446 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



faunas apart. It is fully realized that in the course of time it may be 

 shown that these maps err on the side of too much restriction of the conti- 

 nental seas. It was thought, however, that by the present method of 

 representation more certain progress would be attained than by assuming 

 universal continental synthetic seas, which some paleontologists believe 

 have not led to a proper understanding of the periodic encroachment of 

 the oceans on the land. 



An analysis of fossil faunas indicates that from the earliest Paleozoic 

 times there have been three permanent oceanic realms that have furnished 

 life to the continental seas of Xorth America. In the order of their im- 

 portance these are: (1) The Gulf of Mexico mediterranean, (2) the Pa- 

 cific, (3) the Arctic and the Atlantic. The faunas of the Xorth Atlantic 

 as a rule are restricted to Acadia and to the eastern portion of the Appa- 

 lachian mountains, yet they frequently spread across these folds and mix 

 with the life of the other regions. Those of the Pacific have a far wider 

 range, and often occur as far east as Appalachia. The faunas from the 

 Gulf or Mexican mediterranean are at times clearly tinged with an Atlan- 

 tic facies, but oftener are more of the southern than of the northern 

 European type, while at other times they are without doubt from the 

 South American realm by way of the Pacific. 



AREAL-GEOLOGIC METHOD 



Having ascertained the nature of a fauna and its stratigraphic position, 

 the next point of greatest value is the geographic distribution of the for- 

 mation. Here geologic maps are of the highest importance, especially 

 those that give lists of the local faunas. In this connection the Folios of 

 the United States Geological Survey were most helpful, but all maps 

 issued by the more prominent national and state surveys were scanned for 

 information. The recently published International Geologic Map was 

 likewise found to be very useful, particularly so for outlying regions of 

 the North American continent. 



PETROLOGIC METHOD 



Marine conglomerates unmistakably indicate proximity to land, and are 

 therefore of great value in paleogeography. Marine sandstones are also 

 good indicators for shore conditions, shallow seas, and nearness to land, 

 but are not so reliable as the conglomerates. In the interior region of the 

 continental seas, however, sandstones are of rare occurrence. Mud de- 

 posits point rather to shallow seas, and black shales are thought to denote 

 closed or stagnant arms of the sea, variably foul at the bottom, as in the 

 Black sea of Eussia. Such black shale deposits are the Utica, Genesee, 



