QUATERNARY DEPOSITS OF BRITISH ISLES BROOKS. 283 



rences of land and fresh-water Mollnsca in glacial and interglacial 

 horizons in Germany. He finds that the various molluscan faunas 

 of older Quaternary age can be divided into cold-loving and warmth- 

 loving groups, whose distribution shows that even in the unglaciated 

 region of southern Germany an arctic climate prevailed during the 

 glacial periods. In north Germany the glacial faunas are found 

 chiefly in gravels, in south Germany in loess and sand loess. True 

 interglacial Mollusca occur on two horizons of different age, sepa- 

 rated by glacial deposits. The older one is characterized by P. dihi- 

 viana, Bithynia tentaculata, and Dreissens'm polymorpha, the 

 younger by P. Duboislana, Belgrandia, and Planorbh albus. 



In West Prussia the lower interglacial age of the marine Cardium 

 and Cyprina clays and Eem beds is confirmed by the occurrence in 

 them of Dreissensia poly??iorpha and P. diluviana, In Posen P. dilii>- 

 viana has been found in interglacial deposits associated with Corbi- 

 cula fluminalis, and these two have also been found associated near 

 Odessa and as derived fossils in the middle bowlder clay of East 

 Prussia. 



A. Penck (8) was the first geologist to bring forward evidence in 

 support of a threefold glaciation of the district. Since 1880 a num- 

 ber of other more or less successful attempts at classification have 

 been made, the net result ©f which is very much in favor of Gagel's 

 classification into three glacial and two interglacial horizons (9). 



GLACIAL SUCCESSION IN THE RIVEB VALEKYS. 



Detailed consideration of the glacial succession in the valleys of 

 the north flowing rivers south of Berlin is not necessary, as the 

 conditions are essentially similar to those of the Alps. 



The deposits of the Weser Valley, studied by O. Grupe (10) and 

 L. Siegert (11), may, however, be considered here, as they differ con- 

 siderably, and the differences illustrate the scheme of classification. 

 According to Grupe, the valley of the Weser originated in middle 

 Pliocene, and was cut to a depth of at least 25 meters below the 

 present level of the river. The process of erosion left " Old Pliocene " 

 gravels at a height of 120 to 1G0 meters above valley level. In late 

 Pliocene times the valley was partially filled by clays and sands con- 

 taining Mastodon arvernensis and M. Borsonl. Belonging to the 

 Quaternary there are three gravel terraces — upper, middle, and 

 lower. The upper terrace interdigitates to the north with deposits 

 of the first glacial period on the Porta River, and is accordingly con- 

 temporaneous with this period. This was the maximum glaciation 

 in the district and, correspondingly, the upper terrace reaches the 

 great thickness of 60 to 70 meters at Hameln. The lower part of the 

 middle terrace includes at Nachtigall, peat with Corylus avellana, 



