1-30 Proceedings of the Royal Irish Academy. 



such like. In the "s-iciidty of Galway, the late Mr. J. Beete Jukes 

 pointed out that, in some wide courses or dykes, the outside portions 

 were granitic rocks, while the interior was a felstone, seemingly prov- 

 ing that the change must be due to mctamoi-phic action. 



Thisvarietyof granite often is found associated with felsitic-quartzyte 

 and quartzitic-felsityte, and often appears to graduate into the foimer. 

 For which reasons it seems probable that some at least of those rocks 

 originally were either felstone or felsitic-tulfs.*' 



Mac Culloch seems to have classed together this granite and gTani- 

 toid-felstoue as different varieties of one kind of rock.f Of the latter, 

 he says, " This variety is never perfect, as it contains the same 

 minerals" [quartz, felspar, and mica], " without form ;" while in his 

 first division of the granite, he includes a rock, the description of 

 which would answer for felsitic-quartzyte or quartzitic-granitoid- 

 felstone.| 



Litrusive-ortlwclasic, or hiyldy -siliceous-granite {Oughtm-ard-type- 

 granite). — This rock only occui's as intrusive masses, dykes or pipes. 

 Jukes pointed out that, in ^est Galway, when in mass, it always 

 occupied low ground, while in the adjoining high ground, when it 

 appeared, it was as dykes, pipes, or small protrusions. This pecu- 

 liarity he had also observed in I^ewf oundland, and other places, fi'om 

 which he concluded that, being a deep-seated rock, it was exposed 

 only in low ground, when the superior rocks had been denuded away ; it, 

 however, also extended under the adjoining high gi-ound, as was 

 proved by the dykes or off-shoots fi'om it, but was still unexposed, 

 denudations having failed to remove the covering rocks. According 

 to this view, the deeper and more extensive the denudation, so much 

 larger ought the tract of this orthoclasic-granite to be. 



The rock, as just stated, has two characters — one massive, and the 

 other a veiu-rock, and, when found in these different positions, its 

 composition seems to vary. Under all conditions the rock appears 

 never to contain oligoclase as an essential, and when in veins or pipes 

 it appears usually to be a ciystalline aggregate of orthoclase, quartz, 

 and a greenish mineral, which is probably an eai-thy mica, or perhaps 

 ripidolite, with more or less pyiite or marcasite. The greenish 

 mineral in places is undoubtedly a mica ; but in other places it has 

 very much the aspect of ripidolite, and possibly both minerals may be 

 present. When in tracts, and wide veins or courses, the conspicuous 

 constituents seem to be orthoclase, quartz, black mica, white or green 

 mica, and pyiite or mai"casitc, while the principal accessories are 

 ripidolite, galenite, chalcoppite, barite, calcite, flouiite, molybdenite, 

 &c. These often occui' in nodules, lentils, and geodes, with lenticular 



* It was pre%iously mentioned that some of the rocks classed as felsitic- 

 quartzyte originally may not have been sedimentary rocks. 



t Mac Culloch, pp. 235 and 237. Granite. Second division. A. a. and A. d. 

 ]: Id., p. 234. B. a. 



