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NEW YORK STATE MUSEUM 



Igneous rocks 



The igneous rocks are very numerous, but may be classified 

 in a few groups by mineral composition and texture. The tex- 

 ture indicates usually the conditions of their cooling. If the 

 cooling occurred at a considerable depth, the process was grad- 

 ual, crystals of the component minerals formed slowly and freely 

 and the resulting texture is coarse. If the cooling was in the 

 open air, as in a lava bed, the process was more rapid; there was 

 not sufficient time for crystals to form, and the resulting 

 texture is fine or glassy. 



The first class is called plutonic, the second volcanic. Plu- 

 tonic rocks abound in the regions where old geologic formations 

 are exposed, since there, either the intrusions did not reach the 

 surface or the surface material which cooled as lava was re- 

 moved by long erosion, and we see only those parts which were 

 deeply covered while cooling. Examples of this are seen in the 

 Palisades of the Hudson; the granite mountains, Anthony's 

 Nose, Storm King, Breakneck and other peaks of the highlands, 

 and in Mt. Marcy, Whiteface, etc., of the Adirondack chain. The 

 volcanic rocks are chiefly exposed in regions of the newer forma- 

 tions because of the deep-seated plutonic masses have not yet 

 been brought to view by erosion. The only good exposure of this 

 character in New York is the mass of red porphyry or trachyte 

 at Cannon's Pt., near Essex, on Lake Champlain. 



This statement involves the theory that every volcanic mass 

 has beneath it, or connected with it, a plutonic mass of the same 

 general chemical composition.«' 



The names of a few important igneous rocks and their essential 

 compositions are given below according to the classification of 

 Eosenbusch. & 



a The accurate classification of rocks dates from about 1873, with the develop- 

 ment of methods of study with the microscope. Most of the older hooks in English 

 are much behind the present German standard of progress. 



b Mikroskopische Physiographie der Mineralien und Gesteine. 



