KiNAHAN — Oti Gninitie a>td other Inyenite Rocks. 121 



Foliated feUtone or gneissyte and granitoid felstone. — These rocks may- 

 occur cutting across tlie stratification of the metamoi-phic sedimentary 

 rock as dykes, or they may be in masses or beds. The most common 

 variety of the foliated-felstone is a gneissoid-rock consisting of layers 

 or leaves of quartz, felspar, or felsite and mica, apparently similar to 

 the rock described by Cotta, and called gneissyte or irruptive gneiss.'^' 



In varieties, amphibole or ripidolite may partially or wholly replace 

 the mica ; pyrite also is often present. Some f oliated-felstones are 

 schistose ; usually the rock consists of layers of felsite and mica, amphi- 

 bole or ripidolite ; there are, however, rocks that are aggregates of 

 quartz and felsite or felspar — the constituents of both the gneissoid 

 and schistose varieties depending altogether on the composition of the 

 original rocks. 



The lines of foliation usually are parallel, or nearly so, to the walls 

 of the dyke, but at other times they are oblique or even spheroidal. The 

 most prevalent structures in fel^tones were enumerated when describ- 

 ing those rocks ; and on a comparison of the foliation in gneissyte with 

 the lines of structure in f elstones, a remarkable similarity will be found, 

 consequently it does not appear unreasonable to suppose that the folia- 

 tion in the gneissyte has been induced by the most prominent lines of 

 structure in the original rocks. In places, the foliated rocks can be 

 traced until they graduated into rocks in which foliation has not been 

 developed. Gneissyte was principally noted as dykes among the gneiss 

 and schist, but in the neighbourhood of Barna there is a foliated felstone 

 in the porphyritic-oligoclasic-granite ( Galtvay-type-granite). Its occur- 

 rence in such a place seems to be remarkable, and hard to explain, never- 

 theless, the writer of this essay on a former occasion has suggested an 

 explanation for the phenomena.! 



Others of the metamorphosed f elstones are devoid of foliation. These, 

 may be called granitoid-fehtone from their appearance. J They are a 

 granular compound of felsite and quartz ; or of felsite, quartz, and mica; or 

 of felsite, quartz, mica and felspar, and from the latter the rock graduates 

 into a granite. In a few localities amphibole, or a mineral that appeared 

 to be ripidolite, was noted in addition to the mica. In more than one place 

 these rocks can be traced until they graduate into a felstone, very little 

 if at all altered. Most of these rocks seem to have been originally highly- 

 siliceous-felstones {petro silex) ; therefore, they for the most part have 

 orthoclase for their felspar ; there are, however, other granitoid-f elstones 

 that appear originally to have been basic-felstones [euryte), and these 

 were changed into rocks in which oligoclase, amphibole, and such like 

 minerals are essentials. 



* Cotta, 1. c, p. 234. 



t Joiirnal of the Eoy. Geol. See, Ireland, 1871, vol. iii., p. 1. They may, 

 however, be foliated normal felstones, as normal foliated intriisive granites (as at 

 Newry, Co. Down) occur in places. 



X Macculloch describes rocks among his granites that evidently are similar to 

 these rocks. 



