138 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
these rocks form a belt down the Atlantic coast from Newfound¬ 
land to Georgia. In this portion of the Blue Hills the ground 
mass of the rock in small patches is microgranitic, with quartz and 
feldspars, but for the most part it presents a granopliyric texture 
forming rosettes or parallel bands of fine hair-like fibers or needles, 
usually surrounding the feldspars. As suggested by Miss Bascom 
(’93, p. 817) in the case of the volcanics of South Mountain, Penn., 
this appearance may represent the intermediate stage between the 
splierulitic and completely micropoikilitic 1 crystallization, caused by 
the breaking up of the radiating splierulitic fibers in the process of 
devitrification. Such is the “ flexuose structure ” of Rosenbusch 
(’92, p. 380). The needles of these rosettes or surrounding bands 
seem to be partly of feldspar, but chiefly dark silicates; the latter 
frequently tending to form darker margins about the feldspar pheno- 
crysts. Particles of the dark silicates, or else of one of the iron ores, 
too minute for identification, are also scattered through the rock. 
Phenocrysts are not abundant. , 
Quartz is usually with well-marked crystal outlines, but is also 
corroded and sometimes with inclusions. In some of the sections 
the quartz shows cracks and undulatory extinction due to movement 
of the magma after the quartz crystallized. 
Orthoclase, on the whole, seems to be quite as abundant as quartz 
in the phenocrysts. Usually it presents sharply defined, rectangular 
crystals of rather low polarization colors, and but little altered. 
Strains and microperthitic intergrowth are noticeable, frequent 
twinning on the Carlsbad law, and minute, perfectly formed feldspar 
crystals of similar form intergrown in the larger ones, with the same 
orientation. The cracks are filled usually with the microgranitic 
type of ground mass. 
Grains of a decomposition product, yellowish in the center with 
dark opaque rims, are present. Rutile in tiny brilliantly polarizing 
needles with very perfect terminations is scattered through the 
ground mass of some of the sections, — particularly those approach¬ 
ing the microgranitic ground mass. 
Zircon occurs in small amounts. 
Hematite forms a few blood-red grains. 
Biotite appears sparingly but usually as an alteration product 
bleached by departure of the magnesia molecule. Some of the dark- 
1 The term micropoikilitic (Williams, ’93, p. 176) is applied to those minerals which have 
a pronounced mottled appearance under the microscope, caused by numerous inclusions 
of different orientation. 
