442 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



which is represented in the photograph by a horizontal line, can be traced, 

 through these quarries for half a mile, yet the two deposits can not be 

 readily separated by any other means than the entombed fossils. Such 

 disconformities are numerous and are of general occurrence in the central 

 portion of the United States and Canada, where they have led to the dis- 

 covery of various land barriers so necessary to a proper interpretation of 

 faunal provinces. 



It has been assumed by some geologists that where such disconformities 

 occur the sea has been continuous and has failed to deposit sediments, or 

 has even scoured away parts of the sea-bed, as is the case with the present 

 gulf current when it is forced between narrow passages like that between 

 Florida and the Bahamas. The question may be asked of those making 

 this assumption : Why is it that a sea which has not laid down strata for 

 thousands or perhaps hundreds of thousands of years suddenly begins to 

 deposit sediments? In this connection it should be noted that during 

 Louisville time a clear sea was the agent for the precipitation of coral- 

 reef limestone, and that very much later, in Onondaga time, it was fol- 

 lowed by another sea with identical physical characters. During the in- 

 terval, if the sea were present, it did not accumulate material nor remove- 

 an appreciable thickness of the limestone by leaching. On the other hand, 

 scouring of the sea-bottom is known only where the Gulf stream flows 

 swiftly, a condition which is exceptional in existing seas. 



Neither can it be admitted that the land interval was less in time than 

 the fossils indicate, nor that extensive sheets of limestone have suffered 

 erosion. If the latter were true, outliers of these missing horizons would 

 be found, for the land was so low that the wearing away could not have 

 removed them completely over hundreds of miles of extent. Since late- 

 Ordovicic time the Cincinnati region has been above the sea, yet it has lost 

 less than 300 feet by erosion, and probably the greater part of that has 

 been taken away since the Pleistocene elevation. It is possible, however,, 

 that a thin Mississippic formation may have covered the Cincinnati area ; 

 but in any event less than 400 feet have been eroded since the close of the- 

 Ordovicic. 



Currents as fauna distributors. — The statement has been made that it 

 is not necessary to assume the presence of land barriers to have distinct 

 faunas in a wide spread supposedly continuous sea, but that such will be- 

 kept separate and distinct by currents having decided differences in tem- 

 perature. In other words, two definite faunas may exist side by side in 

 the same marine waters, this condition being certainly in force at present 

 in the Atlantic ocean off the coast of America. On the continental shelf, 

 as far south as cape Hatteras, lives the fauna of the cooler waters due to 



