ON THE CONSTITUTION OF THE GRANITES OF DONEGAL. 53 
granite, which on the whole was gneissoid and bedded, gradually change at.a 
certain place into a perfect granite, and then, in complete uninterrupted 
continuity, pierce the rock in the form of a dyke. Another instance is men- 
tioned of a granite rock occurring in the schistose rocks ‘ partly in very regular 
layers, partly as isolated knolls and lumps, and partly as a multitude of veins, 
which in several places run through large portions of the neighbouring moun- 
tains as a close network.’ In spite of this, however, this granitic rock showed, 
in many places, a gneissoid structure. The relations of the hornblende schists 
and greenstones resemble those of the granite. The hornblende schist is 
regularly interstratified with the gneiss, mica-schist, and other rocks.” 
We have been induced to make this long quotation, owing to the great 
analogy which it shows to exist between the district which comes under our 
notice and the Scandinavian peninsula. We have learnt from Mr. Jukes 
that he has observed similar phenomena on the coast of Newfoundland ; .and 
they were observed and noticed by Sir R. Griffith upwards of twenty years 
ago, when he was examining the county of Donegal for the purpose of pub- 
lishing his Geological Map. 
The fact that in Donegal this gneissose granite is apparently intrusive in 
many places is abundantly proved by the occurrence of granite dykes cutting 
across the limestone at Drumnaha Gap, near Fintown, and in Dunlewy 
marble-quarry. It appears also in a striking manner at Pollnacally, near 
Trawenagh Bay, where the granite is intrusive into quartz-rock, and sends 
veins into hornblende-rock ; and also at several localities on the south shore 
of Arranmore Island. 
Among the argillaceous beds which lie near the granite are found several 
of anthophyllite slate, which pass gradually into soapstone and potstone, as at 
Crohy Head, Gartan Abbey, Convoy, and elsewhere. At Crohy they are 
associated with a mass of light-green serpentine, which unfortunately does 
not occur in a sufficiently massive condition to be available for commercial 
purposes. The soapstone too is rendered impure by the presence of iron 
pyrites, to such an extent that its utility as a lubricating agent is seriously 
impaired, 
Arranmore Island consists mainly of quartz-rock of the micaceous variety, 
with interstratified and intrusive igneous rocks. At the S.E. corner of the 
island, from Leabgarrow to the chapel at Ilion, we find a coarse-grained red 
granite, which takes a fine polish, and is remarkably free from joints. One 
block, measuring superficially 90 x 20 feet, is exposed on the sea-shore. 
Between this point and Torboy at the S.W. corner of the island, a considerable 
portion of the coast is formed of white sphene granite, like that at Ardara, 
At Tordhu it contains some syenite and gneiss, the whole forming in places 
a network of veins, as was so well described by Keilhau in the quotation we 
have already made from ‘ Gea Norvegica.’ 
This granite, as well as the other rocks of the island, and the granite near 
Dunglow, is penetrated by numerous dykes, some of ordinary trap, others of 
amygdaloidal pitchstone, and some of felstone-porphyry. 
In the 8.W. part of the county, extending down to Teelin Head, we meet 
with mica-slate, abounding with iron-pyrites. Through this rock, especially 
in the neighbourhood of Ardara, there is disseminated a large quantity of 
syenite passing into hornblende rock. 
The granite of Bluestack and Barnesmore, as has been said before, differs 
in its character from that of the Gweebarra Valley, as it appears to consist of 
red orthoclase and quartz, with very little mica. It is traversed by numerous 
* Gea Norvegica, p. 367. t Ibid. p. 348. 
