
are
to occur
known
the
amongst
3, p. 420).
The question of the presence or absence of the Erins Quartzite must be left open
therefore
till
has
the mapping
been
carried
further
to the
north-west. 12.-INTRUSIVE TYPES OF EPIDIORITES. The extrusive types have been discussed above, but there are, in addition, numerous exposures of hornblende schist, some of which are definitely intrusive and differ in texture from the rocks of similar composition
previously
described.
They are extensively developed amongst the schistose grits of theDart area,
along
the main
watershed
of
where
the Sperrins
form
they
bold
crags, and in the south-eastern portion of the limestone series. Most
of
the main
exposures
are
on
indicated
the published
maps
of
the Geological Survey, but additional areas have been discovered on the eastern
side of the Glenshane
case, however,
at a point
Pass
about
and
in other
two miles
localities.
north-east
In at least oine
of Newtownstewart,
the rocks mapped as epidiorites, here definitely intrusive into the lime stone, are quite unfoliated and unaltered dolerites, probably of tertiary age. The joining up of isolated exposures where the outcrops are narrow and exposures infrequent is of necessity somewhat conjectural, but the trend directions
general
and
the variety
of type have been carefully
borne
in mind in the reconstructions. junctions
The
with
the Stranagalwilly
the schists
Burn,
and east
as seen on the western of
the Glenshane
Pass,
ridge
of Dart,
etc.,
in
are sharp
without gradual passage. As
in Scotlarnd,
the epidiorites
are on the whole
more
in the nature
of
to the This is not only shown by their general conformity sills than dykes. to which attention was strike and dip of the associated schists, a feature (4, p. 25, 5, p. 12, 6, p. 12), frequently drawn by the earlier surveyors but it is also clearly seen, for example, on the western portion of the Dart
summit ridge where small inliers of epidiorite can be discerned below the gently
inclined
schistose
grits,
and
in the section
at the old Priory
Church,
Dungiven, illustrated in P1.VII, fig. 2. On intrusion clearly
hand they are occasionally out by Egan has also been pointed
the other shown
in P1. VII
fig. 2.
Evidence of transgressive. and Nolan (5, p. 12) and is
HARTLEY-Dalradian Rocks of the Sperrin Mountains.
159
One point in the occurrence of these rocks is somewhat difficult to explain, that is their apparent local disappearance, or at any rate diminution, in the area to the south of Dungiven. That this is a real absence
and not due
to the lack of exposures
seems
to be indicated
by the
almost continuous sections supplied by the Finglen, Glenedra, and other rivers. The most probable explanation is that the epidiorites actually represent a comparatively limited number of sills, and that they appear more numerous to the east and west owing to the close folding, with the ground surface here running not only approximately parallel to, but also closely adjacent to, their surface. Microscopic Petrology.-The great amount of foliation and alteration to which these rocks have been subjected has left few, if any, traces of ophitic or other igneous texture. If it were not for the field relations they would in many cases be described rather as amphibolites or even hornblende schists than epidiorites. The minerals present include horublende, felspars, ilmenite, and leucoxene, also epidote, chlorite, quartz, biotite, and garnets. The hornblende occurs in plates with ragged edges about 0 5 mm. in length and breadth. Both the colour and pleochroism are distinct and strong. Occasionally, however, the amphibolite element is represented by pale-green actinolitic needles similar to those found in the basic lavas. The felspar is chiefly albite in small granules scattered amongst the quartz. The crystals are untwinned and not distinctly porphyritic, except in one locality on the Finglen River where they reach 0 6 mm. in diameter. At this particular exposure oligoclase showing lamellar twinning is also present. Ilmenite is usually plentiful either as scattered grains or as pseudo morphic replacements after pyroxene. In many, though not in all, cases the ilmenite is partially or entirely replaced by sphene and leucoxene. Epidote and chlorite are never-plentiful and sometimes entirely absent. The occasional lack of the former mineral amongst the Scottish epidiorites is a feature
noticed
by Wiseman
(15, pp. 359 and 387).
The chlorite when present frequently shows anomalous interference colours, browns and purples, comparable to those recorded by Phillips (10, p. 244) and Wiseman Quartz
occurs
(15, pp. 360, as a granular mosaic
361, and 365). which
builds
up
small
lenticles
amongst the amphibole. Mica
is represented
by brown
biotite.
The green
variety
has not been
observed. occur within that portion of the epidiorites which red garnets on Plate the line marked lies within VIII. by crosses in connection with these epidiorites The two chief features of interest Small
are, firstly the nature of the original material fromwhich they were derived, and secondly
the grade
of metamorphism
to which
they have
attained.
[Q 2]
Proceedings of the Royal Irish Academy.
