File:The Quarterly journal of the Geological Society of London (14096462793).jpg

SCHISTOSE ROCKS OF NORTHERN DONEGAL.
223
Donegal. 'The revelation made by the rocks themselves was both a
surprise and a disappointment. I have visited Prof. Hull's most
critical sections, and some districts not described by him.
Pig. 1. — Section in Bar
Gap and on Lough Greenan,
S.E.
Lough G-reenan. Lough Salt.
I
I
G. Granite.
L. Limestone.
Ms. Mica-scliist.
Hs. Hornblende-schist.
Section in Barnesheg Gap (fig. 1, north-west end).
It was in this wild ravine, near a wooden barrack erected for a
body of soldiers appointed to protect a cottage opposite, that I fi.rst
touched the granite. The rock surfaces on each side were well
rounded by ice-action, and the colouring and weathering suggested
some bits of Hebridean scenery in Scotland. The first blow of the
hammer destroyed my preconceptions. The rock was grey in colour,
uniform in texture, consisting of a well-crystallized, coarse-grained
aggregate of quartz, black mica, and felspar, mostly orthoclase.
There were no signs of bedding, and, but for a roughly linear
arrangement of the mica, I should have declared I had before me as
typical a granite as I had ever seen.
Towards the junction with the schists at the south-eastern end
of the Gap, the granite grows lighter in colour and less coarse in
grain. At the entrance of the gorge we reach the important part
of the section. Masses of schist lie between masses of granite, and
at a hasty glance the two appear to be interstratified. I first came
upon a thin band of well-crystallized mica-schist, dipping at a
moderate angle to the south-east, with granite on both sides of it.
A little further on, I reached hornblende-schist, the hornblende
(consisting of small black sparkling crystals) predominating over the
quartz. This rock at first was rather puzzling. It had no regular
dip, but rose vertically like a dyke, with irregular sides, as if it
had thickened abruptly downwards. But this diorite-like mass was
distinctly foliated, and I at first thought that I had before me a
case of pressure-foliation in an igneous rock. The foliation, how-
ever, was not parallel to either of the margins of the mass or at
right angles to the direction of the thrust which had contorted
the region, but sloped at a medium angle to the south-east, in
accordance with the prevailing dip of the schists outside the granite.