WHITE : PETROGRAPHY OF THE BOSTON BASIN. 
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Apatite abounds in long prismatic crystals with hexagonal section. 
Pyrite is scattered through the section. 
The rock varies greatly in texture and degree of alteration in 
different portions. The plagioclase is clear when fresh, but most 
of it has become milky. The alteration products are not definitely 
determinable. Chlorite from the neighboring bisilicates has soaked 
in and given further alteration to epidote, forming a “greenstone.” 
Neither macroscopically nor microscopically is any definite demar¬ 
cation between the diorite and the including granite perceptible; 
the diorite seems simply to develop wherever the orthoclase of the 
granite diminishes, and the chloritic products lend a green color to 
such masses. Similar variation is noted in New Brunswick by 
Matthew (’94). 
Diorite.— The greatest development of the diorite itself, how¬ 
ever, is seen at Whitman Pond. The granite adjacent to the mass 
of unmixed diorite partakes of the dioritic character and is well 
exposed in a small quarry on the east side of Whitman Pond. The 
rock here is a dioritic granite or grano-diorite with a considerable 
proportion of both orthoclase and quartz. The hornblende is largely 
uralitic, having light green and yellow pleochroism and frequently 
associated particles of magnetite. The hornblende seems to have 
largely replaced any original biotite. A few small crystals of zircon 
and apatite were noted. 
The diorite itself is of a rather strongly basic character, of a dark 
green color and quite coarsely crystalline. It contains well-twinned 
plagioclase feldspars and, unlike the adjacent granite, is entirely 
quartzless. A secondary, greatly strained and bent hornblende 
occurs all through the rock and is its most prominent feature. 
There is also an original brown hornblende, which in most cases, 
however, is so bleached that it is easily mistaken for a variety of 
pyroxene. Crystallized apatite occurs, also epidote scattered through 
the rock. 
As previously noted, the dioritic granite is a feature especially 
characteristic of the contact zone. The lamprophyric types, rich 
in lime, magnesia, and iron, would naturally tend to form peripheral 
zones on the great granitic batholites, until finally as here found the 
entire mass attains the composition of diorite, according to Pirs- 
son’s (’95) rule that the “ highly differentiated magma of one 
locality maybe the common type or main one at some other center.” 
This fact is well exhibited in the development of dikes in the gran- 
tie, as well as bosses, along the shore near Marblehead, Mass. 
