408 TIIE CAMBRIAN. [bull. 81. 



limits. A near contemporaneity in rocks may be proved, but not in the transitions 

 from one rock to another. For example, the Devonian age has a very different series 

 of periods and epochs in North America from what it has in Europe, and there is 

 even considerable diversity between the epochs of New York and the Atlantic slope 

 and those of the Mississippi Valley. The Carboniferous, Reptilian, and Mammalian 

 ages also have their American epochs and their European, differing from one another ; 

 and the differences between the continents increase as we come down to more modern 

 times. There are Tertiary and Cretaceous rocks in America as well as Europe, but 

 there is little reason for the assumption that the transitions from one set of Tertiary 

 or Cretaceous strata to another were, in the two, contemporaneous. The point should 

 be proved, not assumed. We add, therefore, 



Sixthly. It is an important object in geology to ascertain as nearly as possible the 

 parallelism between the periods and epochs marked off on each continent, and 

 study out the precise equivalents of the rocks, each for each, that all the special 

 histories may read as parts of one general history, and thus contribute to the per- 

 fection of one geological system. 1 



MISSISSIPPI VALLEY. 



The Lower Paleozoic rocks of this region were correlated with those 

 of New York by Messrs. Owen, Locke, Hall, and others, usually with- 

 out stating the data except by implication (e. g., speaking of the Tren- 

 ton limestone of New York as a limestone carrying certain fossils and 

 superjacent to a marked formation, the Calciferous sandrock of New 

 York or the Lower Magnesian limestone of Wisconsin, etc.). Thus the 

 correlation is based upon stratigraphic position, presence of similar 

 fossils, and lithologic characters. 



Hall. — The following observations by Prof. Hall are a good illustra- 

 tion of the method by which the Potsdam sandstone of New York was 

 correlated with the lower sandstone of the Mississippi Valley section : 2 



In comparing the older rocks of New York and of the East generally with those of 

 the West it should not be forgotten that there is a long interval on the line of the 

 northern outcrop of these ancient strata between the St. Lawrence and the western 

 limit of Michigan on the Menomonee River, where we can expect little aid from pale- 

 ontology. The i'ossiliferous beds of these ancient formations in Wisconsin lie to the 

 west of what appears to have been a great promontory at the time of their deposi- 

 tion, stretching southward from the region of Lake Superior far into the ancient sea. 

 The disconnection caused by this promontory between the East and the West would 

 of itself prepare us to expect a fauna differing in a great degree from beds of corre- 

 sponding age on the opposite sides. 



It has been shown by the investigations of the Canadian Survey that not only the 

 Potsdam sandstone but all the fossiliferous beds below the Birdseye and Black River 

 limestones are absent from Kingston, on Lake Ontario, to Lacloche, on lake Huron. 

 From Lacloche to Lake Superior there is a sandstone coming in below the Birdseye 

 limestone, which, from its position, may be considered as of the age of the Chazy 3 for~ 



1 Manual of Geology, 1st ed., New Haven, 1863, pp. 126-128. 



Preliminary notice of the fauna of the Potsdam sandstone, with remarks on the previously known 

 species of fossils and descriptions of some new ones from the sandstone of the Upper Mississippi Val- 

 ley. 16th Ann. Rep. Reg. State Cab., Nat. Hist. N. Y., 18G3, pp. 211-213. 



3 The ' Chazy formation ' of the Canadian Geological Survey, in its eastern localities includes a sand- 

 stone which comes in below the greater part of the limestone, leaving from 10 to 20 feet of shale and.lime- 

 stone beneath (Geology of Canada, 1863, p. 123). It is apparently this sandstone of the Chazy forma- 

 tion, having in Canada a thickness of 50 feet, which has become augmented in its western extension 

 while the calcareous part of the formation has partially or entirely disappeared. 



