120 POST-PLIOCENE LAKE TERRACES. [Oh. X, 



loam like that of the Rhine at the Porta Westphalica, near Minden, at 

 the height of 500 feet above the river-plain of the Weser, in which the 

 same three shells were conspicuous. 



If in some places mollusks of purely aquatic species of such genera 

 as Lymnea, Planorbis, and Paludina, occur near the base of the loess, they 

 probably indicate ancient ponds and lakes marking the course of old de- 

 serted river channels, which were afterwards silted up. 



In parts of the valley of the Rhine, between Bingen and Basle, the 

 fluviatile loam or loess now under consideration is several hundred feet 

 thick, and contains here and there throughout that thickness land and 

 amphibious shells. As it is seen in masses fringing both sides of the 

 great plain, and as occasionally remnants of it occur in the centre of the 

 valley, forming hills several hundred feet in height, it seems necessary 

 to suppose, first, a time when it slowly accumulated ; and secondly, a 

 later period, when large portions of it were removed, or when th*e 

 original valley, which had been partially filled up with it, was re- 

 excavated. 



Such changes may have been brought about by a great movement 

 of oscillation, consisting first of a general depression of the land, and 

 then of a gradual re-elevation of the same. The amount of continen- 

 tal depression which first took place in the interior, must be imagined 

 to have exceeded that of the region near the sea, in which case the 

 higher part of the great valley would have its alluvial plain gradually 

 raised by an accumulation of sediment, which would only cease when 

 the subsidence of the land was at an end. If the direction of the 

 movement was then reversed, and, during the re-elevation of the con- 

 tinent, the inland region nearest the mountains should rise more rap- 

 idly than that near the coast, the river would acquire a denuding 

 power sufficient to enable it to sweep away gradually nearly all the 

 loam and gravel with which parts of its basin had been filled up. 

 Terraces and hillocks of mud and sand would then alone remain to 

 attest the various levels at which the river had thrown down and after- 

 wards removed alluvial matter. 



Post-pliocene lake-terraces in Switzerland. — In Switzerland terraces 

 of drift are found at different levels above the present rivers and lakes, 

 which correspond to the older gravels (Nos. 3 and 4, fig. 106), and 

 they contain the remains of the mammoth, reindeer, and other 

 mammalia, many of them extinct or no longer inhabitants of Europe ; 

 together with shells, all of them of species still living. Skirting the 

 Lake of Geneva are the deltas of numerous torrents which bring 

 down mud, sand, and pebbles to the lake, so as to make annual addi- 

 tions to the littoral accumulations. " If," says M. Morlot, " we follow 

 up the course of any of these streams to the height of 150 feet above 

 the lake, we encounter another and more ancient delta, about ten 

 times as large, evidently the monument of a more protracted period, 

 when the water stood for ages at that higher level, and when the physi- 

 cal geography of the country differed considerablv from that now estab- 

 lished." 



