76 NEW YORK STATE MUSEUM 



most of the igneous rocks that are classed in the trade as granite 

 by the prevalence of the basic plagioclase feldspars and the higher 

 percentages of the iron magnesia minerals, while it contains no 

 quartz. Some of the so-called " black granites," however, are 

 trap. The name is sometimes applied to fine-grained rocks of 

 granitic or syenitic composition and sometimes even to rocks of 

 sedimentary derivation, but such usage is misleading and inde- 

 fensible. 



The particular value of trap is due to its hardness and toughness. 

 Its fine, compact, homogeneous texture gives it great wearing 

 powers and it is eminently adapted for road metal and concrete of 

 which heavy service is required. The principal product, therefore, 

 is crushed stone. It has been used to some extent, also, as paving 

 blocks, but these are rather difficult to prepare, since trap very 

 seldom shows any capacity for parting comparable to the rift and 

 grain structures of granites. 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 State is properly a diabase. Its 

 mineral composition varies somewhat in the diflferent occurrences, 

 but the main ingredients are plagioclase, feldspar and pyroxene, 

 with more or less of amphibole, olivine, magnetite and sometimes 

 biotite. The texture is characteristic, for the feldspar forms lath- 

 shaped crystals which interlace and inclose the pyroxene and other 

 ingredients in the meshes, and it is this firmly knit fabric which 

 gives the stone the qualities of strength and toughness. 



The largest occurrence of trap in New York is represented by 

 the Palisades of the Hudson and the continuation of the same 

 intrusion which extends southward through New Jersey onto 

 Staten Island and is also encountered in the interior of Rockland 

 county. The Palisades are the exposed edge of a sill or sheet of 

 diabase that is intruded between shales and sandstones of Triassic 

 age. The sheet is several hundred feet thick, in places nearly 1000 

 feet, and in general seems to follow the bedding planes of the sedi- 

 mentary strata which dip to the west and northwest at an angle of 

 from 5° to 15°. The outcrop is narrow, seldom over a mile, and in 

 places is limited to a single steep escarpment. The principal 

 quarries are near Nyack and Haverstraw at the base of the cliffs. 

 Other quarries have been opened near Suffern, Rockland county, 

 on an isolated intrusion, and also near Port Richmond, Staten 

 Island, at the southern end of the Palisades sill. 



