312 W. F. SMEETH. 



Na 2 (1'3%); while similarly, thealbite will contain 1-9% of CaO, 

 the bulk of the CaO being in the glass and in the lime-soda micro- 

 liths. Figures derived in this way cannot of course be trusted 

 very far, but they are interesting from their general agreement 

 with the conclusions of microscopical observation. 



A few words may not be out of place here as to the nomenclature 

 of this rock. The well denned and abundant phenocrysts form a 

 striking feature, and recall the appearance of vitreous members of 

 the Dacite group, and this suggestion is somewhat furthered by 

 the presence of hypersthene as the sole ferro-magnesian constituent. 

 The high percentage of Si0 2 however, combined with the excess 

 of potash over soda, and the fact that the majority of the porphy- 

 ritic felspars are sanidine, necessitates the rock being placed in 

 the acid group. It therefore becomes a quartz-felspar-pitchstone. 



I am not aware that hypersthene has before been observed in 

 so acid a rock. 



In certain rhyolites intergrowths of enstatite and bronzite have 

 been occasionally noticed in monoclinic pyroxenes. These latter 

 are seldom conspicuously developed in the more acid lavas, and 

 may usually be referred to the less ferruginous varieties. The 

 presence therefore of hypersthene in this rock as a distinct con- 

 stituent and in well marked grains, is, I think, worthy of special 



note. 



Perlitic and other Cracks. 



It would not be necessary to more than allude to the perlitic 

 structure, so well exhibited in the glass of this rock, were it not 

 that certain cracks similar to those in the quartz crystals have 

 been claimed to be perlitic also.* In view of this it may be advis- 

 able to consider somewhat in detail the nature of the cracks to 

 which the term perlitic is applied, and as a starting point I will 

 first describe the structure which may be easily produced with 

 great perfection in Canada balsam. This artificial structure is 

 admitted to resemble very closely the perlitic cracking of lavas 



* The Occurrence of Perlitic Cracks in Quartz, W. W. Watts, m.a., f.g.s. 

 —Q.J.G.S., Aug. 1st, 1894. 



