272 " NEW YORK STATE MUSEUM 



sold for interior decorative work. The Becraft limestone with its 

 round and crescentic fragments of crinoids is worked near Catskill 

 for building and ornamental use. 



Serpentine is classed with marble in the decorative stone trade, 

 but is quite different in its mineral and chemical nature, being a 

 hydrated silicate of magnesia that is formed by the alteration of 

 basic rocks in which pyroxene and olivine are important ingredients. 

 Its ornamental quality depends upon the translucency and varied 

 color of the serpentine, ranging from clear white to dark opaque 

 shades of green. Large bodies of this rock occur on Staten island 

 and near Rye in Westchester county, but there has been little 

 quarrying at either locality. 



SANDSTONES 



Under sandstones are included the fragmental rocks, composed 

 of sand grains compacted or bound together into a coherent mass. 

 The size of the grains may vary from particles of almost microscopic 

 size to those an inch or so in diameter, and this variation is recognized 

 by dividing the group into silts, sandstones proper, and conglome- 

 rates. Some sandstones have been simply compacted by pressure, 

 but most of them are bonded by some cementing substance that 

 has been deposited in the open pores between the grains. Upon 

 the nature of the cement and its abundance depend to a large extent 

 the color and strength of the stone. Silica, iron oxide, iron sulphide, 

 iron carbonate, calciimi carbonate and clay are the commoner 

 kinds of cement in sandstones. Silica produces the hardest, most 

 durable bond and when the grains themselves have a similar com- 

 position the material is known as quartzite. Iron oxide affords a 

 permanent bond, but iron sulphide decomposes readily on exposure 

 to the atmosphere and sandstones in which it enters to any extent 

 are unsuitable for constructional purposes. 



Of the sedimentary rocks which occur in New York State, sand- 

 stone is one of the most widespread geographically but in economic 

 importance it ranks second to limestone. Nearly all of the recog- 

 nized stratigraphic divisions above the Archean contain sandstone 

 at one or more horizons. The kinds chiefly quarried are the Potsdam, 

 Hudson River, Medina and Devonian sandstones. A few quarries 

 have been opened also in the Shawangunk conglomerate and the 

 Clinton and Triassic sandstones. 



Potsdam sandstone. The Potsdam, a member of the Cambrian 

 system, is the oldest and lowest of the formations that contain 

 sandstones in extensive areal development adapted to building 



