g6 NEW YORK STATE MUSEUM 



is shipped by boat, as the barge canal passes directly through the 

 district. For paving blocks the sandstone seems specially adapted 

 by reason of its qualities of wearing evenly and not becoming readily 

 rounded or " turtle-backed," which is a fault in many stone pave- 

 ments. The use of blocks is superseding asphalt in paving between 

 car tracks in city streets. There are a large number of quarries 

 in the section from Medina to Holley, but many of these are closed 

 at present. The tendency in the district has been toward a con- 

 centration of operations at a relatively few quarries which are well 

 equipped and actively worked, in contrast with the small and 

 scattered enterprises in the eastern bluestone districts. 



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 rocks of granite or syenitic composition and even 

 to rocks of sedimentary derivation, but such usage is misleading 

 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 adopted for road metal and for 

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

 this State to some extent 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 



