KiNAHAN — Oil Grcuiiiic and other Ingenite Rocli^. 123 



tains mucli water, probably next ; and the silex, which the heated 

 water would longest hold in solution, last." Nevertheless, this 

 opinion does not seem to be universal, as Cotta and others place such 

 rocks as elvanyte among the granitic-rocks.* 



The base may be gTeen, grey, purple, brown, blue, or even yellow- 

 ish ; the latter colour, however, seems generally, if not always, to be 

 due to weathering. The base usually gives the colour to the rock, but 

 it may be more or less modified by the colour of some of the contained 

 minerals. ' All the highly siliceous varieties, and some of the basic, 

 weather with a more or less smooth, flowery outline, so characteristic 

 of the felstones, and not with the rugged, rough aspect of a granite ; 

 some amphibolic or pyroxenic varieties, however, weather rough. 

 The quartz bleds, globules, or crystals, scattered through the mass, 

 range from mere specks to the size of peas, or even larger, in some of 

 the highly siliceous varieties being so numerous as to give the rock a 

 pisolitic appearance. They internally have a glassy appearance and 

 fracture ; on some of the blebs the crystal faces can be recognised, 

 while others appear to be minute coated balls. The latter often have 

 a radiated structiu'e, and in some there is a minute hollow in the 

 centre. f 



The vai'ieties in composition of the elvanytes that were observed in 

 tliis country are highly-siliceous {quartzitic-elvanyte), having numerous 

 orthoclase crystals {orthoclasic-elvanyte), or, perhaps, oligoclase crys- 

 tals {oligodasic-elvanyte) ; mica in some may be so abundant as to give 

 a character to the rock [micaceous-elvanyte), or that mineral may be 

 wholly or partially replaced by amphibole or ripidolite {hornllendic- 

 elvanyte and chloritic-elvanyte).^ The highly-siliceous elvanytes have 

 orthoclase for their felspar. The basic elvanytes usually have a 

 trialinic felspar or the lime orthoclase (adularia), with which are 

 associated such basic minerals as amphibole, pyroxene, and the like. 



In some of the elvanytes in the neighbourhood of Galway, and also 

 in the island of LettermuUan, many of the orthoclase crystals (flesh- 

 colour) are enclosed in an envelope of light green felspar (oligoclase ?), 

 somewhat similar to the Finland rock that has been called Rapakivi 

 granite {RapaMvi-elvanyte) . 



* Jiikes classed in his elvanytes the rocks called by Cotta " granitic-porphyry" 

 and " quartz-porphT,T3\" In the latter group Cotta, as well as Jukes, seems to include 

 some rocks that evidently are quartzitic-felstones ; but as such are not hypogene- 

 rocks, they do not properly belong to this group. 



t The latter can be seen with a pocket-lens. I have tried to get a section of 

 one to examine under the microscope, but as yet have not succeeded, as out of 

 numerous slices of elvanyte none contained this peculiar form of the quartz. — \_Note 

 in pressJ] 



% Micaceous-elvanyte would be called by some geologists «»>«-^rff^;; this further 

 shows that the tenn is not a class-name. Homblendic-elvanji^e would, by some, 

 be called syenyte ; this, for those in which the felspar is only orthoclase, is correct ; 

 in many of them, however, orthoclase is not the felspar, consequently, for such, rocks 

 the name would be incoiTect. 



