On the Laurentian Rocks of Donegal and of other parts of Ireland. 583 
and whitish orthoclase felspar, also a pale grayish triclinic species (probably oligo- 
clase) quartz, black, white, and bronze mica, and hornblende. !n some instances 
beds of considerable thickness occur almost entirely composed of red or pink felspar, 
containing traces of the metal antimony (as determined by Mr. E. T. Hardman), 
while in others the beds are almost entirely composed of hornblende and mica.” 
These remarkable red gneissose beds are bounded on the east by a fault which 
brings the quartzites and schists of the Lower Silurian period against the Laurentian 
beds. On the west they form the shore of the Atlantic where they are laid open to 
view, much flexured and contorted. 
(3.) Galway District.—To the north of Galway Bay, and extending inland for 
some distance, is a wild tract of country—indented by numerous arms of the sea— 
full of lochlets—rocky, and (except where bosses or ridges of rock rise above the 
general surface) deeply overlaid by morass and peat. It strongly resembles some 
tracts in Sutherlandshire formed of Laurentian gneiss or schist, such as that 
stretching from the base of Ben Foinaven and Ben Arkle to the coast, or the tract 
about Kylesku and the shores of Loch Dow. In both districts the rocks are 
glaciated and boulder-strewn, and in both they consist of similar materials; and, as 
I now feel confident in affirming, they are of similar geological age. 
The rocks composing the Galway tract have been fully described by Mr. Kinahan.* 
They consist of beds of gneiss, composed of “ pinkish” or flesh-coloured felspar 
(orthoclase), more or less porphyritically developed ; greenish, or yellowish waxy 
felspar (oligoclase), quartz; black, green or white mica, with other minerals.+ These 
beds pass into others in which hornblende and mica predominate, and are tra- 
versed by veins “of a coarsely crystalline variety of granite that answers the 
description for Cotta’s ‘pegmatite.’ Along with foliated and porphyritic red gneiss 
occurs an “ intrusive orthoclase or highly silicious (gray) granite” of later date. 
The tract occupied by this old Galway gneiss is bounded along the north by the 
quartzite mountains, known as “The Twelve Bins of Connemara,” formed of beds 
which, together with the overlying schists and limestones, represent the metamor- 
phosed Lower Silurian beds of Donegal and the North Highlands of Scotland.[ At 
the point crossed by the section of the Geological Survey (Sheet 25) the boundary 
between the two sets of rocks is a fault; to the south of which, and extending to 
Kilkieran Bay, there occurs a great thickness of schistose beds, chiefly hornblendic, 
with others of red gneiss and hornblende rock, and having at the base of the whole 
the porphyritic granite or gneiss of Avoch, consisting of large crystals of pink 
orthociase felspar, greenish oligoclase, quartz, and black and white mica, and forming 
* Expl. Memoirs of the Geological Survey. Sheets 93, 94, 105, and 114. 
+ This is Mr. Kinahan’s “Galway type granite.” 
} This is the view expressed by the Director-General of the Survey, Professor Ramsay, as far as regards 
the comparison with the Scottish area, after a visit paid to the district in 1877. See Preface to Expl. 
Mem., Sheets 93, 94, &c. The relative position of the Laurentian and Lower Silurian beds, is represented 
in the Horizontal Sections, published by the Survey, Sheet 25. 