160
Wiseman in his valuable discussion of the epidiorites of the Highlands suggests
(15,
p.
that
371)
the
amphibole
and
chlorite
were
formed
simultaneously from pyroxene and plagioclase felspar with the extraction of alumina
from
No
doubt
lime.
the anorthite
and liberation has been operative
of albite,
molecule
the same process
here,
silica, and as in any case
the origin of the albite in these intrusions must be essentially different from
its mode
would
account
of formation for
in the detrital
the superfluous
silica
schists. and
The quartz
the sphene
and
and sphene epidote
for
the liberated lime. The comparative scarcity of these lime-bearing minerals and the occsional presence of unaltered ilmenite would, however, suggest even in default of analyses that the intrusive epidiorites were originally much in soda
richer
than
in lime.
As regards the metamorphic grade the evidence would seem to point to a horizon
just
either
above
or
just
below
the garnet
The
isograd.
optical characters of the hornblende, the scarcity of chlorite (cf. 15, pp. 361 and 380),
the nature
of the mica
(cf. 15, p. 387), and the occasional
presence of small garnets all point in this direction. Although the presence of garnet does not necessarily imply a high but may
of metamorphism
grade
depend
on the original
of the
composition
rock,'4 yet, ifWiseman's criterion (15, p. 407) for the Highland epidiorites be accepted, is indicative zone
and "the presence of garnet combined with -an albite felspar situated at the commencement of the garnet of an epidiorite
as defined
Tyrone
would
agreement
with
of some of the epidiorites sediments," pelitic a also be in lie within conclusion would this zone. Such the character of the mica schists which bound the epidiorites in normal
on either side. 13.-RELATION is little
There
BEWEEN THE EXTRUSIVE AND ITRUSIVE that,
doubt
although
it has not
been
EPIDIORITES. found
to
possible
the two types of amphibolites into each other, or epidiorites directly and probably contemporaneous. they are closely associated in the field and similar degree of metamorphism Their close proximity this conclusion. would support They are both on the whole " epidiorites" trace
to use in its original sense to signify a metamorphosed
the field term introduced igneous
rock (extrusive
intrusive
types were
by C. W. Von Giimbel5 or intrusive) containing
hornblende. Although survey maps
14Cf. C. E. (1923), ^Die
p.
198;
and
extrusive either
Tilley, also
pal?olithischen
from each other or, in many
''Met. references
not separated on the Irish cases, from the surrounding
of Start Area," Jour. Geol. Quart. Soc, to a paper by Wiseman by J. Suzuki (15, p.
Rocks
Eruptivgesteine
des
Fichtelgebirges,
Munich
(1874),
Ixxix
vol. 378). p.
9.
HARTLEY-Dalradian Rocks of the Sperrin Mountains.
161
schists, the surveyors recognised that both types might be present, and J. R. Kilroe suggested (6, p. 12) that, though generally intrusive, some of
them might
schists.
This
be
lavas
on
is certainly
account
the case
of
their
in Scotland,
general where
conformity
the presence of pillow structure in the epidiorites of Argyll and Kintyre
(12, p. 83), and now holds
to the
B. N. Peach
also for the epidiorites
showed
(14, p. 43) of Northern
Ireland.
14.-FAULTS. Excluding the boundary and overthrust faults to the south-east which have been
already
there are
four
in the form
treated
in a previous
important
faults
of a somewhat
the inside of the two V's
communication
in the Sperrin
sprawling
which
W, and each up this letter.
build
(19, pp. 263-264),
area, which fault Of
are arranged downthrows
these four
to
faults
those running east-west through Gortin, and north-south towards Magherafelt have the most important effect on the structure. They down in each
throw
case
towards
the
inside
of
the area
and bring
in the
two
tongues of Carboniferous which run respectively up the basin of the Owenkillew schists
and
These
recent
certain
been
probably
times,
degree
part
of the Roe valley, while or semi-inliers.
them
beyond
the
up as inlies along one of whic
brought
two faults,
the other most more
the higher
are again
control
the curved
and along movement has certainly i itiated or renewed in Tertiary or oI the distriet and also to a topogrnphy
either
the
character
of
the rock outcrops
shown
on Plate
VI II. somewhat the map running doubtfully and Omagh. between Dungiven Only the end portions, where it is bounded as faults on the original on one side by the Carboniferous, were marked of the Green Beds anld the maps, but from the effect on the outcrops The
third
fault
is shown
on
that it is most probably continuous extent confirmed is to a certain suggestion this suggested line crosses the three main by the important gaps by which this line the shattering of the rocks ridges of the Sperrin Group. Along transverse feature of erosion. has apparently given rise to an important east of Sawel where one of the only two The pass'6 at Cloghornagh roads crosses the main Sperrin passable range, Barnes Gap, and Gortin epidiorites
between
it appears
these
points.
to the author This
course. all three of these served Gap all lie along its suggested Though and were in consequence and widened, they as glacial overflows deepened marked in the must have initially depressions ridges and therefore -have been presumably
16This Barnes
Top
lines of weakness.
is continued depression and Meenard.
to
the
north-east
through
a prominent
gap
between
162
Proceedings of the Royal Irish Academy.
