120 P. BASCOM — VOLCANICS OF NEPONSET VALLEY 



spherulitic fibers are negative. The fragments are very heterogeneous 

 in size and character, and the rock may be termed an agglomerate. 

 These agglomerates are of a green color, the result of the production of 

 pinite, and contain in some cases fragments crowded with white kaolin- 

 ized spherulites from the size of a pinhead to a pea. 



Between Mother brook and the Providence railroad there is exposed a 

 tuffaceous volcanic. From the same locality comes a specimen of crushed 

 and recemented granite. The slide shows broken granitic quartz and 

 feldspar cemented by a fine grained silicious crystallization. Pinite is 

 abundantly developed in this cement and gives its color to the rock. 

 This specimen shows no volcanic fragments. The aporhyolitic tuff, which 

 was not sectioned, is a purple and gray rock, free from pinite, with plain 

 evidence of its clastic character on the weathered surface. 



At the corner of River street and Glen wood avenue there occurs a frag- 

 mental rock that might be classed with igneous conglomerates. The 

 rock is of a medium green color, and is of a compact character on the 

 fresh surface, but exhibits its clastic character on weathered surfaces. 

 The fragments are subangular and rounded. The slide shows them all 

 to be of an igneous origin — quartz, orthoclase, and plagioclase, aporhy- 

 lite, and aporhyolitic ash. Pinite is more or less developed. 



The localities where pinite is the predominating alteration product are 

 the following : Near the crossing of Blue Hill avenue and the New Eng- 

 land railroad ; on Blue Hill avenue south of Brook street, Milton, and 

 on Central avenue, Milton ; at the quarry* near Mount Calvary cemetery ; 

 at the intersection of Glenwood avenue and River street, Hyde Park; 

 Stony Brook reservation, Hyde Park ; between Mother brook and Provi- 

 dence railroad, and, finally, south of the New England railroad, on Nor- 

 folk street, in Mattapan. 



In the latter locality it is very characteristically developed. The de- 

 velopment of pinite is much more marked in the tuffaceous than in the 

 massive volcanics, though not absolutely confined to the former. 



Stony Brook reservation embraces some two square miles of the acid 

 volcanics. The material collected from this area exhibits gradations 

 into a highly silicious granite. Passing from south to north, the forma- 

 tion is first a green non-porphyritic aphanitic rock, which possesses a 

 very fine grained groundmass, with phenociysts of quartz and albite. 

 The alteration is mainly to pinite and epidote. Farther to the north 

 the rock becomes less fine grained and fresher, the phenocrysts larger 

 and more numerous. The feldspars are perthitic, and the quartz gran- 

 ulated or with undulatory extinction. The micropoikilitic structure 

 shows itself in the groundmass. The rock approaches a granite in ap- 

 pearance. It is a fresh medium grained light gray rock. 



