742 ME. r. ETJTLET ON PEELITIC EELSITES, 



in solution, substances readily taken up by water filtering through 

 overlying rocks. 



If beneath the calculated percentage composition of kaolin we 

 place an analysis of epidote, and subtract from the kaolin the amount 

 of silica, alumina, and water present in the epidote, we see that the 

 kaolin would require to have 31 per cent, of its constituents replaced 

 by ferric oxide, lime, and magnesia. 



SiOg. AI2O3. FesOg. CaO. MgO. H^O. 

 KaoHn 4640 39-68 13-92=100 



^^(laLdswl)^;?!!^ 21-13 16-85 23-58 0-17 ... =100-10 



^^sont)!(VoL^^^^ 28-90 7-43 24-30 O'lO 0-63=100-43 



That such a change may take place seems very probable, since 

 water which has filtered through rocks moderately rich in lime and 

 iron would suffice to bring about a conversion of this kind. 



It is difficult to say whether, in a felsitic rock, the conversion of 

 the felspathic constituent into epidote would take place directly from 

 the alteration of the felspar, or indirectly from it, by the alteration 

 of kaolin ; but the alkalies present in the felspar would not be in- 

 volved in the fabrication of epidote, and it seems highly probable 

 that the conditions requisite to effect the alteration of felspars would 

 entail the early removal of their alkalies and the consequent formation 

 of kaolin. Such changes may occur in felsitic rocks, especially in 

 those which, by reason of their perhtic structure, offer exceptional 

 facilities for the percolation of water. 



As regards the blowpipe characters of the rocks which form the 

 subject of this paper, thin splinters fuse on the edges to a white 

 frothy glass in some spots and to a brown or greenish-black in others, 

 the latter being evidently due to the fusion of parts more strongly 

 charged with epidote. 



By ordinary transmitted light the epidote ranges from almost 

 colourless to yellowish-green, and where the granules are closely 

 massed together, the green colour is deeper, becoming brownish-green, 

 and in such spots the section transmits very little light. Between 

 crossed nicols some of the sections of the larger crystals show oblique 

 extinction and the interference-colours are strong. The majority 

 of the sections, however, give parallel extinction. The pleochioism 

 is not strongly marked. In sections parallel to 100, l)=nearly 

 colourless, c =pale yeUo wish-green. 



That the two specimens here described show perlitic structure 

 there can be no doubt : but, owing to the prevalence of epidote, it 

 is rendered unusually obscure, and I have therefore given a more 

 detailed description of the microscopic characters, in the hope that 

 it may encourage search for this structure in felsitic rocks, even 

 when alteration is far advanced. In the cases here given very slight 

 additioQal change would have obliterated all traces of perlitic 

 structure. It is possible that some epidosites result from alterations of 

 this kind. 



