132 Proceedings of the Royal Irish Academy. 



appears to lose all its quartz and mica, and to merge into a crystalline 

 felspathic mass {felsitic granite).'^ The aspect of this rock is well 

 described by Cotta — " a rock of compact textm'e, about the hardness 

 of felspar, "with dull or smooth conchoidal or fissile structure ; colour, 

 yellowish, reddish, grey, greenish, or bluish, weathering -white." The 

 loss of the quartz and mica is, however, more apparent than real, for 

 on a close examination with a lens both may be detected, always veiy 

 minutely and often sparingly developed. In another variety the rock 

 appears to lose its felspar and to become a crystalline granular compound 

 nearly solely of quartz Q.ndiVin.(iQ.[quart%itic granite).] This rock undoubt- 

 edly is very quartzose, and perhaps, although closely allied to them, 

 ought to be excluded from the true granites ; for although in general a 

 little felspar can be detected, that mineral often appears to be absent, 

 and when present, presents more the appearance of an accessory than an 

 essential. In some places even the mica also is unapparent, the rock 

 being very similai' to the rocks at Bray Head, Co. ^"icklow, and called 

 quart%-rock by Jukes. These quartzose-rocks are peculiar, and hard to 

 explain. At CurTaun, immediately south of Maam Bay, the jS^. W. arm 

 of Lough Corrib, and in other places, the quartzose rock undoubtedly 

 graduates into the intrusive highly-siliceous or oithoclasic granite, but 

 in some places the relations between the quartzose-rocks and those 

 associated with it are veiy obsctu'e. At Canrower and Croaghna- 

 cloosh, both in the viciidty of Oughterard, masses of this rock occur, seem- 

 ingly in connection with the Oughterard-type-granite, but the coat of 

 bog that covers so much of that country prevents them from being pro- 

 perly examined. In both of these places the quartzose-rock is foliated, 

 and apparently identical with quartzyte that in other parts of this 

 coujitiy is undoubtedly metamorphosecl sedimentary rock. However, in 

 these two places the quartzose rock quite disagrees, as to strike and 

 dip, with all the schists and gneiss in its vicinity, and as the exposures 

 are in ii-regrJar patches, they scarcely could have been brought into 

 their present positions by faults.;}: 



* Felsite-rock of Cotta, p. 220. 



t Seemingly tlie rock called greisen "by Cotta, pp. 207 and 321. 



j All the details in relation to tkese exposiu-es of quaitzose-rock are given in 

 tlie Geological Survey Memoir', ex-sL.eet 105, pp. 33, 39, to whicli tlie reader may 

 be refeiTed. 



