PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 143 



case of the light-green lavas the color is owing, partly to the presence of epidote 

 but chiefly to that of pinite, 8 a secondary product derived from the alteration of 

 the feldspathic groundmass and the feldspar phenocrysts; the gray rhyolite 

 owes its color to a freedom from iron oxide and to a comparatively fresh feld- 

 spathic character. 



The fabrics found in these lavas are the granular, fluxion, trachytic, por- 

 phyritic, amygdaloidal, and spherulitic. 



The granular fabric, consisting in a homogeneous quartz-feldspar mosaic, 

 usually combined with flow-fabric, characterizes much of the lava; associated 

 with this presumably secondary crystallization are microscopic colorless spheru- 

 lites, usually occurring in bands; the radiating crystals of the spherulites are nega- 

 tive and feldspathic. When feldspar predominates in the groundmass the tra- 

 chytic fabric is developed; 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, which is only rarely and never con- 

 spicuously the case, the vesicles are now filled with cryptocrystalline silica; 

 crystals attached to the concentric walls of the lithophysal vesicles are replaced 

 by silica. 



The perlitic fabric is associated with the spherulitic, which is found most 

 abundantly in the district west of West Street and in the mass constituting High 

 rock, in Hyde Park. The slides of this material are crowded with spherulites 

 spherical in form or polyhedral because of mutual interference; where the radiat- 

 ing fabric is well preserved, the fibers are both positive and negative; small pheno- 

 crysts of perthitic orthoclase often occur in the center of the spherulites. In many 

 cases the original branching and spherulitic fabric is indicated in ordinary light only 

 by the iron dioxide (hematite), while in polarized light an extremely fine granular 

 quartz crystallization replaces the original fabric. In the groundmass associated 

 with these altered spherulites are found flow fabric, perlitic parting, and a second- 

 ary micro-poikilitic fabric. The micro-poikilitic fabric may be combined with 

 the spherulitic, or may replace it altogether, or may be confined to the ground- 



mass. 



Chemical Composition. 





The following analysis of the acid volcanics of the Neponset Valley basin waf 

 made from composite samples carefully collected for the purpose. For com- 

 parative examination there are tabulated with it analyses of rhyolite and of 

 keratophyre 9 from Marblehead Neck, of a soda granite 10 from the neighborhood 

 of Cristiania and of a soda rhyolite 11 from Berkeley, California. 



■ W. O. Crosby, Relations of the Pinite of the Boston Basin to the Felsite and Conglomerate, 

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

 • H. S. Washington, op. cit., pp. 292-293. 



u pi! C ' Brogger ' Die Eruptivegesteine des Kristiania-gebietes, vol. I, p. 127. 

 Charles Palache, Soda Rhyolite North of Berkeley. Bull. Dent, of Geol., vol. 0, p. 67. 



