PALEOGEOGRAPHIC MAPS OF NORTH AMERICA 41 



many; but there is danger in carrying the refinements too far on the 

 basis of paleontologic correlation alone, since it is still difficult to 

 distinguish between synchronous and homotaxial faunas or floras. 

 It may be hoped that these paleogeographic studies will themselves 

 assist us to a better understanding of the evolution of life conditions 

 and thus lead to a solution of some of the problems of correlation with 

 the aid of biologic evidence. 



I. LOWER CAMBRIAN NORTH AMERICA 



The map of lower Cambrian North America presented herewith 

 conforms to the outline developed by Mr. Walcott in the course of 

 his studies. East and west of the central land mass are relatively 

 narrow sounds limited on the oceanic side by islands or land masses 

 of indeterminate extent. The old land area of Appalachia is believed 

 to have covered the region of the West Indian Islands, it being well 

 established that a somewhat extensive land extended to the southeast 

 of the Appalachian trough, and it being plausible that that land 

 lay between the Atlantic deep on the northeast and the deeps of the 

 Caribbean and Gulf of Mexico. In the adaptation of marine cur- 

 rents to oceanic and continental features, it is inferred that the 

 return waters from the Arctic occupied the sounds along the inner 

 continental margins. The distribution of these currents suggests 

 that the habitat of the lower Cambrian fauna of the Appalachian 

 trough on the east and the British Columbia-Nevada trough on the 

 west was determined by the cool waters flowing southward. This 

 view of dispersion of the faunas from the north is not shared by 

 Mr. Walcott, who presents the alternative hypothesis of a connection 

 of the faunas around the southern margin of the continent. The 

 fauna of the Nevada basin appears to belong to warmer waters than 

 that of British Columbia, inasmuch as it contains corals. The land 

 areas of lower Cambrian time throughout the northern hemisphere 

 appear to have been large. There is evidence in the character of the 

 sediments and in glacial deposits in China that there were marked 

 contrasts of climate. 



2. LATE MIDDLE AND UPPER CAMBRIAN NORTH AMERICA 



The map of late middle and upper Cambrian North America 

 represents an expansion of the area of the epicontinental sea which 



