GEOLOGY OF THE NORTHERN ADIRONDACK REGION 367 



Feet 

 Bluish black limestone like no. 3, but less pure, contain- 

 ing Maclurea magna Les., O r t h. i s p e r- 

 V e t a Con., Strophomena incrassata 

 Hall, Orthis disparilis Con., or O. p o r c i a 

 Bill 75 



Total thickness of B 350 



Group C (Upper Chazy) 



Dove-colored, compact limestone, in massive beds, con- 

 taining a large species of Orthoceras, P 1 a c o p a r i a 

 (Calymmene) multicosta Hall, Soleno- 

 pora compacta, and a large Bucania 60 



Dark impure limestone, in thin beds', abounding in 

 Rhynchonella plena; at the base a bed 4 or 5 

 feet thick is filled v^ith various forms of Monticulipora 

 or Stenopora 125 



Tough, arenaceous magnesian limestone, passing into 

 fine grained sandstone 17 



Total thickness of C 202 



Aggregate thickness of the Chazy on Valcour island. . 890 



In the same papers the authors show that the diminution in 

 thickness of the formation southward is brought about by dis- 

 appearance of the lower and upper divisions, so that in the more 

 southerly exposures, only the middle division remains, and that 

 this then rapidly pinches out to disappearance. To the north- 

 ward the work of the Canadian geologists has shown that the 

 formation rapidly changes in character in that direction, land 

 wash entering much more prominently into its make-up than is 

 the case along Lake Champlain. 



It is thus seen that the Chazy is a comparatively local forma- 

 tion, laid down in an arm of the sea which occupied the present 

 line of the Champlain valley, whose upper end Limited its waters 

 on the south. Its breadth however, specially on the New York 

 side, was much greater than the present limits of the outcrops 

 would indicate. The southern end of the basin was depressed 

 for a much shorter time than the central portion, and its deposits 



