152 PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 



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 

 effusive, much jointed, with a development of chlorite on the joint faces, and 



frequently amygdaloidal. 



This rock is characterized by comparative freedom from the alteration pro- 

 ducts—chlorite, calcite, and kaolin. The feldspar is accordingly fresher, and 

 extinction measurements show that both orthoclase and an acid plagioclase are 

 present. These feldspars with scanty calcite and much epidote and magnetite 

 constitute the rock; the fabric is micro-ophitic combined with flow movement. 



At the same locality there occurs a dike of the basic igneous rock which is 

 lighter colored than the volcanic and shows much greater alteration of feldspar 

 and matrix; the alteration, which has been carried too far for the determination of 

 species, consists for the most part in the production of cloudy and granular 

 epidote and of chlorite; magnetite is present; the fabric is trachytic where not 

 obscured by secondary products. 



On Norfolk Street, near Cook's Court, occurs a great mass of lava, which 

 resembles the basic lava already described. It is very fine-grained and without 

 peculiar fabrics ; the original constituents are more or less completely replaced by 

 calcite, chlorite, epidote, and quartz; the groundmass polarizes but faintly, and 

 may have originally been in part glass; the feldspathic microliths are obscured 

 in outline and in specific character. 



On Morton Street, near Codman, in Dorchester, the basic volcanic occurs as 

 an aphanitic dark green rock, obscurely mottled with areas of fine red jasper, 

 which also coats the walls of cracks. The slides show the same fabricless or 

 faintly ophitic groundmass, with epidote, chlorite, quartz, leucoxene, and the iron 

 oxides as constituents, with altered plagioclase phenocrysts, and olivine pheno- 

 crysts completely replaced by hematite. 



On Delhi Street, in Mattapan, occurs a volcanic, very like the Morton Street 

 rock both megascopically and microscopially . As in the Morton Street volcanic 

 the alteration to chlorite and epidote is so far advanced as to disguise the 

 original constituents ; one untwinned feldspar phenocryst whose substance has 

 not completely disappeared shows an extinction on 010 of 14°; the groundmass 

 polarizes very faintly and seems to be composed of altered orthoclase; there is 

 considerable magnetite in the rock and some secondary quartz. 



A basic volcanic exposed in a high ledge on the west side of Central Avenue, 

 Milton, is coarse-grained and amygdaloidal, and greatly altered to epidote, chlo- 

 rite, quartz, and calcite. 



Where the feldspar is still comparatively fresh, the fabric is trachytic and 

 porphyritic ; again extinction angles show that the feldspar is orthoclase and an 

 acid plagioclase of approximately the composition Ab 4 Anx or oligoclase. In this 

 locality the rock exhibits a marked cleavage, parallel to which it breaks in 

 parallelopipeds about one-quarter of an inch in thickness. 



