360 



HISTORICAL GEOLOGY 



was laid down and that chiefly in a restricted sea extending 

 from the St. Lawrence region to Virginia. This is called the 

 Helderberg limestone because it is well exhibited in the Hel- 

 derberg Mountains of eastern New York. At the same time, 

 apparently, a bay extended up from the south into Tennessee 

 and Indian Territory. 



Oriskany sandstone. As the clear sea with its limey 

 bottom spread slowly westward into the Mississippi Valley 

 and perhaps south to Alabama, some radical change in the 

 middle Atlantic states allowed coarse, 

 sandy sediments to be spread out over 

 the Helderberg formation. The result- 

 ing Oriskany sandstone is several hun- 

 dreds of feet thick, and the sand of which 

 it is made represents the decomposition 

 of a vast amount of solid rock. (How 

 might this be accounted for (1) by cli- 

 matic change, (2) by diastrophism ?) 



Onondaga limestone. Gradually the 

 deposition of sands became restricted, 

 and the sea which occupied the Appa- 

 lachian depression was again clear. In 

 & a second limestone, called the Onon- 

 daga, was deposited over the Oriskany 

 sandstone. The warmth and shallow- 

 ness of the Onondaga sea are shown by the abundance of 

 corals (Fig. 372) and other animals which frequent coral 

 reefs. By this time, also, the northwestern part of the con- 

 tinent, from Alaska to Alberta, was covered by the waters 

 of the northern ocean. They were apparently not cold 

 waters in those days, for reasons not yet well understood. 

 Hamilton shales. Muddy sediments succeeded the 

 Onondaga deposits in the East, and even in Illinois the lime- 

 stone is less pure. The Hamilton shales are usually dark and 

 bituminous, implying an abundance of minute plants as well 

 as the animals whose shells abound in the same beds. The 



'organ-pipe coral (Sy- 

 ringopora) from the 



