648 NEW YOEK STATE MUSEUM 



Guelph division of tlie Magara, and the Cambro-Silurian (of 

 Westchester county) yield the best dolomites. 



PROSPECTmG 



The points to be considered in prospecting are topography, 

 vegetation and appearance of outcrops. 



Limestones as a rule weather quite readily, but the presence 

 of impurities may exert an important influence in this respect. 

 Pure limestone is easily soluble in carbonated water, consequently, 

 when a bed of soft, pure limestone is inclosed between two harder 

 rocks, dipping at high angle, the limestone may be dissolved away, 

 leaving a valley between the more resistant layers. This fact 

 is often noticed in Westchester county, where the section involves 

 a basal gneiss, a limestone, and an overlying mica schist; the beds 

 have been much folded, and the dip is often steep. Most of the 

 valleys in this county exhibit gneiss on one side and mica schist 

 on the other, while the intermediate limestone has been cut down 

 by weathering to form a valley. (PL 2) 



Many limestones contain sandy layers or chert nodules, or in 

 some cases silicified fossils. In such instances the weathering of 

 the rock is extremely irregular, the lime carbonate being dissolved 

 out, while the silicified portions stand out in bold relief on the 

 weathered surface. Limestones of great purity, however, may 

 at times weather unevenly, solution for some reason not well 

 understood, taking place along certain lines, thus producing a 

 series of reticulated gashes extending inward from the surface 

 of the rock. 



In magnesian limestones the carbonate of lime is dissolved out, 

 while the carbonate of magnesia yields but slowly to solution, 

 the result being that the rock breaks doTVTi into a series of sand- 

 like grains. The Guelph limestone of western 'New York shows 

 this. Many dolomites, however, owing to the coarsely crystalline 

 structure and insolubility, disintegrate rather than decompose. 



