GEOLOGIC FORMATIONS OF NEW YORK 



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in the gorge of the Mohawk protruding through the Hudson 

 river shale and Trenton limestone. 



Beneath the metamorphic rock® of the Archaean and intersect- 

 ing them, are found what are known as Plutonic a rocks, the pe- 

 culiarity of which is, that they are not found in layers or strata, 

 but in solid masses, and appear to have been forced up from 

 below in a plaistic condition. They form the central mass of 

 the Adirondacks, and large areas of them are found in the High- 

 lands and in many parts of New England. They were once gen- 

 erally called ' primary ' or ' primitive ', as it was believed that 

 they were the original crust of the earth, first formed in the cool- 

 ing of its melted mass, but it is now doubted whether, if such a 

 crust exists, it can be identified, and many geologists think that 

 most of the granites and other plutonic rocks are only re-melted 

 and altered forms of older ones. That many such masses are 

 so, is certain; and whether we can find any which are portions 

 of an original crust of the globe, is at least very doubtful. 



Containing no fossils, these rocks have their chief interest in 

 their value for economic uses in building and other purposes, 

 and in the cabinet specimens of the minerals which they so often 



contain. 



The Archaean rocks cover two separate tracts of country in 

 this state, one in its southeastern part known as the Highlands; 

 the other lying in the central portion of the great Adirondack 

 wilderness. 



Various kinds of rocks are mingled over most of these 

 areas, seeming often to change or gradually pass into each other. 

 The metamorphic masses of gneiss, etc. are more fully exposed 

 (as a general rule) around the edges of the tracts, where they 

 pass under the lower strata of f ossiferous rocks; while the 

 granite, hypersthene and other plutonic masses are more fully 

 developed near the centers of these areas and among the highest 

 of the mountains. 



' Throughout the Archaean districts there are many dykes, or 

 veins of trap or other igneous rock penetrating masses of a dif- 

 ferent character. Not infrequently, a mountain or hill shows 



a Plutonic, from Pluto, king of the infernal regions in Pagan mythology. 



