146 PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 



exhibits a fragmental texture, which is defined in ordinary light by red pigment 

 and obliterated in polarized light by homogeneous crystallization; the original 

 fragmental character is, however, indubitable. South of Norfolk Street and the 

 New England Railroad a light green volcanic, breaking readily into slabs, shows 

 the fragmental texture in spite of a uniform alteration of the replacing granular 

 mosaic to pinite, which gives the light pea-green color to the rock. In other 

 instances where the fragmental character of the rock is obscure in the hand speci- 

 men its obliteration is aided by the alteration of the secondary quartz-feldspar 



crystallization to pinite. 



Where this mineral is not developed the tufaceous and brecciated character 

 is always apparent in the hand specimen ; the angular fragments exhibit a variety 

 of shades, — pink, red, purple, green and other tints, — and often a fluxion arrange- 

 ment; the material of the fragments is either quartz, feldspar (orthoclase, albite), 

 aporhyolite or spherulites. The fragments of aporhyolite in many instances 

 exhibit remarkably well preserved perlitic parting or fluxion fabric or "aschen 

 structur"; in the tuff on Blue Hill Avenue south of Brook Street the fragments 

 are mainly of aporhyolite, which has been recrystallized and largely altered to 

 pinite and epidote, with orthoclase only of the original constituents remaining. 

 Some fragments have a secondary spherulitic crystallization, in which case the 

 spherulitic fibers are negative. Where the fragments are very heterogeneous in 

 size and character, the rock may be termed an agglomerate ; these agglomerates 

 are of a green color, owing to the production of pinite, and contain in some cases 

 fragments crowded with white kaolinized spherulites varying in size from a pin- 

 head to a pea. 



Between Mother Brook and the Providence Railroad there is exposed a 

 tufaceous volcanic; from the same locality comes a specimen of crushed and 

 recemented granite, composed of broken granitic quartz and feldspar cemented 

 by a fine-grained siliceous crystallization ; pinite is abundantly developed in 

 this cement and gives its color to the rock, which includes no volcanic frag- 

 ments. The aporhyolitic tuff is a purple and gray rock, free from pinite, with 

 evidence of its clastic character disclosed on the weathered surface. 



The localities where pinite is the predominating alteration product are the 

 following : Near the crossing of Blue Hill Avenue and the New England 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 ot 

 Glenwood Avenue and River Street, Hyde Park; Stony Brook Reservation, 

 Hyde Park; between Mother Brook and Providence Railroad, and, finally, south 

 of the New England Railroad, on Norfolk Street, in Mattapan, where it is very 

 characteristically developed. The development of pinite is much more marke 

 in the tufaceous than in the massive volcanics, though not absolutely confined o 

 the tufaceous material. 



Rhyolitic Tuff.— Interbedded with lava flows and agglomerates in the Wes 

 Roxbury volcanic neck is an exceedingly fine grained slaty rock obscurely showing 



