QUARRY MATERIALS OF NEW YORK 35 



Variations in the character of the igneous rocks are produced 

 by pegmatitic and aphtic segregations and dikes, by quartz veins, 

 and by inchisions of foreign materials that have been involved in 

 the mass during its progress toward the surface. These are detri- 

 mental to uniformity of the product, or may necessitate the dis- 

 carding of much material in the quarry work. They are not so 

 important in case the stone is to be used for engineering work in 

 which appearance is a minor consideration. 



With the sedimentary rocks, the dip or inclination of the beds 

 is a matter of importance.. With ordinary quarry materials exploita- 

 tion under cover is impracticable on account of the cost, though 

 it may be adopted in the case of marble or slate. The thickness 

 and succession of the beds, the presence of shale partings, varia- 

 tions of texture and color, and the spacing of the joints are features 

 to be noted. When the beds lie nearly flat and their edges are not 

 exposed in nearby stream valleys, it may be necessary to prospect 

 the beds by test holes. For that purpose, a diamond or shot drill 

 is used and the cost of securing cores by such method may be 

 expected to amount to several dollars a foot ; ordinarily, only shal- 

 low holes are necessary, but the expense is proportionately large 

 on account of frequency of moving and setting up the drill. 



The sedimentary rocks, unless broken and faulted by dynamic 

 agencies, may be expected to extend over wide areas. It is not 

 safe, however, to rely on the continuity of individual layers for 

 any considerable distance without evidence in the matter. In the 

 clastic rocks like sandstones, especially, the character of the beds 

 may change quite rapidly, or the layers may wedge out to be suc- 

 ceeded by others of different color or texture. This feature is well 

 illustrated by the Medina sandstones which are subject to rapid 

 variations along the strike, the heavy and valuable beds becoming 

 thin or shaly within short distances, though on the dip they are 

 apparently more persistent. The use of the core drill will often 

 effect a large saving in the development work of quarry properties. 



The value of observations in the field as to the durability or 

 weathering qualities of stone is not of much consequence. At most, 

 they can be used only to compare the relative resistance of different 

 materials when exposed to similar conditions. That the conditions 

 depend much upon the topography and the character of the soil 

 covering appears very evident and the variations in these respects 

 may overbalance the factors inherent in the stones themselves. 

 Thus the evidences of weathering are more apparent in valley 

 bottoms where the process of decomposition and disintegration is 

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