682 MR. F. E. COWPEK EEED ON THE [NOV. I9OO, 



mosaic, and a few large irregular patches of clear quartz-grains, is 

 a felsite from near Garrarus. The mosaic is sometimes blurred 

 (sheet of banded felsite west of Kilfarrasy Island, p. 664). 



The clear micropoikilitic groundmass composed of small patches 

 passes often into a granular or coarsely microcrystalline 

 aggregate or mosaic in which the true characters of the 

 micropoikilitic type are absent (Type D). Such are several from 

 Garrarus. In many cases the mosaic is blurred, and phenocrysts 

 may be wholly absent, as in the case of the sheets between 

 Foilnaneena and Knockmahon. 



Granophyric as well as microcrystalline and granular struc- 

 tures are found associated with the micropoikilitic occasionally 

 (Type E), as in a rock from the cliffs on the east side of Sheep 

 Island with large porphyritic rounded quartzes and a very few 

 large decomposed orthoclases. The columnar felsite from Kilfar- 

 rasy Island (p. 666) has a groundmass of this type, but contains 

 only a few large felspar-phenocrysts. 



The dykes cutting across the augite-porphyrite near Knock- 

 mahon (p. 668) are of this granular-mi crop oikilitic type, occasion- 

 ally with obscure pseudospherulites ; and in the columnar 

 felsite of the ' Bishop's Library ' granophyric structure is combined, 

 and the groundmass also contains large rounded orthoclases and 

 smaller plagioclases with crowds of crystallites. 



A mosaic of small granophyric patches, clear or blurred, 

 forms a special type (Type F) of groundmass, but is connected 

 closely with the micropoikilitic type by transitional varieties. Some 

 of these contain large phenocrysts of felspar showing microperthite. 

 Typical examples of this granophyric mosaic are found in the late 

 intrusive felsite from Newtown Head and other localities (Garrarus, 

 Morageeha, etc.). These have usually phenocrysts of orthoclase, and 

 granular patches of clear secondary quartz. But some are devoid of 

 phenocrysts or contain a few small pseudomorphs of quartz after 

 felspar, or of a greenish substance after felspar with crowds of 

 crystallites (Lady's Cove, Garrarus). Especially beautiful examples 

 of the granophyric mosaic are found in slides containing a few 

 small porphyritic plagioclases (extinction-angles 16° to 18°), as in 

 the horny grey felsites on the west side of Dunabrattin Head, and in 

 the pink felsite on the west side of Sheep Island, which contains as 

 well a few very large imperfect orthoclase- and quartz-phenocrysts. 



In other cases the felspar-phenocrysts have themselves been 

 more or less completely replaced by a granophyric mixture similar 

 to that which forms the groundmass, but slightly coarser in texture 

 (as, for example, the red felsite by Sheep Island). 



Another type (Type G) of groundmass is characterized by the 

 presence of more or less numerous felspar-microlites or laths. 

 In one example from the east side of Dunabrattin Head (p. 666) 

 the groundmass is micropoikilitic, but crowded with small laths 



