THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 21 



resulted from a mixture of the two processes. In the first case the group of 

 animals was isolated for a long time and will bear evidence of its history in the 

 presence of archaic forms and peculiar specializations such as always arise in 

 isolated communities. Cases are rare where it can be shown that epiconti- 

 nental seas existed so long undisturbed that actual evolutionary changes are 

 apparent. Such a case perhaps is the Gaspe region in northeastern Quebec. 

 Ulrich asserts that evolution has always taken place (in the Paleozoic inverte- 

 brates at least) in the ocean basins and not in epicontinental seas ; that the 

 faunal changes noted and attributed to evolutionary processes are due to a 

 misapprehension of the composite nature of the beds, and that the changes 

 are due to a retreat of the fauna to the ocean basins and its return after 

 undergoing an evolution there. ^ If epicontinental beds can be actually 

 isolated which show the long-continued and uninterrupted presence of the 

 sea, far-reaching conclusions as to the conditions of adjacent land and sea 

 are usually forthcoming. 



In the second case the evidence is more readily detected. The sudden 

 appearance of a new fauna or of new types in an old fauna is almost invariably 

 due to migration. The source of and the route of the migration become 

 at once of interest and may be revealed in many ways. 



In marine beds the appearance of new forms may perhaps be correlated 

 with the advance of the strand as in overlaps, etc., or it may be due to 

 rapid submergences of a land area following the breaking-down of barriers 

 by the action, slow or rapid, of physiographic forces. It is not improbable 

 that the sudden diversion of the Colorado River left many fresh-water forms 

 in the layers of mud it deposited in the Salton Sink. Noble has indicated 

 a somewhat similar action, but of marine waters, in Paleozoic time, revealed 

 in the section of the Grand Canyon of the Colorado.^ 



The partial submergence of the continent of North America in Cambrian 

 time is a similar case in point.^ 



The third case, where resident and migrant faunae are mingled, is by far 

 the most common. Classical examples of this condition occur in the various 

 troughs of the Appalachian Basin, where new faunae repeatedly penetrated 

 through channels, the Quebec, Levis, etc., from the North Atlantic, and 

 across the weak spot in the barrier of Appalachia near Chesapeake Bay, or 

 around the southern end of Appalachia from the mid-Atlantic. 



^ Ulrich, E. 0., Revision of the Paleozoic Systems. Bull. Geol. Soc. Amer., vol. 22, pp. 



495-505. 1911- 

 " Noble, L. F., The Shinamo Quadrangle, Grand Canyon District, Arizona. The Hotatau 



Conglomerate. Bull. 549, U. S. Geological Survey, 1914. 

 ' Walcott, C. D., Abrupt Appearance of the Cambrian Fauna on the North American 



Continent. Smiths. Misc. Coll., vol. 57, No. i, pp. 1-16, 1910. 

 ^ Weller, Stuart, The Paleozoic Faunas of New Jersey, New Jersey Geological Survey, 



vol. 3, 1903. 

 Ulrich, E. O., and Chas. Schuchert, Paleozoic Seas and Barriers, Bull. 52, N. Y. State 



Museum. 



