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NEW YORK STATE MUSEUM 



Trap 



Trap is a name commonly applied to a series of igneous rocks, 

 distinguished chiefly by their basic composition, their fine grain, 

 and, because of their basicity, by a dark color. In mineral com- 

 position they dififer from granite in the total absence of quartz and 

 by the more basic character of their feldspars. They consist 

 mainly of plagioclase (soda lime feldspar) and pyroxene. In 

 diabase, by far the most common rock type of the series, the feld- 

 spars have crystallized out in needle or lathlike forms, forming 

 an interlocking network, with the pyroxene filling up the interstices. 



Trap appears usually in two forms of occurrence: (i) as dikes 

 which are usually vertical, wall-like masses of rock, which have been 

 intruded while in semifluid condition into fissures, from some 

 molten mass below; (2) as sheets which are intrusions of similar 

 character but which have been forced between the bedding planes 

 of sedimentary strata and have a horizontal position. 



Of the first type, the dikes, there is an abundance around the 

 borders of the Adirondacks especially in Clinton and Essex 

 counties. However, owing to their small size or their inaccessi- 

 bility, they are with the exception of one near Greenfield, Saratoga 

 CO. and one at Little Falls, Herkimer co. of no present commercial 

 importance. 