To ends
judge on
effect
on
character preceded have
its effect
on
this
the outcrops
the Carboniferous,
fault
of some of
initially
occurs
a wrench
in
the
Craig
Cunningham
and 645), Clough
Loch
or
in that
direction,
its two but
its
the Dalradian
as well as the horizons, a lateral element may have and that it may movement
torsional-tear
Tay
and
of
area, 17, p. 112), Elles
(in Tilley,
adjoins
which
downthrows
of the slickensiding, suggests that or been associated with this vertical
been
which
from
the north-west,
(9, p. 90), and Barrow
fault, the
similar type
and Tilley
to the one described (16, pp.
by 637
(20, p. 133).
The fourth fault running south-east through Dungiven downthrows to over at any rate most of its length. the north-east Its main importance lies in the light which it throws on the character of the folding as discussed
in the following section. 15.-STRUCTURE. Taking
a broad
genreral
view
of
the whole
of
structure would seem to be comparatively simple. marked angles
show a general
on Plate VIII
inclination
as a rule comparatively
being
low (about
the Sperrin
area
the
The directions of dip
towards
the north-west,
the
20 or 30 degrees), though It must be noted outcrop.
they become vertical south-east of the limestone are true dips and not foliation dips. that all the dips marked on Plate VIII They have been obtained from those sections which show close interbanding of beds of divergent texture and composition, from the thin bands of mica schist
between
types previously
the
from the alternating and pelitic limestone, psammitic to on p. 155, or from the narrow siliceous bands
referred
in the MNullaghearn Group. In a few cases, such as Barnes Gap, the dip can be seen on a large in others such as the Butterlope Pass where scale even from a distance, across a valley the contact of two different beds can be mapped accurately it is deduced
from
the relation
between
outcrop
and
contour
lines.
so far as the author 's that, the direction observations go, the strike of foliation and, in consequence, of foliation dip agrees with the true dip, but the two may vary considerably as regards the angle of inclination. it might be assumed Since the dips are to the north-west (without In
this
connection
it may
be noted
of relative age of the beds) that the Newtown into the question and the Black Schists of is at the top of the succession Group The whole succession could then represent either at the base. Broughderg or the south-western half of a simple anticline the north-western portion entering stewart
of a syncline. to contradict such a view there appeared Although nothing definitely was commenced in the south-east of the area, yet, as the mapping when north-westwards and the rock types berame more the work progressed varied
and
the outcrops
more
continuous
and
better
exposed
along
the
HARTLEY-Datradian Rocks of the Sperrin Mountains.
163
higher ridges and the intersecting glacial overflow channels, it became obvious that such a simple view would require some modification. The first difficulty was the Black Schist which was found typically developed high up on the southern flanks of Carnanelly. Though this apparently represented an infold of the band to the south there was no evidence as to whether the structure was anticlinal or synclinal. The same difficulty as to a simple succession arose in the Butterlope area to the north of Plumb Bridge where a series of limestone bands, parted by bands of mica schist, is brought in between two belts of extrusive epidiorites, the northerly one being lenticular and dying out to east and west; and where, in addition, the Schistose Grits of theDart Group appear to overlie the main epidiorite band which in its turn rests on limestone. Since the Butterlope Pass, formerly a spillway from the glacial lake in Glenelly, represents one of the two glacial channels which traverse the main
Sperrin
range,
it cuts
a deep
rocky
gorge
along
the dip with
the
road across the pass reaching a maximum elevation of less than 800 feet, In consequence while the hills on either side rise to 1300 feet or more. of
this
was
the mapping
be carried
could
the case.
usually
This
fact,
in a more
out
together
detailed
the numerous
with
form dips,
than varied
rock types and good exposures, suggested the possibility of constructing this line, but, when this was done, it was not the facts without the all illustrated possible type of folding in Fig. 2 in which the limestone occurs as a syncline between the epidiorites. lavas of Craig based A further section to the east through the pillow
a detailed
section
across
to meet
found
on the relationships shown in Fig. 2 seemed also to fit the facts better than
Al.
the earlier
and
simpler
first adopted.
hypothesis
IV_ ....
~-
~
~
~
~
~
~~
N
-
FIG. 2.-Section across the Butterlope Pass looking east-north-east. The pebbly conglpmerates referred to on page 166 occur at the point marked X and about
three-quarters
of
a mile
from
the
road.
Proceedings of the Royal Irish Academy.
