154 PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 



described, so far as the writer knows; the writer therefore proposes for this 

 subrang the name neponsetose; this magmatic name means that the rock is 

 dosalic, dolenic, domalkalic, and persodic. 



Andesite Pyroclastics . — Associated with the massive andesite at Cooks 

 Court and on Rockville Street near Blue Hill Avenue are andesitic tuffs : at 

 Cooks Court, where the rock is composed of angular fragments of plagioclase, 

 quartz, orthoclase, or basic and acid lava fragments, the tufaceous character 

 is distinctly brought out by weathering; on Rockville Street, where a dark purplish- 

 colored tuff is composed of heterogeneous fragments of andesitic lava, porphy- 

 ritic or non-porphyritic or amygdaloidal, of aporhyolite and of jasper; these 

 fragments are contained in an exceedingly fine-grained siliceous cement; the 

 tuff though retaining original texture is completely altered mineralogically to 

 chlorite, epidote, calcite, hematite, and magnetite. 



Andesite Dikes. — Associated with the andesitic flows and cutting indiffer- 

 ently the granite, the micro-granite, the rhyolite, and the aporhyolite are basic 

 dikes of a more recent age, ranging in width from an inch up to twenty odd feet. 

 Material was obtained for study from exposures of such dikes at Coburn Street 

 entrance of Stony Brook Reservation, on Bearberry Hill, at the end of Gordon 

 Avenue, Hyde Park, and northwest of Winter Pond, Stony Brook Reservation. 



Specimens from these localities show a dense, fine-grained to aphanitic, 

 purplish gray or greenish purple rock with a scanty and inconspicuous feldspar 

 phenocrysts. The weathered surface is a greenish brown and the rock is some- 

 times faintly mottled with green owing to chloritization. 



The rock, originally very feldspathic is now almost completely metaso- 

 matized, with the fabric still preserved. The secondary minerals are epidote, 

 calcite, chlorite, kaolin, and the iron oxides; epidote occurs in cloudy, granular 

 aggregates and in transparent crystals; light green chlorite occurs in irregular 

 allotriomorphic areas or in pseudomorphs after feldspar; the occurrence of 

 calcite is similar; cracks traversing the rock are filled with calcite and quartz; 

 aggregates of submicroscopic, brilliantly polarizing dust were considered to be 

 kaolin; the iron oxides, magnetite and hematite, together with epidote grains, 

 outline what were once lath-shaped feldspars and record the original ortho- 

 phyric fabric. 



The dike at the Coburn Street entrance to Stony Brook Reservation fur- 

 nished the only material from which any clue to the character of the original 

 constituents could be obtained. In this rock extinction determinations obtained 

 on the feldspars, which constitute some 75 per cent, of the rock, showed that 

 orthoclase, a little anorthoclase probably, and labradorite of the composition 

 Ab a An 4 were present. The original ferromagnesian mineral, which was a subor- 

 dinate constituent, is in every case entirely replaced by secondary products, 

 epidote and chlorite. Slender columnar crystals of apatite are still preserved 

 as is often the case when other constituents are completely altered. 



