GEOLOGY OP THE NORTHERN ADIRONDACK REGION 



307 



Feet 



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

 ing Maclurea magna Lea., Orthis per- 

 v e t a Con., Stroph o m e n a i n c r a s s a 1 a 

 Hall, Orthis disparilis Con., or O. porcia 

 Bill 75 



Total thickness of B 350 



Group C (Upper Chazy) 



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



raining a large species of Orthoceras, Placoparia 

 ( C alymmene) m u 1 1 i c o s t a Hall, Soleno- 

 pora compact a, and a large Bucania 00 



2 Dark impure limestone, in thin beds, abounding in 



Rhynchonella plena; at the base a bed 4 or 5 

 feet thick is filled with various forms of Monticulipora 

 or Stenopora 125 



3 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 