164
a study
From
of these and other
sections
it became
that
evident
there
was a considerable amount of fairly large scale isoclinal folding present causing local inversions of the apparently simple succession. not only with the smaller structures is in accordance Such a conclusion seen in some of the hand specimens but with the general opinions generally and illustrated for rocks in Cowal and elsewhere accepted for similar prepared
on Plate XVIII,
example
by Dr. B. N. Peach,
for "Chapters
on
the Geology of Scotland." In view of the somewhat limited height of the sections available the possibility extent
was
that
north-west example, hopeful
that
entertained
the actual but
rather
sheet
the
dips
towards
isoclinal
appeared
folding
the various
the south-east
the quartzites.
overlay
line of attack
of
In order to be
might
be of such an
were
groups
not
actually
for the limestones, to test this hypothesis the most and
that
to investigate
the effects
of pitch
and dip faulting on the character of the outcrops and direction of the strikes. Both pitch and faults might be expected to bring within reach of observation horizons otherwise inaccessible either owing to thickness of cover or the removal of beds by denudation. The Effect of Pitch. In the Sperrin area the beds as shown by the dips pitch definitely to the east or north-east in the east part of the map, and slightly south-west in the west, the culmination of the fold occurring in the centre. The north-easterly pitch'7 is themore strongly developed of the two, the south westerly one is only apparent in the extreme south-west corner near Omagh where it brings in an arc of Black Schists above the Tyrone Series, and it does not seem to persist so strongly towards the north. The dominant strike
would
structure
thus perhaps
be best described
as consisting
of a
north-easterly pitch interrupted by local south-westerly undulations. This direction of pitch agrees with that found in North Antrim 1, Plate
II), while
it is opposed
to the south-westerly
p. 87) and Kintyre
(2, p. 103, 3, p. 428). E. B. Bailey (2, PI4 1), the pitch
To
by Dr.
is again
pitch
judge
from
of Cowal the map
north-easterly
(cf. (9,
given around
Loch Tay. The
in the Sperrins with the axial pitch in both directions a centre shows which the similar at the repeats doming inlier of schists and gneisses in the shown by the central
outward
culmination culmination
and it is in consequence tempting Igneous Series to the south-east, to ascribe it to the early Devonian overthrusting. radial faults which certain that the post-carboniferous It is however and Gortin areas on the tongues of the Dungiven bound the Carboniferous
Tyrone
17 This
pitch
was
noticed
by Portlock
(25,
p.
171)
in 1843.
H1ARTIEY-Dalradian Rocks of the Sperrin Mountains.
165
north-east and south-west respectively, and indicate in each case a considerable drop, must also have been instrumental in producing or at least modifying this axial pitch. The feature is thus at least in part post-carboniferous, and, so far as the north-easterly one which affects the basalts is concerned, late or post-tertiary. Again it will be noted that the summits of the main Sperrin range increase in altitude from each end and rise from 1800 feet towards Sawel at the centre with a height of 2240 feet. This suggests that both the pitch elements are fairly recent. Similar processes may have determined the form of the inlier when the axial tilting would then in each case be most probably late Tertiary and associated with the downward movements which produced the depressions of the Foyle valley and Lough Neagh. Whatever its date, however, the pitch is distinct and one may now return to investigate the light, if any, which it throws on the question of the extent of the local overfolding. This evidence appears decisive in favour of the view that the general dip is towards the north-west. To the east of Dungiven there is strongly marked
of the limestone
bending
of the beds begin
to advance
group
towards
the south and
in this direction
steadily
the outcrops
as they come within
the influence of the Dungiven depression. A similar effect though not so pronounced is shown in the south, where what
is probably
a continuation
of
the
same
occurs
depression
in the
neighbourhood of Draperstown. To
the west
the curvature of the strike though appreciable is not This may so definite as it is at the eastern end of the range. be due to the fall in the ground level compensating the axial depression. at all, the strike curvature so far as it is evident is south Nevertheless, to adds the above conclusions. wards and this support perhaps
quite
The which
only
important
faults
which
of downthrow the direction south-east runs north-west
which doubtful
one between
the former Dalradians
definitely
bring schist against schist, and of is at the same time known, is the one
and the somewhat through Dungiven and Omagh. At its north-western end to the north-east, and its effect on the in the southward of the apparent transgression
Dungiven downthrows
is immediately a distance
This again confirms of some two miles. the of north-westerly as to the dominance dip supplied by the pitch. of outcrops Though the displacements produced by the Dungiven-Omagh seem to in same also the their support fault would is point direction, limestones
for
evidence
weakened which
may
The
through ignorance have taken place
section
shown
as to the exact
amount
along this line. on Plate VIII is intended
the suggested structure, but it must lines of the minor folds The main
of lateral movement
to give
a general
of necessity
be somewhat
shown
been
have
obtained
idea of
diagrammatic. chiefly
from
166 the
Proceedings of the Royal Irish Academy. (suggested
of Dart
synclinal)
sill near
epidiorite
of black
area
of
to the summit
and Sawel,
schists
the main
the vertical
and from
from
on Carnanelly, Sperrin range
and then overturned
the
and north fold exposed
on the Butterlope. The general turning towards the west or north-west at higher elevations is more conjectural, as the only evidence available for this construction is the assumption
that
the
fold
connecting
of Strabane
the pillow-lavas
the same horizon as those of Craig. them must
and Altigarvan
lie at
If, however, this hypothesis is correct,
be as shown,
that
and
is anticliwal
not
synclinal, since it narrows to the north-east wlth a north-easterly pitch. The extension of the beds below ground-level towards the south-east is also conjectural. Such a construction would, however, place the beds in their position time
believed
in with
fall
the
to be stratigraphically
type
of
fold
observed at this level in Scotland. The further question as to the relative not
yet
give
been
touched
a convincing
and which
upon,
answer
the older
beds
and
correct
(the Ben
Lui
ages of
it is perhaps
to the question
not
as
at the same
and
fold) which the various possible
to which
are
has
been
groups
has
at present
to
the younger
of the series.
