CRYSTALLINE SCHISTS INTO IGNEOUS ROCKS EN" CO. GALWAY. 523 



enclosing schists. lEost of the mica is in long, narrow, ragged flakes, 

 almost like bits of frayed string, and suggests great compression. 

 The rock is now a sort of quartz-schist. What it was originally, it 

 is hard to say ; but I see no reason why it may not have been a 

 hornblendic granite, like the granite of the district. 



4. Age oe the Igneous Rocks. 



The Silurian conglomerates of Killary Harbour are rnainhy com- 

 posed of large rounded fragments of igneous and metamorphic rocks. 

 Amongst these is a coarse-grained granite, with a great deal of 

 plagioclase and a little altered biotite. The constituents of this 

 rock are the same as those of the typical G-alway granite, but the 

 felspars are smaller. Pebbles of quartz-felsite are also abundant. 

 The ground-mass of this felsite is devitrified, the quartz-crystals are 

 large and clear, and there is a small proportion of biotite. This 

 description will also apply generally to the ordinary quartz-felsite 

 near Galway. The conglomerates clearly prove that in early 

 Silurian times the adjoining land largely consisted of igneous rocks, 

 closely resembling those still found in the region, and we may there- 

 fore fairly conclude that the granite and felsite, with the still older 

 diorite, are of Pre-Silurian age. The metamorphic schists enclos- 

 ing the intrusive masses and veins are, of course, of still greater 

 antiquity. 



5. The Galway Gneiss. 



This rock forms a triangular area about two miles each way, with 

 the town of Galway situated in the centre. It is bounded on the 

 west by granite and felsite, on the north-east by the Carboniferous 

 limestone, and on the south by Galway Bajr. It is usually coarsely 

 crystalline. The common minerals are quartz, felspar, and horn- 

 blende, with epidote as accessory. The quartz and felspar form the 

 ground-mass. Immersed in it are numerous dark, speckled blocks, 

 suggesting the diorite fragments in the granite further west. These 

 block-like masses are often arranged in a roughly linear manner, 

 but frequently they are irregularly distributed. Hornblende also 

 occurs in bands or masses, displaying a foliated structure. In the 

 latter case, the rock is like true gneiss. Rarely could I find con- 

 tinuous seams of the hornblende. A flaky appearance sometimes 

 occurs on rather a large scale, long slender tongues running out 

 irregularly from patches of foliated hornblende into the grey grani- 

 toid ground-mass. 



There is no true bedding in the Galway gneiss, so far as I saw ; 

 but the seams and masses of hornblendic rock usually lay with their 

 longer axes dipping at a high angle to the N.N.W. Comparing 

 this gneiss .with the diglomerates already described, a similarity of 

 origin is at once suggested, the apparent dip being accounted for by 

 tangential pressure. As this was the first district I visited in Con- 

 naught, the significance of the phenomena did not then appear, 

 and my work proceeded on other lines ; but the hints afforded by the 



