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Ch. 1: Introduction

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16               GEOLOGY OF NEW YORK CITY
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
Ch. 1: Introduction Page of 281 Ch. 1: Introduction
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