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NEW YORK STATE MUSEUM 



As shown on the map, the four rock types are crystalline liine- 

 stone with schists, gneiss, granite, and Potsdam sandstone. 1 



The term, schists* is here used to include a variety of rocks, 

 such as quartzite, hornblende and mica schists, hornblende, 

 pyroxene and mica-gneisses, etc. These rocks may occur singly, 

 but mosi frequently arc intimately associated with one another 

 and with the crystalline limestones in long belts. While In the 

 belts farther south the crystalline limestone is more abundant 

 than the schists, the reverse is true in the region here treated. 



These belts are characterized by their heterogeneity and in 

 particular by rapid changes in the nature of the rocks across the 

 strike. The whole aspect of such a belt is that of a highly 

 metamorphosed series of sedimentary rocks, and as such if* rocks 

 arc classed with the utmost confidence. The quartzites and 

 marbles represent originally pure sandstones and limestones, 

 while most of the schists and gneisses are the metamorphosed 

 shales, impure sandstones and limestones, etc. However, it is 

 probable that some of the schisis are altered igneous rocks, 

 (perhaps, in part, contemporaneous volcanics) which are inter- 

 mingled with the sediments and so thoroughly metamorphosed 

 that their original character is entirely obliterated. Such rocks, 

 if present, must be treated as part of the sedimentary series, 

 and as ye I no effort has been made to determine their existence. 



Scattered areas of unchanged intrusive rocks are known to 

 occur within the schists at many points, but, as a rule, are loo 

 limited in extent to appear on a map of small scale. 



The gneiss of the area under consideration is fairly uniform in 

 character, though, as shown below, not all of the same age. In 

 general it is of medium grain, and pink, red, or gray in color. 

 In texture it ranges from strongly foliated to entirely massive, 

 in the latter case becoming essentially a granite. 



The foliated and massive varieties shade into each other, and 

 in most cases are clearly different phases of one and the same 



diabase dikes identical in character with those of the Admiralty 

 islands previously described by the writer (N. Y. acad. sci. Trans. 13: 

 209-14) are abundant on Grindstone island, and occasionally are seen else- 

 where. Small intrusions of hyperite, also, are sometimes encountered. 