of the author is that the order given the opinion Nevertheless table on Plate VIII, with the Quartzitic Group of Newtownstewart
in the repre
This opinion is based in part rocks, is the correct one. which occur in the schistose grits of pebbly conglomerate the under Group only a few feet above their junction with These bands of on the north-western slope of Oughtnager.
the older senting on the thin bands of
the Dart
lying
lavas
the upper edge is less base line, while a well-marked into the overlying there is a gradual passage grits. occur in the Loch Awe Series at area conglomerates In the Knapdale wrote J. F. G. Wilson Bay. Kilmory Of these conglomerates (12, p. 64) individual bands of conglomerate in every The as follows:have, conglomerate well defined
instance,
have
and
a well
defined base they decrease
start, but direction the conglomeratic
very
and bottoms contrast
the
conglomerates. that
the pebbles come in thickly in number quite gradually, so that bands have no definife boundary-the line and
at the in this
tops bands are thus strongly and the conglomeratic contrasted, not uncommonly in unfolded is of type one meets with to escape the inference difficult In fact it is exceedingly
of
seams are right way these conglomeratic evidence was, as stated by Dr. Bailey
This
up." (2, p. 97), suggestive
rather
than conclusive, but nevertheless gave valuable support to the testimony obtained bottoms
by Peach (12, p. 68) of the pillow lavas.'8
from
the difference
18 that even this line of evidence It may be noted, however, For the No. Geol. Soc. Proc. 1304, p. 38. (1936), on Sedimentation/' "Treatise cf. W. H. Twenhofel, evidence
Cf.
Abs.
between
has
been
value London
of
the
tops anid
recently queried. the conglomerate p. 210. (1932),
HARTLEY-DalTradwnRocks of the Sperrin Moutntains. In
of
the opinion
the author
seen
the features
167
in the conglomerates
of Oughtnager correspond very closely to those described from Kilmory Bay,
perhaps
and are therefore
evidence.
worth
be admitted,
It must
recording
however,
of more
in default
that Dr.
definite
attaches
Bailey
little
weight to this particular exposure. Such
a
conclusion,
if
correct,
place
would
the Black
of
Schists
Broughderg as the youngest, the Newtownstewart Quartzites as the oldest members
of
the Sperrin
deduced
from
the general
Series, order
and
would
mean
that
the
succession one.
Only
be in agreement
with
is an inverted
of superposition
locally are the beds in their correct stratigraphical position. This
conclusion
as to the actual
would
succession
the views of Dr. E. B. Bailey
(2, p. 101) as to the structure
beds
is opposed,
in Scottish
correct
areas.
by A. Geikie,
It
G. Barrow,
J. W.
however, Gregory,
of the equivalent
to the order
held
to be
and other workers.
STRUCTURE ANDPHYSICAL 16.-THE RELATION GEOLOGICAL BETWEEN FORM. From
what
has
been
already
mentioned
it will
be
realised
that
the
of beds differing in in a series of in ldness, outcrop and therefore which composition, arcs which trend on the swing round rom a south-easterly concentric one oni the wet, and that the beds have a general east to a south-westerly
Sperrin
area
broadly
consists,
eaking,
of
a series
inclination towards the north or north- est. Two important radial depressions hay opened out the lines of attack these lines the two post cretaceous dip rivers the Owenreagh and the Roe have cut deep salients, to the characters of the and paying, as such rivers do, but little attention to the
forces
of
river
denudation
and
along
rocks which they traverse have somewhat obscured the main structural lines. In
of their
the development
and Coney Glenlark Glenelly, structure relationship between
the rivers of especially strike tributaries, we find a more marked Glen, however, and form. These rivers have picked out
the schists, and horizons amongst the softer and more micaceous beds. courses follow very closely the line of strike of these particular hard
schistose
and tough hornblendic are epidiorites grits and massive and remain to build up the prominent ridge of tlle main
carefully
avoided
Sperrin The
range. relationship
In traversing of a specially when
they
foundation.
their The
it is evidently rapidly. progressing to find them following the strike for considerable distances and then,
is not complete, but it is frequent
the streams favourable
lose it, taking
horizon a sudden
jump
across
to return
to their original
168
Proceedings of the Royal Irish Academg.
