MARYLAND GEOLOGICAL SURVEY 47 



that came in from the east or south from those that invaded the continent 

 from the north or west. 



The underlying principles of the science of paleogeography and the 

 methods employed in the preparation of paleogeographic maps have been 

 discussed in detail by Ulrich. 1 In brief the study of first, fossil faunas 

 and floras, and second, of the phenomena expressed under the general 

 name of diastrophism, afford the data for such maps. In the study of the 

 ancient life forms, conclusions of value are reached first, by determining 

 the areal distribution of certain associations of species of land and water 

 organisms, and second, by the discovery of their place of origin. 



The second method of study, based on diastrophism depends upon the 

 idea of essentially permanent depressions and elevations of the earth's 

 surface. According to this view the surface of the continent can be 

 divided into (1) positive areas that have been rarely if ever submerged, 

 this being shown by the distribution of the sedimentary rocks around them ; 

 and (2) negative areas which often received deposits from waters of one 

 or another of the oceanic basins whenever by subsidence they were brought 

 below sea level. A paleogeographic map therefore is produced by plotting 

 the isolated occurrences of a definitely identified fossil fauna and connect- 

 ing them with the ocean of their origin by sea ways within the negative 

 areas. 



From the study of the criteria of paleogeography it becomes apparent 

 that the Paleozoic epicontinental seas occupied mostly small, shallow, 

 often disconnected, basins, communicating with the nearest oceanic basin. 

 In general they must have been much like Hudson Bay, which may be 

 regarded as a modern representative of an American interior continental 

 sea. Many of these land basins were filled and emptied many times, 

 occasionally receiving their water from the Atlantic and at other times 

 from the Arctic, and ofttimes from the Gulf of Mexico. Naturally, with 

 each change in the source of the waters, the geographic pattern differs 

 considerably, and at times fundamentally, from the next preceding. In 



i Bull. Geol. Soc. Amer., vol. xxii, 1911, No. 3, pp. 281-680, 5 pis. and Compte 

 Rendu, XII session du Congres geologique international, pp. 593-667. 



