96 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. 3 
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 
