Joly — On the Minerals of the Dublin and WkMow Granite. 61 



the upper and cooler layers of rock, they have attained a suf- 

 ficiently low temperature. 



In these veins the crystals of beryl, forming in deeper and 

 hotter regions than the tourmaline, and taking toll from the pass- 

 ing waters, grow and gather in bunches; the zone of solidification 

 retreating downwards as cooling progresses. Similarly, tourmaline, 

 forming always higher in the vein than beryl, but, like it, ever 

 forming deeper and deeper in the granite, covers up finally with 

 a schorlifferous covering the beryl already deposited. 



These beryl and schorl veins may be seen in perfection at 

 Grlencullen. Sometimes they are euritic in texture : more gene- 

 rally porphyritic, when they yield beryl and schorl, intermingled 

 with overgrown crystals of felspar. 



If it is allowable to reason on these lines, it is perhaps sufficient 

 to seek for the cause of the alteration experienced by the beryls in 

 a change of temperature, it may be, of the upwelling waters, where- 

 by dissolution and replacement of the beryl was brought about; or 

 it may be in a change of constituents — more highly alkaline water. 

 Or, finally, both causes may have operated. 



Those other changes — cavities eaten out, chlorite developed 

 near the surface of the crystals, kaolinizing of orthoclase and 

 beryl — are most probably changes of tertiary formation. It is 

 probable that water action, at the ordinary temperature, has 

 effected some of these changes. Thus the most advanced cases of 

 decomposition have been taken from the wettest veins in the 

 quarry. I have, from these veins, removed hexagonal shapes, 

 which, crushed between the fingers, crumbled into a rusty-brown 

 kaolin. 



Percentage Composition of the altered Beryls. 



It is interesting to note the extent to which replacement by 

 orthoclase is carried in some cases. This may be investigated in 

 three ways : by specific gravities, by specific heats, and by chemical 

 analysis. 



1. The specific gravity of beryl from Grlencullen was found to 

 be 2 "722 ; the specific gravity of orthoclase from Grlencullen, 2-510. 

 The specific gravity of the mixed minerals was, in the specimen 

 dealt with, 2*625. The weight of this specimen was 50-400 grams. 



