146 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
The biotitic granite is of limited occurrence, of a white or pink 
color, resembling those on Long Island Sound, and is very finely 
crystalline. The hornblendic granite of Quincy has a typical 
granitic texture, the feldspar being chiefly orthoclase. The most 
striking feature is the occurrence of numerous blue acicular crystals 
of glaucophane. Segregated patches of dark silicates, resembling 
those of the Maine granite, occur in places. By imperceptible 
gradations the latter granite, in the Blue Hills Reservation, passes 
into a porphyritic aporhyolite, with well-developed flow structures 
in the ground mass; or in which, in some cases, the ground mass is 
microgranitic. 
The porphyritic type seems to represent the normal composition 
of the magma which formed the batholites, having been the outer 
crust in cooling. The granite has crystallized within this crust and 
the fiuidal aporhyolite is a later ejectment through the crust, subse¬ 
quent to the differentiation of the magma. 
This later aporhyolite shows all the types of compact, jaspery, 
fiuidal, and coarsely banded texture, seen elsewhere along the 
Atlantic coast, and is always entirely devitrified. Spherulitic texture 
is rare and only minutely developed, and the beds of volcanic tuff, 
ashes, and breccias, often found overlying on the Maine and New 
Brunswick coasts, are lacking. 
Melaphyr forms the great flow on Hough’s Neck which Wolff 
has already described as a dike, but which on account of its relations 
to the conglomerates seems rather to be a tilted sheet. The upper 
part of the flow is filled with amygdules. In the base of the flow 
these are lacking and the rock becomes a true basalt. Olivine is 
rarely evident, and much of the rock is now a diabase. 
The dikes, although plainly of different ages, are nearly all 
diabases, differing little in microscopic appearance and usually much 
decomposed. There is in some a strong gabbroitic development. 
One dike, newer in age than the granite, contained a rose-colored 
augite, resembling garnet except for its low refraction. 
Granite dikes occur cutting the slates. The great points of interest 
are the relation of the various rocks to one another and the transi¬ 
tions noted in the phases of the conglomerate, — the passage of 
granites into true porphyries and subsequent fiuidal aporhyolites, — 
and the development of peripheral phases of diorite. 
