WHITE : PETROGRAPHY OF THE BOSTON BASIN. 
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masses of the melaphyr, whereas the lower contains none, and is 
interbedded with seams of brown fine-grained sandstone. From the 
complex character of the upper conglomerate and the extensive 
masses of older rocks, chiefly igneous, and many of them crystalline, 
the field characters, rather than the microscopal slide, must be relied 
on in description. 
GRANITOID ROCKS. 
Plutonic Rocks with hypidiomorphic granular ground mass, 
without phenocrysts. 
Although variable in detailed characters, the granites of the area 
considered may be divided both by microscopic and macroscopic 
characters into four types, viz: — 
Type 1 : Dioritic, medium coarse granite, often with segregations 
of diorite and becoming diorite proper in the extreme phase of basic 
differentiation. 
Type 2: Biotite granite (Granitite). An even-textured rock 
with fairly equal proportions of quartz, feldspar, and biotite; pre¬ 
sumably forming, with Type 3, the normal phase of the magma. 
Type 3 ; Typical Quincy coarse hornblende granite, which shades 
into the porphyritic series of rocks (Aporhyolites). 
Type 4: Hornblende granitite, presumably forming a contact 
zone over Type 3. A fine-grained, evenly crystalline granite, 
principally feldspathic. 
Type 1. Dioritic Granite. — Texture medium coarse and usually 
associated with inclusions or segregations of diorite. It occurs in a 
limited area near the contact with the Braintree argillites and also 
on Mt. Pleasant, Weymouth, and about South Weymouth, as well 
as in the vicinity of the Whitman Pond diorite area noted later on. 
It is similar to the granite of Sullivan, Maine ; light colored with 
grains of various dark green silicates scattered through it. 
Microscopically its texture is thoroughly granitic. The feldspars are 
of both kinds. Plagioclase is always present and is the chief one ; 
sometimes also seemingly microcline. Zonal decomposition of the 
feldspar is evident in some instances. Frequently microperthitic 
intergrowth is shown, with jagged edges. 
Quartz occurs in considerable quantities. Mica is chiefly biotite 
of a brown color often bleached in places to pale green by incipient 
decay due to loss of the Mg molecule of the biotite. The biotite is 
