OSBORX. REVIEW OF THE PLEISTOCENE 237 



A summary of the correlation presented in this chapter is embodied in 

 the Table of Osborn and Eeeds above. A summary of the very diverse 

 opinions on this subject is embodied in the Table of Wiegers below. 



A very strong reason for abbreviating our estimates of the period 

 which has elapsed since the appearance of man of the pre-Chellean cul- 

 ture stage in Europe is found in the relatively unchanged condition of 

 the river valleys of the Somme in northern France and of the Vezere in 

 Dordogne, in which the earliest human cultures occur. The Vezere has 

 not materially changed since Acheulean times. The pre-Chellean, Chel- 

 lean and Acheulean specimens found in the Somm'e valley are also con- 

 nected with the present river system. Both on the Somme and the 

 Marne the Chellean and pre-Chellean cultures occur on the "lowest ter- 

 races." Again, there is no faunal break between late Chellean and early 

 Acheulean times nor between late Acheulean and early Mousterian times. 

 The first great faunal break is that produced by the Fourth glaciation. 



In favor of Penck's contention as to the earlier geologic age of the 

 Chellean is the occurrence of pre-Chellean and Chellean palseoliths in 

 association with a very primitive mammalian fauna such as is character- 

 istic of Second Interglacial times. 



ELEVATION AND SUBSIDENCE OF LAND IN QUATERNARY TIMES 



The relations of the mammals of Europe with those of Asia on the east 

 and Africa on the south were profoundly affected in Pleistocene times by 

 the periods of elevation of the continental shelf, resulting in the creation 

 of new land connections which facilitated migration, or of subsidence 

 which cut off and isolated many migrating forms from their centres of 

 origin and dispersal. The maximum elevation, as represented in the 

 accompanying diagram (Fig. 1), never occurred in all portions of the 

 continent of Europe at the same time, because there were oscillations 

 both on the northern and southern borders of Europe and Asia, 



The beginning of the Pleistocene Epoch is one of elevation and is re- 

 markable for the broad land connections between Europe, Africa and 

 Asia. It represents the last stage in that vast community of mam- 

 malian life which during Pliocene times distinguished the entire region 

 of Europe, Asia and Africa. 



The theoretical relation which elevation and subsidence respectively 

 bear to the glacial and interglacial stages and phenomena is, broadly 

 speaking, as follows : 



ELEVATION, emergence of the land from the sea, broad land connections 

 facilitating migration, retreat of the ice caps, periods of erosion of the 

 river valleys and formation of terraces. 