The feature of dip slopes and escarpments also shows signs of development. The watersheds of both the Sperrin Mountains and the Munterloney Hills lie closer to their southern than their northern boundaries. The southern slopes are in each case the steeper, less drift covered, and more sharply incised. At the northern boundary of the area mapped the development of strike features is again reflected in the topography. The grits and epidiorites are s4cceeded by the soft and easily denuded limestones, the course of which is represented by the low-lying ground occupied over much
of
its length
Quartzitic
Group
by
the Inver
gives
rise
River.
Further
to a ridge
of higher
north
again
ground,
the lower
which
includes
the thousand-foot rounded summits of Crockdooish, Slieveboy, and Straid. 17.-THE METAMORPHISM. The general metamorphic position of the intrusive epidiorites having been
already
discussed
on page
160,
it is only
necessary
to refer
to the
position of those schists which were originally of detrital origin. The greater part of the area occupied by the detrital schists lies within the biotite zone of metamorphism, but garnets are developed in two distinct areas,
the boundaries
of which
In the first place the centre
wide,
length with
are marked occur
garnets
along
by small a belt
crosses
about
on Plate
a mile
and
VIII. a half
roughly over the greater part of its coincides At its south line of outcrop of the Green Beds.
of which
the main
western and north-eastern ends, however, where the effects of pitch become apparent in diverting the outcrops of the stratigraphical divisions towards the south, the metamorphic isograds continue their original directions almost without change. Along
the centre
of the belt
is an oval
shaped
area,
the rocks of which
lie within the biotite zone. garnetiferous locality occurs forming a triangular in length between Mountfield and Greencastle. This area is bounded-to the south by the main boundary fault. The garnets of the northern small pinkish belt are always garnets. than 0 5 mm. and are as a rule of more They never reach a diameter Further
south another
area about
five miles
half
that size.
about
Since
they are
so small
as to always
require microscopic
for their determination, and since, in addition,
the garnet isograd must few, the line marking but sufficient slides have been somewhat tentative, is correct. that the main outline
comparatively be here ensure In 30 mm. evidence
the Mountfield in diameter,
area
the garnets
frequently
of considerable
margined
retrograde
examination
the exposures are of necessity examined
to
are much 20 to larger, sometimes with a broad chlorite rim, strong
metamorphism.
HARTLEY-Dalradian Rocks of the Sperrin Mountains.
169
The question has been considered as to how far the above described distribution of the garnets supports the views put forward by Elles and Tilley
(16 and
17) as to the structure
of the varying
grades
of
regional
metamorphism. These authors have suggested that in North Kintyre the garnet isograd forms, like the beds, a recumbent fold closing towards the south and containing towards the north a core of higher metamorphic grade.
16, Fig.
(Cf.
2, p. 629.)
In the Cowal
area
(16, Fig.
1, p. 627)
only the lower edge of the garnet zone comes sufficiently low to touch the surface, so that this particular belt is represented by a thin veneer of garnetiferous material resting on less metamorphosed sediments. The zones like the beds (?) are claimed to be inverted. The conditions shown in Cowal would appear to be possible in the case of the Sperrins. Taking a general view of the most northerly of the garnetiferous belts it is seen to correspond roughly with the southerni slope of themain Sperrin range. It occupies high ground, but the garnets are always small. They show no signs of increasing size with increase in the depth
of
the sections,
but
on
the contrary
tend
to disappear,
and
to be
replaced by biotite. In the centre and broken
of the belt also
up by
lenticles
the garnet
of biotite
grade.
band It
is thin and discontinuous is only
towards
the ends
where the axial depression (equivalent to a topographical elevation of the ground) comes into play that the garnetiferous area is broader and more continuous. The southern garnetiferous area around Mountfield is, however, in a different category. It is almost everywhere confined to low ground and its outcrop
forms a broad V running down the valley of the Owenreagh. or south the beds are traced along the strike in either a north-west east direction we pass into garnet free rocks lying at a higher topographical zone cannot a simple veneer, but it level. Here the garnet represent
When
underlie the biotite schists unless it has been faulted or folded down is in This is a possibility, for the area it occupies to its present position. outlier intimate association with a Carboniferous lying to the south-east
must
dropped
It might also represent an overfolded of this particular zone, or a narrow wedge lying along the from the north-west fault which has been overthrust by schists
down
"outlier" boundary
amongst
older
rocks.
of a lower grade.
Nevertheless the simplest view of the character of the metamorphism and
the one
it increases,
that most readily meets the facts observed in intensity with structural not decreases, 18.-SUMMARY
in the field
is that
depth.
