PLEISTO CENE L OAM Y DEPOSITS. 1 5 9 



limits." It is found at all levels up to heights of 300 and 400 

 feet above the valleys. In some places it overlies ancient river- 

 gravels, while along its northern limits it appears to rest upon, 

 and now and then to be covered by certain accumulations which 

 are known as "Northern Drift," and of which I shall speak 

 farther on. Murchison and his associates state that the materials 

 of this " drift," consisting of stones derived from the north, are 

 reduced to small size, and mixed with the debris of local rocks 

 as they approach the northern margin of the black-earth, by 

 which deposit they are succeeded if not overlapped. At one 

 place, however, they observed " erratics " or travelled stones of 

 northern derivation superimposed on the black-earth. 



Many theories have been advanced in explanation of the 

 phenomena presented by the various accumulations — the loams, 

 loamy clays, loss, and black-earth — which we have now passed in 

 brief review. The loss of Central Europe especially has given 

 rise to many speculations, and will probably continue to exercise 

 the ingenuity of geologists for years to come. At one time it 

 was supposed to be of marine origin, a view advanced by Bennig- 

 sen Forder, but which has long been abandoned, and the lacustrine 

 hypothesis in its various forms has shared no better fate. The 

 earliest exponent of the latter was Hibbert, 1 who believed that 

 the Ehenish loss had accumulated in a wide freshwater basin that 

 formerly occupied the broad and open part of the Ehine valley 

 above Bingen, prior to the time when the present outlet had 

 been sufficiently deepened to permit any overflow in a northerly 

 direction. The hypothetical lake was supposed indeed to have 

 then drained to the south. After its bottom had received a 

 great accumulation of fine mud, the Alps were, according to 

 Hibbert, suddenly upheaved, and the drainage of the lake was 

 thereby instantaneously reversed. The whole of its contents 

 were now discharged in one enormous diluvial rush, and swept 

 through the straits at Bingen, which were deepened as the 



1 History of the Extinct Volcanoes of the Basin of Neuwied on the Lower Rhine, 

 chap. xxv. For an account of the reversal of the drainage in that region, see 

 an interesting paper on the origin of the valley of the Rhine by Prof. Ramsay 

 Quart. Joum. Geol. Soc, vol. xxx. p. 81. 



