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NEW YORK STATE MUSEUM 



characteristic structures of the rock being here easily made out 

 with the eye. 



These rocks show much variation from place to place, due in 

 part to local differentiation during cooling; in part to mutual 

 corrosive effects of adjacent minerals on each other, both during 

 the original cooling of the rock and as a result of subsequent 

 metamorphism ; and in large part to varying severity of meta- 

 morphism. Where least metamorphosed, a simple original min- 

 eral constitution is usually shown, the rock consisting essentially 

 of plagioclase feldspar (usually labradorite) , augite and magne- 

 tite; to these hypersthene is frequently to be added, and rarely 

 olivine. These primary feldspars and augites invariably hold a 

 multitude of minute inclusions, the augite specially containing 

 them in such numbers that it would often be impossible to make 

 out the color of the mineral were it not for the fact that a nar- 

 row outer zone is usually free from them. Nor is the feldspar 

 far behind in this respect. The inclusions in the augite are 

 mainly opaque and consist probably of magnetite or ilmenite. In 

 large part the feldspar inclusions consist of small augites. The 

 structure is rather prominently ophitic in most cases, that is, the 

 feldspar is in long, lath-shaped crystals, separated by and par- 

 tially embedded in the stout augite crystals. 



From the extinction angles shown by the feldspars from various 

 occurrences, it is quite certain that they show a range in com- 

 position from andesin to anorthite, with labradorite the prevail- 

 ing variety. The augite is of a pale, gray-green shade, nearly 

 colorless in thin sections. 



In addition to the foregoing, even the least metamorphosed 

 rocks show much granular material, and rocks which consist 

 mainly or wholly of this, with little or no preservation of the 

 original character, are far more common than those of which the 

 reverse is true. This granular material is, to some extent, due 

 to corrosive interaction of the original minerals of the rock. This 

 is most apt to take place between magnetite and feldspar but also 

 occurs between the pyroxenes and feldspar. In general nothing 

 of the sort takes place at pyroxene magnetite contacts. The main 

 new minerals produced by this action are garnet and a peculiar 

 brown hornblende, with some quartz and often biotite accom- 

 panying. These are found arranged zonally between the two 



