62 NEW YORK STATE MUSEUM 



The best iiudicator of the position of the gypsum is the Bertie 

 wateriime, which is found above the deposits in exposures ailong 

 the sides of valleys or hills, or to the south of them when the 

 surface is flat. It is much more resistant to erosion than the Salina 

 shales, and together with the overlying limestones can often be 

 traced in outcrop by the character of the topography. A very 

 noticeable escarpment formed by the Hmestones extends across Erie, 

 Genesee and Monroe counties, where it is known as the " ledge." 

 The Salina shales occupy the plain between this escarpment and the 

 parallel one to the north formed by the Niagaran limestones. 



The absence of a protecting cover of limestones leaves the gyp- 

 sum open to the attack of weathering agencies which may result 

 in the partial or complete removal of surficial deposits. This seems 

 to be the prevaihng condition in the western section where the 

 gypsium is very rarely seen in ontcroip. 



The sampling of gypsum m.ust be conducted with care and intel- 

 ligence. The successive layers or strata may show wide variations 

 in purity, and it is generally better to sample each separately so 

 that the series of analyses will reveal their individual character. 

 Sometimies it may be found practicable to work only certain beds, 

 leaving the poorer material in the roof or floor o^f the mines. In 

 sampling the pocket deposits of friable shaly gypsum, close atten- 

 tion is required that the mass of fibers or crystals may not be un- 

 fairly sorted from the impurities. 



The beds of rock gypsum can be explored to best advantage by 

 core drilling. It is difficult in most cases to^ form an accurate esti- 

 mate of their quality and thickness from exposures, except where 

 these result from previous quarrying or mining operations. The 

 sites of the driill ho/les should be selected with due allowance for 

 weathering and solution of the gypsum near the surface. Besides 

 affording accurate samples for analysis the cores will give valuable 

 information as to the character and thickness O'f the covering. 



The core drill is absolutely essential fo^r exploration in Geniesee 

 and Erie counties, since the surface in that section is almost level 

 and the deposits lie at depths of from 40 tO' 80 feet. Its advantage 

 over the churn drill is so obvious and decisive that there can be 

 little excuse for the continued use of the latter for such work. 

 After the glacial material is once passed, no difficulty need be an- 

 ticipated in securing cores of the limestones, shales and gypsum 

 with a 2-inch diamond drill. As a rule the glacial drift of western 



