122 F. BASCOM — VOLCANIC8 OF NEPONSET VALLEY 



remains the general prefix for raetamorphic rocks, representing all kinds 

 of alterations in any rock. 



It covers the original definition of the prefix, and embraces all those 

 rocks which under that definition have received the prefix "apo." It 

 will include, in addition, those altered efl'usives which may or may not 

 have been perfectly glassy, but which, with altered constituents, still 

 show the structures of lavas. Under this usage the prefix may be applied 

 with increased confidence. With this terminology the acid volcanics of 

 the Neponset valley become apo-soda-rhyolites. 



Basic Volcanics 

 occurrence and petrogra phi c cha ra cters 



At the crossing of Blue Hill avenue and the New England railroad 

 both volcanics are exposed in the railroad cut. The basic rock occurs 

 as a dike in the acid eruptive. It is a compact, fine grained, dark green 

 chloritic rock, confusedly traversed by joint-planes. The weathered 

 surface and the joint-planes are iron-stained, and the latter are so numer- 

 ous as to render it difficult to obtain a fresh fracture. 



The specimens from this locality show a nearly complete alteration 

 of mineral constituents and a preservation of structure. The alteration 

 products are calcite, chlorite, quartz, epidote, and kaolin. 



Epidote occurs as a cloudy yellow aggregate, filling the interstices of 

 the lath-shaped feldspar, the outlines of which still remain. The feld- 

 spar substance is so completely replaced by calcite, chlorite, kaolin 

 scales, and quartz as to render it impossible to determine the species. 

 Magnetite is sparingly distributed and there are some remnants of iron 

 pyrites undergoing alteration to limonite. The structure is trachytic 

 and inconspicuously porphyritic. Flow movement is indicated by the 

 arrangement of the lath-shaped feldspar microliths. 



West of Oakland street and south of the New England railroad, not 

 far from the preceding locality, the basic volcanic occurs as an aphanitic 

 dark purplish rock, much jointed, with a development of chlorite on the 

 joint faces. - 



This rock is characterized by comparative freedom from the alteration 

 products — chlorite, calcite, and kaolin. The feldspar is correspondingly 

 fresher, and extinction measurements indicate albite as the species. 

 There is scanty calcite and much epidote and magnetite present. These 

 minerals, with the plagioclase, constitute the rock. The structure is 

 micro-ophitic combined with flow movement. 



At the same locality a dike of the basic igneous rock occurs. The 

 rock is lighter colored than the above and differs in showing much 



