LOWER SILURIC SHALES OF THE MOHAWK VALLEY 67 



thickness of the Schenectady beds in this region is due to their 

 formation in a sinking trough parallel to and in front of the Green 

 Mountain-Taconic fold system. The Schenectady beds strike for 

 this reason from north-northeast to south-southwest. 



5 The uppermost portion, about 300 feet, of the " Frankfort " 

 beds at their eastern edge in Albany county has been found to con- 

 tain an entirely different fauna and also to be lithologically some- 

 what different. This division has been termed here the Indian 

 Ladder beds. These rapidly disappear westward, being absent at 

 Cobleskill and probably formed originally a narrow belt extending 

 north-northeast to south-southwest; they are hence for the greater 

 part buried under Devonic rocks in the south, and lost by erosion 

 north of the Helderbergs, and only exposed to view in the Helder- 

 berg escarpment. The Indian Ladder beds contain the fauna of the 

 Southgate division of the Eden shale and are hence of the age of 

 part of the Frankfort beds but entirely different from them in their 

 faunal aspect. 



6 The Brayman shales which were originally referred to the 

 Clinton and later were correlated with the Salina, appear to be the 

 residual pyritiferous clays of the long hiatus corresponding to the 

 time interval from the upper Trenton Schenectady shale to the 

 Cobleskill limestone. They seem, however, sufficiently closely con- 

 nected with the underlying sandstone at the top of the Schenectady 

 shale to be considered as mainly formed in Upper Ordovicic time. 



7 In the Hudson valley a belt of slates, grit and sandstones 

 extends between the Canajoharie shale and Normanskill shale. It 

 is distinguished from both by its fauna, and from the Canajoharie 

 beds in the character of the shale and the greater intercalation of 

 arenaceous beds. This shale which in the Hudson valley is well 

 exposed on Green island, opposite Troy, and at Mechanicville, has 

 received the name Snake Hill beds from its very fossiliferous out- 

 crops at Snake hill, on the east side of Saratoga lake. It exhibits in 

 its fauna evidence of being older than the Canajoharie shale and 

 younger than the Normanskill shale and also distinct relations to the 

 basal Trenton of the Ottawa basin, and properly belongs to the 

 Levis basin where it follows the Normanskill shale. 



8 The Canajoharie, Utica and Frankfort shales seem to corres- 

 pond to the three divisions of the Martinsburg shale of Pennsyl- 

 vania, Maryland and Virginia, or to be a northern continuation of 

 that formation ; and the Frankfort shale can also be correlated with 

 the middle part of the Eden shale of the Cincinnatian in Ohio. 





