I 



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-Tacoriic 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 

 I 3obleskill and probably formed originally a narrow belt extending 

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

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

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

 )erg escarpment. The Indian Ladder beds contain the fauna of the 

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

 Dart 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 

 ime interval from the upper Trenton Schenectady shale to the 

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

 lected with the underlying sandstone at the top of the Schenectady 

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



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

 ;xtends between the Canajoharie shale and Normanskill shale. It 

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

 »eds in the character of the shale and the greater intercalation of 

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

 :xposed on Green island, opposite Troy, and at Mechanicville, has 

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

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

 ts fauna evidence of being older than the Canajoharie shale and 

 ounger than the Normanskill shale and also distinct relations to the 

 )asal Trenton of the Ottawa basin, and properly belongs, to the 

 ^evis basin where it follows the Normanskill shale. 



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

 »ond to the three divisions of the Martinsburg shale of Pcnnsyl- 

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

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

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



3 



