A
superb example of all gradations and sorts of metamor-phism is found in
the Chester amphibolites and serpentines of Massachusetts. The series
(Emerson) consists of amphibole rocks, pyroxene rocks, enstatite rock,
serpentine, dolomites, and steatites, and can be traced from the Hoosac
Tunnel across the State. It is filled with granitic intrusions, and
both regional and local (contact) metamorphism are displayed. Professor
Emerson says, where the granites are present, " the intenser
metamorphism of the schists is indicated by their coarser
crystallization, the lack of sericite, and the great abundance of
cyanite, which, of all the purely aluminous silicates, indicates the
strongest metamorphic agencies."
A
capital example in these Chester series of the effect of heated water
solutions (which makes up a stage in the metamorphism) are the
enstatite beds (silicate of magnesia with iron), which Emerson
concludes were formed from a somewhat ferruginous dolomite "permeated
by heated siliceous solutions set in motion by the large granite
batholites upon their intrusion into their present positions." This
resulted in the formation of a ferruginous silicate of magnesia, or
enstatite.
Take
one other example from the same classic region, the famous Chester
Emery Bed. It seems to Prof. Emerson that the emery magnetite vein was
"originally a deposit of lim-onite, which was formed by the replacement
of limestone," and into which alumina was carried. Metamorphism then
formed magnetite and corundum (emery), and with silica the
corundophyllite (a greenish mica-like mineral composed of silica,
magnesia, alumina, iron). That was the first generation of minerals.
The corundophyllite continued to be formed in the abundant fissures
"produced by the continued intestinal movements of the mass," along
with tourmaline, epidote and pyrite. That was the second generation of
minerals.
The
corundophyllite continued to form in larger plates with rutile,
brookite, menaccanite, calcite, diaspore, margarite, and epidote. That
was the third generation. The fourth
