90 NEW YORK STATE MUSEUM 



TRAP 



The quarrying of trap is a somewhat specialized branch of the 

 stone industry which may be treated with advantage under a sepa- 

 rate head. Trap is not a distinct rock type, but the name properly 

 belongs to the fine-grained, dark-colored igneous rocks that occur as 

 intrusive sheets or dikes. In mineral composition it differs from 

 the other igneous rocks classed in the trade as granite, by the prev- 

 alence of lime-soda feldspars and higher percentages of the lime, 

 magnesia and iron minerals and correspondingly lower amounts of 

 silica, with little or no free quartz. The name is sometimes applied 

 to fine-grained igneous rocks of granitic or syenitic composition and 

 even to rocks of sedimentary derivation, but such usage is mislead- 

 ing and indefensible. 



The particular value of trap is due mainly to its hardness and 

 toughness. Its fine, compact homogeneous texture gives it great 

 wearing powers and it is eminently adapted for road metal and for 

 concrete of which heavy service is required. It has been used to 

 some extent in this State as Belgian blocks. As a building stone it 

 finds very little application, probably on account of its somber color. 

 The expense of cutting and dressing trap is also an obstacle to its 

 employment for building or ornamental purposes. 



The trap quarried in New York is properly a diabase, made up 

 of plagioclase feldspar in lath-shaped crystals and pyroxene as the 

 main constituents, and amphibole, olivine and magnetite as sub- 

 ordinate minerals. The largest occurrence is represented by the 

 Palisades of the Hudson, which begin near Haverstraw and extend 

 southward into New Jersey. The Palisades represent the exposed 

 edge of a sill or sheet of diabase intruded between shales and sand- 

 stones of Triassic age. The sheet is from 300 to 800 feet thick and 

 about 70 miles long. Most of the trap quarried in this State has 

 been obtained from this region, chiefly from the vicinity of Haver- 

 straw and Xyack, but to some extent from near Richmond, Staten 

 Island, where the sheet has its southern termination. Smaller 

 occurrences of diabase are found in the Adirondacks and the border- 

 ing area. There are countless numbers of trap dikes in the interior 

 of the Adirondacks, but few have any considerable thickness and in 

 general they are too remote from the market to be profitably 

 quarried. In the outlying region the dikes at Greenfield, Saratoga 

 county, and at Little Falls, Herkimer county, are the most notable. 

 Quarries have been opened at the former locality and the trap is 

 crushed for road metal. 



