142 PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 



spherulites either crowd the rock to the almost complete exclusion of a matrix 

 or they are associated with, and imbedded in, a light green groundmass; their 

 tints are pink or red or light-greenish yellow or all these colors arranged in con- 

 centric zones. None of these types are conspicuously porphyritic but they fre- 

 quently exhibit small and inconspicuous phenocrysts. 



Specimens of the massive rhyolite were obtained from exposures at the 



crossing by the New England Railroad of Blue Hill Avenue, and at Cook's Court, 



near Prospect Street, in Mattapan. At the latter locality fluxion banding and 



cleavage are notable. Specimens were also obtained from Blue Hill Avenue in 



Milton, where the occurrence is of a similar character and from Grew's wood, 



Hyde Park, where there is a ledge of porphyritic rhyolite. At the intersection 



of Arlington and River Streets, Hyde Park, occurs a red, eutaxitic rhyolite; the 



eutaxitic character, due to relatively silicious lenticular segregations, colored a 



deep red by hematite and influenced in form and arrangement by flow movement, 



is very conspicuous on the weathered surfaces; these lenses are often partially 



replaced by red jasper and present a superficial resemblance to amygdules, from 



which they may be distinguished by their lenticular form, which is unlike the 



spindle shape of the genuine amygdule. At High Rock, and in Grew's woods, 



Hyde Park, spherulitic lava is abundant ; at Central Avenue and on Columbine 



Road, Milton, there is exposed a deep purple rhyolite, characterized by marked 



flow structure. 



Petrographic character: The primary constituents which are still preserved 

 in the acid volcanics are the alkali feldspars and quartz. There is some indication 

 that members of either the pyroxene or amphibole group may have originally 

 been present in very minor amounts, but no limemagnesian or ferromagnesian 

 constituents remain. 



Feldspar is the predominating constituent, forming some 60 per cent, of the 

 rock and occurring in two generations ; in the first generation the feldspar appears 

 as small and scattered phenocrysts; as a component of the groundmass the 

 feldspar may be granular, or lath-shaped, or acicular and radiating; the lath- 

 shaped feldspars do not often show polysynthetic twinning and usually possess 

 a parallel extinction; in the granular feldspar polysynthetic twinning is not un- 

 common and the microperthitic fabric is a conspicuous characteristic ; extinction^ 

 indicate that orthoclase, anorthoclase and albite are the species represented 

 which albite is the predominating species. . ,, 



Quartz occurs rarely as a phenocryst, but is always a constituent o 

 groundmass. The secondary constituents are magnetite, pinite, epidote, kao , 

 quartz, sericite, hematite, calcite and leucoxene. « g 



To the iron oxides, which are disseminated as microscopic dust as we 

 in coarser aggregates, is owing largely both the record of early fabrics an ^ 

 color of the rhyolites: all the red material owes that color to the P res . enC ° nt 

 hematite; piedmontite occurs too rarely and too scantily to figure as a P 1 ^ 6 ^ 

 the purple rhyolites contain both magnetite and hematite as pigments* *" 



, m 



