206 BAILEY WILLIS 
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. 
IT. 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 
probably was not at any time actually reached. The middle Cam- 
