ACID VOLCANICS 117 



parallel arrangement peculiar to acid lavas. In some localities, notably- 

 High rock, Hyde Park, the lava is locally an aggregation of spherulites. 

 These spherulites vary in size from that of a pea to a butternut. The}'' 

 are pink or red or light greenish yellow and often exhibit concentric 

 color tints. They may crowd the rock to the exclusion of a matrix or 

 they are imbedded in a light green ground-mass. 



The massive volcanics are exposed at the crossing of Blue Hill avenue 

 and the New England railroad and at Cooks court, near Norfolk street, 

 Mattapan, where flow structure and cleavage are marked. On Blue 

 Hill avenue, in Milton, the occurrence is of the same character ; on Stony 

 Brook reservation it occurs with a granitic facies. In Grew's wood, Hyde 

 Park, there is a ledge of very inconspicuousl}^ porphyritic volcanics. 

 At the intersection of Arlington and River streets, Hyde Park, occurs 

 the red amygdaloidal volcanic. High rock, Hyde Park, is a mass of 

 spherulitic lava. At Central avenue and on Columbine road, Milton, 

 there is exposed a deep purple volcanic, characterized by marked flow 

 structure. 



PETROGRAPHIC CHARACTERS AND MINERAL CONSTITUENTS 



The primary constituents which have been preserved in the acid vol- 

 canics are the alkali feldspars, quartz, and magnetite. 



Presumably either hornblende, biotite, or augite was originally present 

 as a constituent of the groundmass, but no lime-magnesian or ferro- 

 magnesian constituents remain. 



Feldspar occurs both as small scattered phenocrysts and as a compo- 

 nent of the groundmass. Here it is sometimes granular, sometimes lath- 

 shaped, and sometimes in radiating fibers. The lath-shaped feldspars do 

 not show polysynthetic twinning and usually possess a parallel extinc- 

 tion. Microperthitic structure is a marked feature of the phenocrysts. 

 Polys3mthetic twinning is not uncommon. Extinctions indicate that 

 albite, orthoclase, and anorthoclase are the species represented. 



Quartz rarely occurs as a phenocryst, but is a constituent of the ground- 

 mass. That the magnetite is often titaniferous is indicated by an altera- 

 tion to leucoxene. It also alters freely to hematite. 



The secondary constituents are pinite, epidote, kaolin, quartz, sericite, 

 hematite, and leucoxene. 



In all the red material hematite is disseminated as a microscopic dust, 

 giving color and preserving structures. Piedmontite occurs rarely in 

 small quantities in the feldspars and along cracks. 



The purple volcanics contain both magnetite and hematite as pigment, 

 while in the case of the light green volcanic the color is due partly to the 



