426 GEOLOGY. 



The Onondaga limestone of the interior, often described under 

 other names (Corniferous, 1 etc.) is usually not thick, rarely more than 

 100 or 200 feet, but it nevertheless represents a long interval of time, 

 since limestone accumulates but slowly. 



Onondaga limestone (or the equivalent of the Ulsterian series) 

 occurs east of the Appalachian belt in northern New England and 

 Canada where it has a distribution similar to that of the Helderberg. 



Fig. 194. — Section showing the relations of the Devonian system between Porcupine 

 Mountain and the mouth of the Saskatchewan River. Length of section, about 

 125 miles. (Tyrrell, Geo]. Surv. of Canada.) 



At Gaspe, 800 feet of limestone, overlain by 1000 feet of sandstone, 

 have been referred to the epoch. Though these beds may be the 

 time-equivalent of the Onondaga of the interior, they do not contain 

 the fauna characteristic of the formations of the interior, and their 

 exact correlation is open to question. 



The equivalents of the Onondaga beds, or of other subdivisions 

 of the Middle Devonian of the east, have not been differentiated in 

 the western part of the United States, nor in the great northwest terri- 

 tory of Canada, though the Devonian system is represented in both 

 these great areas. 



The map (Fig. 193) showing the outcrops of the Onondaga series 

 represents an extensive outcrop in the northwestern part of the con- 

 tinent. This representation expresses the belief that the Devonian 

 of this region includes the equivalent of the Onondagan. It probably 

 includes also the later parts of the system. No Devonian formations 

 are known to underlie the Great Plains of the United States. 



The Hamilton or Erian series. — After a considerable period, during 

 which there were relatively clear seas over the northeastern interior, 



1 The name Corniferous was originally given to the Onondaga formation be- 

 cause of the local (New York) abundance of chert nodules which stand out (horn- 

 like) when the limestone is weathered. These nodules are sometimes a foot 

 or more in diameter, and often contain abundant shells of microscopic plants 

 (diatoms, etc.), spicules of sponges, and teeth of annelids. Chert nodules are, 

 however, hardly more characteristic of this formation than of many others, 

 and the name is therefore an unfortunate one, and is gradually being set aside. 



