WHITE: PETROGRAPHY OF THE BOSTON BASIN. 
133 
segregations are completely chloritized and although of small size 
are very numerous. Larger segregated patches, as in the case of 
the gray granite tend to the porphyritic texture. Smoky quartz, 
which is about half as large a proportion of the rock as is the 
feldspar is packed into the interspaces. Less dark silicate is present 
than in the gray granite. Microscopically the rock shows a coarse 
granitic texture — chiefly of orthoclase and quartz with a little 
plagioclase. There is hardly any dark silicate, but greenish 
particles of some alteration product, probably epidote. The feld¬ 
spars are fractured and milky white, seldom showing twinning. 
Small octahedral magnetites occur in streaks between the 
feldspars ; and also a red limonite alteration product, to which the 
color of the rock is probably due. Similar dust-like particles of 
magnetite occur in the Iron Mountain rock referred to. 
Upon theside of Rattlesnake Hill, the granite, as it approaches 
the aporhyolite, becomes finer and begins to assume the porphyritic 
texture. Macroscopically it is gray, coarsely crystalline, apparently 
composed almost exclusively of closely packed feldspar crystals, with 
brilliant tabular faces up to 8 mm. in size, with a very little dark 
silicate filling the spaces, and with no apparent ground mass. The 
quartz is not visible. 
Microscopically it exhibits a microgranitic texture, partly con¬ 
fused by the crushing effects which the rock has undergone, which 
renders it difficult to get a straight extinction in the irregularly 
formed and often uncertainly defined phenocrysts. 
* Orthoclase is the principal feldspar and is rarely twinned. The 
edges of the phenocrysts are usually corroded. The quartz 
strikingly shows the effects of crushing and strain, with frequent 
embayments. Hornblende and tourmaline each occur as pale apple- 
green interlacing needles in both quartz and orthoclase, chiefly the 
latter. Hornblende also forms duller green, pleochroic aggregates. 
Hematite occurs in scattered blood-red grains. Calcite is sparingly 
present. The resorbing of edges of the phenocrysts and the fractured 
ground mass seems to indicate the first transition to the porphyritic 
phase described later. 
Type 4. Hornblendic Graxitite. — The fourth granitic type 
Professor Crosby considers to represent the contact zone of the 
normal hornblendic granite just described. It occurs in East 
Braintree and eastern Quincy along the west bank of Weymouth 
Fore River, as a rule in the vicinity of the slate. 
