544 Mr. Archibald GeiJcie [June 7, 



line direction of movement, and its paste- has been converted into a 

 fine kind of gneiss. 



Having thus traced an original westward boundary to the younger 

 crystalline schists of Irelaud and Scotland, I saw that it would be 

 important to follow their eastern boundary as far as it had not been 

 concealed by later formations. In Galway we found that the quartz- 

 ites, limestones, and schists are succeeded to the south by the large 

 area of Archaean gneiss already referred to. But the boundary between 

 the two groups of rock is one of extreme complication, somewhat like 

 that of the north-west Highlands. Along a line running east and west 

 through the heart of this county from Mannin Bay to Lough Corrib, 

 the two groups have been so dislocated and so thrust between and 

 over each other that much time and patience, with the use of large- 

 scale maps, would be required to map out their respective areas. But 

 the important fact is readily percej)tible that in Galway the uprise 

 of a large Archaean area* gives us a southern limit for the basin in 

 which the younger schists of the north-west of Irelaud were deposited. 



To the east and north-east of the Galway area the country has 

 been overspread with Old Red Sandstone and Carboniferous strata, so 

 that for a long space the older rocks are concealed. Far to the 

 north-east in Tyrone, on the southern borders of the great area of 

 crystalline schists, a mass of dark hornblendic rocks was mapped 

 some years ago by Mr. Nolan of the Geological Survey of Ireland, 

 and referred doubtfully to a pre-Cambrian age. A more recent ex- 

 amination of this mass, v/ith the experience gained over so wide a 

 region among the older crystalline rocks, has enabled us to identify 

 it without hesitation as a characteristic portion of the Archaean gneiss. 

 It rises as a long north-east ridge along the south-eastern margin of 

 the chloritic schists of Londonderry which were deposited against 

 and over it. We discovered moreover that these schists have at their 

 base, resting on the old gneiss, a thick volcanic series consisting of 

 amygdaloidal basic lavas, tuffs, and coarse volcanic agglomerates. 

 The green chloritic material of the schists, not improbably represents 

 the original magnesian silicates in the finer volcanic dust that mingled 

 with the ordinary sediment of the sea-bottom. 



From the evidence now adduced, it is, I think, manifest that the 

 crystalline schists of the Scottish Highlands east of the Great Glen, 

 as well as their continuation into the north-west of Ireland, cannot 

 be regarded as merely the equivalents of the quartzites and limestones 

 of Sutherland and Eoss. They are enormously thicker and more 

 varied in their component members than those north-western strata. 

 Whether even any part of them represents the sedimentary rocks of 

 the north-west seems to me open to serious doubt. My own impres- 

 sion is that they are probably younger than these rocks, and that they 

 once stretched far to the north-west and covered them to a depth of 

 many thousands of feet. That the fossiliferous strata of the north- 

 west Highlands were originally buried under a thick pile of other 

 sediments I have already shown. 



