PLEISTOCENE LOAMY DEPOSITS. 163 



the advance of a great polar glacier or ice-sheet upon Northern 

 Europe and Northern Asia blocked up the drainage of the rivers 

 flowing to the north, and converted the low grounds of Northern 

 France, Southern England, the Netherlands, Northern Germany, 

 vast areas in Eussia, and all Northern Siberia, into wide inland 

 seas of fresh water, in which extensive deposits of silt took place 

 — an opinion which does not appear to have met with any sup- 

 port. It is in fact contradicted by the evidence of the loss itself 

 — the distribution and character of which refuse to be so 

 explained. Nevertheless, it is highly probable that the Euro- 

 pean rivers flowing north actually were impeded by the presence 

 in those regions of a great ice-sheet, as I shall point out in suc- 

 ceeding pages. But whether that obstruction gave rise to the 

 loss of Central Europe is another question. Be that, however, 

 as it may, it is certain that vast deposits of loss have been formed 

 in regions where no such damming of the rivers can be supposed 

 to have taken place. The great loss -deposits of the Missouri 

 and Mississippi, for example, certainly cannot owe their origin 

 to the ponding back of those rivers by glaciers. Neither can 

 we account for the presence of the Eussian " Tchernozem" by 

 any such hypothesis. Murchison and his colleagues maintained 

 that the black-earth was accumulated in the sea by diluvial 

 currents sweeping from the north — a view which does not 

 receive support from the occurrence of any marine organic 

 remains. In whatever manner it may have been formed — ■ 

 whether in the sea or in fresh water — it is clear that neither cur- 

 rents nor rivers could have been dammed back as Belt supposed 

 was the case with the rivers of Northern Europe and Siberia. 



M. de Mercey, after having for some time upheld the 

 theory of the " diluvial " origin of the Union of the plateaux 

 of the north of France, 1 has of late given up that view and advo- 

 cated a very different one. 2 He is now of opinion that the 

 lower portion of the Union (Union bitfeux or Union grossier) is of 

 glacial origin, and that it indicates the former existence in the 



1 Bull. Soc. Giol. France, 2 e Ser. t. xxii. pp, 75, 76, 84, 102. 



2 Bull. Soc. Linn, du Nord dc la France, t. ii.-p. 334. 



