MINERAL RESOURCES OF THE STATE OF NEW YORK 277 



mations. Collectively they are the most widespread of all' in surface 

 development, since practically all of the middle and southern tiers 

 of counties, west of the Hudson river, are underlain by Devonian 

 beds — chiefly sandstones and shales — covering an area of fully 

 one-third that of the whole State. The northern and eastern limits 

 of the Devonian beds may be defined approximately by a line dr n 

 from Port Jervis in Orange county, northeast to Kingston and 

 thence north on the west side of the Hudson river to Ravena below 

 x^lbany where the boundary swings in a broad curve to the north- 

 west following the Helderberg escarpment into Schenectady county; 

 from this locality it continues in a direction north of west, parallel 

 with the Mohawk river, into Oneida county, south of Utica; thence 

 the boundary extends almost due west to Buffalo and Lake Erie. 

 Within the area thus defined the strata all belong to the Devonian 

 with the exception of a few patches of Carboniferous conglomerate 

 in Allegany and Cattaraugus counties on the Pennsylvania border. 

 The predominant rock type over large areas is shale, but in most 

 sections sandstones are found intercalated with the shale or in 

 successive beds which may reach considerable aggregate thickness. 

 The Devonian sandstones are commonly grouped together for 

 commercial purposes under the name of bluestone. This name, 

 it appears, originated among the quarrymen of Ulster county, where 

 the characteristic material in the quarry ledges has a bluish gray 

 color. There is no broad significance to the term, however, for 

 drab, green and purple to red varieties are associated with the blue 

 layers, and outside of the Hudson River district they are quite 

 common. The special properties of these sandstones are sufficient 

 reason to place them in a separate class, and the term bluestone 

 may well be retained as a general group name. 

 ^^iThey occur in thin-bedded layers or flags, which gradually thicken 

 in depth below the zone of frost action. They have a fine, compact, 

 even texture, giving the appearance of a uniform dense mass that 

 in the finer-grained phases is really a silt. In conformity with the 

 closeness of grain the stone has a low porosity and absorbs moisture 

 to a small extent, whereas most sandstones are fairly porous and 

 form the natural reservoirs for storage of water, gas and oil. A 

 further character that is found in many of the quarries is he well- 

 developed capacity for parting along planes parallel to the bedding. 

 By reason of this property the heavier layers may often be split 

 into two or more slabs with smooth even surfaces, so as to be readily 

 converted into curb or flagstone. The expert quarryman is able 

 to recognize the lines of potential parting on the edge of the stone 