AND CONCLUSION.
The present communication establishes a stratigraphical succession for the,Dalradian rocks of the Sperrin district inNorthern Ireland. Although the boundaries
of
the various
divisions
are
in some
cases
not
easy
to fix
170
Proceedings of the Royal Irish Academy.
exactly
owing
to complication
impersistent
lenticles,
south-western
portion
or
by minor
to local
folding
with
the development
lack of exposures,
it is believed
of
a sufficiently definite succession has been established to enable the main structure of the area to be followed from an examination of the map, and at the same time to permit f detailed correlation with other districts on the same stratigraphical horizo A detailed e Dalradian correlation with rocks of Antrim and the of Seotlan
is suggested
and
the general
that
structure
briefly described. The
of
thanks
in the field, also mss.
the author
for useful
MeCallien and
discussion,
to Professor
for help
are due
to Dr.
E. B. Bailey and Dr. W. J. for his helpful criticism J. K. Charlesworth for reading through the and
to the former
in other ways.
To Mr.
the late of Gortin, of construiction of the Londonderry and to to Mr. G. Barnett, of Sixtowns, waterworks, many landowners, and residents farmers, in the district he is indebted To Miss W. J. Sayers for local aid and information. and Rev. E. M. M. A. Robinson,
he
Gumley
and
staff
P. Clarke, in charge
is obliged
investigations
for facilitating certain features of the time available was somewhat limited.
when
of
19.?List
1. Bailey,
the engineering
E.
B.,
North East 163-177.
and
Works
to
which
W.
McCallien,
Antrim."
Trans.
Roy.
above
is made.
Reference
J.?"The
the
vol.
of
Rocks
Metamorphic
Soc. Ed.,
lviii
pp.
(1934),
of 2. Bailey, of the South-West the Structure E, B.?"On Highlands 82-131. lxxviii Jour. vol. Scotland." Geol. (1922), pp. Soc, Quart. * 3. Trans. Rocks of Kintyre." W. J.?"The Metamorphic McCallien, Roy. Soc. Ed., 4. Kinahan, G. H.,
vol. lvi (1929), pp. 409-436. Geol. Surv. and Others.?Mem.
Ireland.
Sheet
17
Sheet
18
(1889). 5. Nolan,
J.,
and Egan,
F. W.?Mem.
Geol.
Surv.
Ireland.
(1884). 6. Wilkinson,
S. B.,
and Others.?Mem.
Geol.
Surv.
Ireland.
Sheet
25
(1887). Sheet 26 (1884). Geol. Surv. Ireland. J.?Mem. 7. Nolan, of South Knapdale." Green Beds W. 8. McCallien, J.?"The 156-167. lxx vol. (1933), pp. Mag, of Cowal." Mem. Geol. Surv. 9. Clough, C. T.?"The Geology
Geol. Scotland
(1897). Min. Green Beds of the Scottish Dalradian." F. C.?"The 239-256. Mag., vol. xxii (1930), pp. of Oban and Dalmally." and Others.?"The 11. Kynaston, Geology H, 45 Sheet Mem. Geol. Surv. Scotland. (1908).
10. Phillips,
HARTLEY-Dalradian
Rocks
of
the Sperrin
171
Mountains.
12. Peach, B. N., and Others.?"The of Knapdale, Geology Jura, and North Kintyre." Mem. Geol. Surv. Scotland. Sheet 28 (1911). ' 13. Hill, J. B., and Others.?' The Geology of Mid Argyll." Mem. Geol. 37 Surv. Scotland. Sheet (1905). of Mid the Seaboard 36 (1909). Argyll." 15. Wiseman, and J. D. H.?"The Central South-West Highland Epidiorites" Quart. Jour. Geol. Soc, vol. xc (1934), pp. 354-417. to in Relation 16. Elles, G. L., and Tilley, C. E.?"Metamorphism structure in the Scottish Highlands." Trans. Roy. Soc. Ed., vol. lvi 14. Peach,
B. N.,
of and Others.?"The Geology Sheet Mem. Geol. Surv. Scotland.
(1930), pp. 621-646. 17. Tilley, C. E.?"Metamorphic Scotland." Quart. Jour. 18. McCallien, Dalradian
W. Rocks
J.?"A
Zones Geol. Soc, Contribution
of Scotland
(1931), pp. 126-133. 19. Hartley, J. J.?"The Irish Acad., vol. xli
and
of in the Southern Highlands vol. lxxxi (1925), pp. 100-112. of the to the Correlation
Ireland."
of North-East
vol.
