WHITE : PETROGRAPHY OF THE BOSTON BASIN. 
135 
Minute crystals of the feldspars frequently occur included in the 
larger crystals. Hornblende, in the same variety referred to under 
granite, in slender bluish transparent needles, extinguishing in the 
direction of elongation, occurs in both the ground mass and the feld¬ 
spar phenocrysts, but presents no definite arrangement. The quartz 
plienocrysts are less plentiful than the feldspars, and are irregular in 
outline and without inclusions. 
At several points in the West Quincy quarry district a handsome 
black rock appears, in “ association with a light gray granite.” 
This black “ granite ” is very similar to the red (Braintree) horn- 
blendic granite, except that the coarsely crystalline and brilliantly 
faced feldspar is perfectly black instead of pink. The quartz is chiefly 
white. The texture, in thin section proves to be strongly porphy- 
ritic, — a ground mass of quartz and small feldspars and hematite 
scales, having large quartzes and feldspars packed into it. Tourma¬ 
line occurs in bunches of blunt needles, as does also hornblende, — 
the latter also in ultramarine or brownish aggregates. The dark 
color of the feldspar, which is monoclinic, proves in this section to 
be due to innumerable minute needle-like hornblende crystals which 
traverse it. 
As we pass down hill into the valley between Rattlesnake and 
Wampatuck Hills, the rock grows dark gray to nearly black, that 
being the characteristic color of the feldspar, — the crystals of which, 
up to 1 cm. in length, form the mass of the rock, as before. A 
little white quartz, but no ground mass, is observable. Under the 
microscope it shows a handsome porphyritic texture, — a finely 
microgranitic ground mass, — the phenocrysts large and cleanly out¬ 
lined and making up three fourths of the rock. Orthoclase, twinned 
on the Carlsbad law, or showing peripheral zonal phases, in crystals 
up to 3-5 mm. in diameter prevails. Plagioclase is less abundant and 
is allotriomorphic. It shows the effects of strains and gives undula- 
tory extinction. Pyroxene forms much-cracked allotriomorphic 
masses of bright green color, pleochroic from brown to green. Silli- 
manite forms handsome, long, transparent crystals showing the 
characteristic transverse cracking. Hornblende as usual occurs in 
fine needles throughout. It also occurs in irregular translucent 
flakes or uralitic, probably from alteration of biotite, and giving a 
light green to yellow pleochroism. Magnetite or menaccanite in 
1 1 have not visited the locality; the words are from Professor Crosby’s label upon the 
specimen. 
