CHAP. XVI. GEOGRAPHICAL DISTRIBUTION OF THE LOESS. 329 



cut back at some former period by the denuding action of 

 the sea. 



Even if the imbedded fossil shells of the loess had 

 been lacustrine, instead of being, as we have seen, terrestrial 

 and amphibious, the vast height and width of the required 

 barrier would have been fatal to the theory of a lake : for the 

 loess is met with in great force at an elevation of no less than 

 1,600 feet above the sea, covering the Kaiserstuhl, a volcanic 

 mountain which stands in the jniddle of the great valley of the 

 Rhine, near Freiburg in Brisgau. The extent to which the 

 valley has there been the receptacle of fine mud afterwards 

 removed is most remarkable. 



The loess of Belgium was called ' Hesbayan mud ' in the 

 geological map of the late M. Dumont, who, I am told, 

 recognised it as being in great part composed of Alpine mud. 

 M. d'Archiac, when speaking of the loess, observes that it en 

 velopes Hainault, Brabant, and Limburg like a mantle, 

 everywhere uniform and homogeneous in character, filling up 

 the lower depressions of the Ardennes, and passing thence 

 into the north of France, though not crossing into England. 

 In France, he adds, it is found on high plateaus, 600 feet 

 above some of the rivers, such as the Marne ; but as we go 

 southwards and eastwards of the basin of the Seine, it dimi 

 nishes in quantity, and finally thins out in those directions.* 

 It may even be a question whether the f limon des plateaux,' 

 or upland loam of the Somme valley, before alluded to, f may 

 not be a part of the same formation. As to the higher and 

 lower level gravels of that valley, which, like that of the Seine, 

 contain no foreign rocks,! we have seen that they are each of 

 them covered by deposits of loess or inundation-mud belong 

 ing respectively to the periods of the gravels, whereas the 

 upland loam is of much older date, more widely spread, and 



* D'Archiac, Histoire des Progres, vol. ii. pp. 169, 170, 

 f No. 4, fig. 7, p. 103. | See above, p. 133. . 



