62 NEW YORK STxVTE MUSEUM 



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

 waterhme, which is found above the deposits in exposures ailong 

 the sides of valleys or ihills, or to tlie 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 gyip- 

 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 whei'e the 

 gypsum is very rarely seen in outcroip. 



The samiplling of gypsum must 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 jnay be found practicable to work only -certain beds, 

 leaving the poorer material in the roof or floor of 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 driM holes should be selected with due allowance for 

 weathering and solution of the gypsum near the surface. Besides 

 afifording accurate samples for analysis the cores will" give valuable 

 information as to the character and thickness of the covering. 



The core drill is absolutely essential for exploration in Genesee 

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

 and the dqposits '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-inc'h diamond drill. As a rule the glacial drift of western 



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