322 



ALBERT JOHANNSEN 



III. 



GLASSES 



Since the glasses can be determined megascopically as such, 

 the usual terms are retained, as pumice, obsidian, pitchstone, etc. 

 For the porphyritic glasses the term vitro phyreids is proposed. 



Tabulating these terms: 



Such a classification can be learned by a student in ten minutes. 

 The rocks are subdivided into as many groups as can be recog- 

 nized megascopically, and unknown specimens classified by differ- 

 ent persons will almost invariably be found to fall into the same 

 groups. Frankly acknowledging, by the names used, the field 

 character of the determinations, there will be no confusion caused 

 if later more exact terms are applied to the same rocks. 



