CHEMICAL COMPOSITION OF THE ACID VOLCANICS 121 



Still farther to the north there is exposed in the road- cut a true granite 

 showing no ferro-magnesian constituent. 



CHEMICAL COMPOSITION 



The following is an analysis* of the acid volcanics of the Neponset 

 Valley basin : 



Si0 2 72.85 



A1 2 3 12.92 



Fe 2 3 . 2.98 



CaO 0.90 



MgO 0.38 



Na 2 7.08 



K 2 3.01 



Trace of MnO and P 2 5 1 N 



Co 2 f 



Loss by ignition (H 2 OCo 2 ). 0.65 



Total 100.77 



The chemical analysis shows these volcanics to be related to the rhy- 

 olites, differing from the normal rhyolite, however, in possessing a high 

 soda percentage. They are closely related to the soda-rhyolite or quartz- 

 keratophyre. 



The predominant feldspar is a soda feldspar. The original constit- 

 uents may be estimated as follows : 



Quartz 18.02 



Orthoclase 17.85 



Albite 60.0L 



Ferro-magnesian constituents and lime feldspar molecule. 3.-19 



Other constituents .63 



Total 100.00 



The chemical relationship to the quartz-keratophyre is borne out by 

 a general resemblance in the thin-section. The trachytic alkali feld- 

 spars, the perthitic and plagioclase phenocrysts, the absence of apatite 

 and of ferro-magnesian constituents ally them with the quartz-kera- 

 tophyres. 



It has been suggested to the writer that the prefix " apo " should be 

 restricted to that kind of metamorphism in which textures and structures 

 are preserved, and not confined in its application, as heretofore, to rocks 

 which were originally glassy. With this the writer fully concurs. 



This usage does not conflict with the present use of " meta," which 



* Analysis made by Dr W. H. Walker in the laboratory of the Massachusetts Institute of Tech- 

 nology from composite samples carefully collected for the purpose. 



