INTERGLACIAL EPOCHS. 277 



and a wide region in Middle Eussia, were submerged, and ice- 

 bergs laden with the debris of Scandinavian rocks sailed over the 

 drowned countries. The occurrence of marine shells here and 

 there in the boulder -clay of Prussia seemed to afford strong 

 confirmation of this view. But, on the other hand, the 

 discovery of land- and freshwater-shells under similar circum- 

 stances appeared directly to contradict the evidence furnished 

 by the presence of the marine mollusca. And the evidence 

 was still farther complicated by the appearance of marine- and 

 freshwater-shells commingled in one and the same section of 

 boulder-clay. Thus in the boulder-clay near Berlin we find 

 such sea-shells as Mactra solida, along with freshwater forms, 

 as Valvata piscinalis. The freshwater-shells abounded here, 

 and with them were associated remains of the Pleistocene 

 mammalia. In other places solitary specimens of sea-shells (as 

 Cardium edule and Buccinum undatum) have now and again 

 been detected ; and the same is the case with the freshwater 

 forms Valvata piscinalis and Paludina diluviana. 1 It is little 

 wonder that this curious commingling of marine and terrestrial 

 relics in the till should have greatly puzzled geologists. 

 Berendt, thinking more especially of the marine fossils, has 

 speculated about the former existence of a wide -spread 

 " diluvial " sea ; while others, overlooking or not knowing of 

 the occurrence of the marine forms, have supposed that the 

 drift-deposits were accumulated in a great freshwater lake. 

 Jentzsch, again, was of opinion that the phenomena would be 

 better explained if we could suppose that a large lake formerly 

 occurred in close proximity to a sea ; and Kunth improved 

 upon this suggestion by inferring that there might have been 

 many inland lakes which by and by would become filled with 

 sea-water as the submergence of the land increased. Lastly, 

 Eoth put forward the view that the freshwater molluscs may 

 have lived in inlets and shallow bays of the sea which were 



1 References to the various authorities for these statements will be found in 

 Dr. Penck's paper on the erratic formation of North Germany, Zeitschr. deutschen 

 yaol. Ges., 1879, pp. 125, 141. 



