INTERGLACIAL EPOCHS. 281 



cular masses belonging to the boulder-clay itself, due perhaps 

 to the action of subglacial waters. Be this as it may, we have 

 in the evidence given above clear proof, as Dr. Penck has ad- 

 mirably shown, of the existence of at least three boulder-clays, 

 separated the one from the other by intercalated deposits of 

 freshwater origin. 



But if we have evidence of the existence of a land-surface 

 in North Germany during interglacial times, we have no less 

 certain proof that the same land- surface has also been submerged. 

 Between the upper and lower clays of the province of Prussia 

 come beds of sand and gravel, which, according to Berendt, have 

 yielded a number of shells of marine molluscs, such as Cardium 

 edule, Nassa reticulata, Gyprina islandica,Mactra solida; and from 

 the same beds Jentzsch has recorded Yoldia (Leda) arctica, and 

 the fresh- or brackish-water form Paludina diluviana. This 

 commingling of discordant species, and the fact that the speci- 

 mens of Yoldia are all much rolled and worn, have led some to 

 doubt whether they really occupy their original bedding-place. 

 But Jentzsch points out that they are widely distributed through 

 the beds in which they occur, and thinks there can be no doubt 

 that they are in place and have not been derived from any pre- 

 existing strata. In these beds we have, according to Penck. a 

 marine littoral accumulation ; they mark the shores of an inter- 

 glacial sea upon which the shells were cast up and rolled about 

 by the waves, and the few freshwater forms that make their 

 appearance have been washed down, he thinks, by streams and 

 freshets from the land. The lower blue boulder-clay of the 

 same province, which is very abundantly charged with well- 

 scratched boulders, has yielded at Elbing sporadic shells and 

 fragments of Dreissena sp. and Yoldia arctica, and it is under- 

 laid in the neighbourhood of the Frisches Haff by a bed of fine 

 stoneless clay, which attains a thickness of nearly 200 feet. 

 Yoldia arctica occurs throughout this clay sparsely, in strings 

 or thin lines. The specimens are thick-shelled like those in the 

 Norwegian glacial clays, and they are well preserved, some of 

 them having still their epidermis. Besides these, Cardium edule 