Geol. Mag.,
lxviii
Proc
Roy. (1933), pp. 218-285. 20. Barrow, of Blair Atholl, Geology Pitlochry G., and Others.?"The Mem. Geol. Surv. Scotland. Sheet 55 (1905). and Aberfeldy." and the of Corrour 21. Hinxman, and Others.?"The L. W., Geology Moor of Rannoch." Mem. Geol. Surv. Scotland. Sheet 54 (1923). Geology
Tyrone,"
22. Harker, London A.?"Metamorphism." (1932). on Dalradian Lavas." Proc. Pillow 23. McCallien, J.?"A Note W. xliii vol. 13-22. Irish Roy. Acad., (1936), pp. and Meall 24. Elles, of Ben Lawers G. L.?"The Structure Geological vol. Jour. Geol. Corranaich." lxxxii Soc, (1926), pp. 304 Quart. 331. 25.?Portlock, parts
on The J. E.?"Report of Tyrone and Fermanagh."
Geology Dublin
of Londonderry (1843).
and
of
R.
Paoc.
I. ACAD.,
VOL. XLIV,
SECT.
1B.
P'LATE V.
(Photo, FIG.
black
the Newtownstewart
of
1.-Quairtzites
hammer occurs exposuIre
sclists.
The
shown. The anid soutlh of Dunnamanagh.
GIroup withl is restiing on the about
two miles
of micaceous tlinii ban(ds Ihigher of the two mica north of the Butterlope
J.J.J!.) and bands Pass
'P6
Fi:
2.
Epidiorites
of
pillow Craig and slhowing strueture se(liments the pillows. ( ) between
HARTLEY:
DALRADIAN
Ro(Ks
the
;OF SPEUIZINS.
(Photo,
R.
sponigy
naturie
J. Welch.) of
tlle
PROC.R. I. ACAD.,VOL.XLIV,
FIG.
1.-Coarse The half
grained
schistose
SECT.B.
grits
sub-angular fragments an inch in diameter.
of
PLATEVII.
the Dart
weathleiring
Group out
as are
exposed white
(Photo, J.J.II.) near Mullaghcarn. quartz
and
(Photo,
FIG. 2.-Junction the Dungiven
aver age
J.J.H.)
of transgressive sill of epidioi ite (below) and bedded limestone of Group
(above).
Old
Piriory
Clhurch,
Dungiven,
HARTLEY: DALRADIAN ROCKSOF SPERRINS.
Co. Derry.
Pioc.
R.
VOL. XLIV,
I. ACAD.,
Geological
Map
SECT. B.
of
THE SPERRIN AREA NORTHERN IRELAND by J. J. Hartley.
M
Sc.
Dunnamanagh
Altigarvan
A
Strabane
Dung'iven
Craignagapole1
\
\\
\
~~Slileveboy\\
T-A
roch8oio C
~~Mllghnen
s ~~~~~~~~~~~~~~~~~~
raig~~~~~~~~~~ic n%,Cr R;%-cr~r
re
PLATE VIII.
I,dv
/,vr
Dung'iven
Le
Edenbridg6
CU.
N.
,'
/\
THE SPERRIN AREA NORTHERN IRELAND by J. J. Hartley.
M
Sc.
Dunnamanagh
Altigarvan
A
Strabane
PlumblfB
Mary\
Newtowustewat~
\~~~~~~~\\ 2 ?essy
~~~~Margin of Felspathic
Garnetiferonls
Areas
12~\
marked
thus xxxx.
Grits.b.*.g.
INDEX. B?laclc
Schist
of
Alullaghcarn
Schists.m
1 oun-naline (;reen
Schists
1Tourmaline
Schists, G rit
lavas
rDungiven
?,-.
Limestone
Glenelly
South,
Glen,tlv
Nortb,
Bands.
of
]X
Dart.
E;pidiorites.(extrusive)
|
Group.
Quartzitic
I~~~r.triisive Epidorites.
Quiartzite
of
Gr-oup
anid
Newltownstewvart
,
of
Beds.|_
SChistose I-'illow
Broughderg.
Group.
J,~
XNw I'
/
\\\~~~~~~~~~~~~~lv ,Mary\
Orn2agh
OWe~~~~~~LiJj "
Mu11a~
~
'~~~~~~~~~~~" ~~~~~' ~
~~ .-~
----
~~~~Sal
Mle
HARTLEY:
DALRADIAN
ROCKS
'OF SPERRINS.
INDEX. Blackc
Schist
of
AMullaghcarn
-
Broughderg.
Schists. Schists
1Tounrnaline
of
Glenelly
South,1
Beds.
Green
'I 'ourmaline
Schist-s
of
SChistose Grit Group
Glettl1v
Nnrtbi,1
of Dart.
I-'illow lavas and Epidiorites.(extrusive) Limestone
Dungiven
Newtownstewart
lr.trnisive
I/hIIh|
Group.
Quartzitic
I Group.
___
Epidorites.
Bands.
Qiiartzite I mestones.
Schistose Grits. Felspathic Grits. Albite
Schists.
I
I
