118 F. BASCOM — VOLCANICS OF NBPONSET VALLEY 



presence of epidote, but chiefl)^ to pinite,a secondary product derived from 

 the alteration of the feldspathic groundmass and from the phenocrysts* 



The gray volcanic is very feldspathic and comparatively free from 

 iron oxides and other pigments. 



The structures found in these lavas are the granular, trachytic, porphy- 

 ritic, perlitic, fluxion, amygdaloidal, and spherulitic. 



A homogeneous quartz-feldspar mosaic constitutes the granular ground- 

 mass, and, combined with flow structure, characterizes much of the lava. 

 Associated with this presumably secondary crystallization are micro- 

 scopic colorless spherulites, usually occurring in bands. The fibers are 

 negative and feldspathic. When feldspar predominates in the ground- 

 mass the structure passes into the trachytic. The feldspar is lath-shaped 

 and shows parallel extinctions or inclined extinctions with a small angle. 



These feldspathic volcanics recall the Westfalen quartz-keratophyres 

 and the bostonite of Marblehead neck. 



Where the lava was originally vesicular the vesicles are now filled with 

 cryptocrj'stalline silica. In one case a lithophysal vesicle shows crystals 

 attached to the concentric walls and now replaced by silica. 



The perlitic structure is associated with the spherulitic. The latter is 

 found most abundantly in the mass constituting High rock, in Hyde 

 Park. The slides are crowded with spherulites which possess a polyg- 

 onal outline as the result of mutual interference. 



In some cases the radiating structure is well preserved and the fibers 

 are both positive and negative. The micropoikilitic structure may be 

 combined with the spherulitic or may replace it altogether, or may be 

 confined to the groundmass. Small phenocrysts of perthitic orthoclase 

 often occur in the center of the spherulites and are distributed irregu- 

 larly through the section. 



In other cases the original radiating and branching spherulitic struc- 

 ture is indicated in ordinary light only by the iron oxide (hematite), 

 while in polarized light an extremely fine granular quartz crystallization 

 replaces the original structure. In the groundmass associated with these 

 altered spherulites are found flow structure, perlitic parting, and a 

 secondary micro-poikilitic structure occur. Orthoclase and an acid 

 plagioclase feldspar are the phenocrysts; calcite and epidote are the 

 alteration products. 



FRAOMENTAL ACID VOLCANICS 



The fragmental volcanics are exposed near the crossing of Blue Hill 

 avenue and the New England railroad. They are especially well dis- 



* \V. O. Crosby : Relations of the pinite of the Boston basin to the felsite and conglomerate. 

 Tech. Quarterly, February, 1889, pp. 248-252. 



