HISTORY OF PALEOGEOGRAPHY 435 



&s they go, but they are too synthetic — that is, as a rule they embrace too 

 much time, and hence do not bring out the oscillatory nature of the conti- 

 nental seas. Willis concedes the validity of the criticism "that each indi- 

 vidual map covers so long a period of time and such diverse conditions 

 that they do not truly represent any special geographic phase of the conti- 

 nent" (p. 204). 



These maps were made on the following basis : 



"A certain period having been selected as that which should be mapped, the 

 epicontinental strata pertaining to that time interval have been delineated. 

 The phenomena of sedimentation and erosion have been correlated, with a 

 view to determining the sources of sediment and topographic conditions of land 

 areas, and from these data the probable positions of lands have been more or 

 less definitely inferred. Thus, certain areas within the continental margin are 

 distinguished as land or sea, and these areas may be defined as separate bodies 

 or connected according to inferences based upon isolated occurrences or upon 

 later effects of erosion. 



"It is assumed that the great oceanic basins and such deeps as the Gulf of 

 Mexico and the Caribbean have been permanent features of the earth's surface 

 at least since some time in the pre-Cambrian" (page 203). 



These principles have also governed the present writer in the following 

 investigation. He has likewise been able to unearth a vast amount of 

 paleontologic knowledge buried in American geologic literature, and dur- 

 ing the past thirty years has gained wide experience in the field and the 

 laboratory. The results thus attained have been freely discussed with 

 many men, geologists as well as paleontologists, and the writer believes 

 that by "selecting narrower time limits and more precise correlations than 

 have been attempted" by Willis, he has taken at least "one of the steps in 

 the advancement or" knowledge" (Willis, 1909, page 204). 



Walcott 17 has a "Hypothetical map of the North American continent 

 at the beginning of Lower Cambrian time." In the same year 18 appeared 

 another paleogeographic map portraying "the beginning of Ordovician 

 time." Three very small maps, also representing Lower, Middle, and 

 Upper Cambric times in North America and ascribed to Walcott, may be 

 found in Lapparent's Traite. Walcott 19 has likewise brought out a valua- 

 ble "Hypothetical map to illustrate the areas of the Cordilleran, Missis- 

 sippian and Appalachian seas." Logan 20 has given an excellent map indi- 

 cating the outlines of the late Jurassic continental sea, with North Pacific 

 connections. As this sea is an independent one, it is named by the present 

 writer Logan sea. 



17 Walcott : Bull. U. S. Geological Survey, no. 81, 1891, plate 3. 



18 Walcott : Twelfth Ann. Rep. U. S. Geological Survey, 1891, plate 45. 



19 Walcott : Proc. Amer. Assoc. Adv. Sci., vol. 42, 1894. 



20 Logan : Journal of Geology, Chicago, 1900, p. 245. 



