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 foree at an elevation of no less than 1600 

 feet above the sea, covering the Kaiserstuhl, a volcanic moun- 

 tain which stands in the middle of the great valley of the 

 Ehine, near Freiburg in Bi'isgau. 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, recog- 

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

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

 envelops 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 " 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,J we have seen that they are 

 each of them covered by dei^osits of loess or inundation-mud 

 belonging respectivel}^ to the periods of the gravels, whereas 

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



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

 t No. 4, fig. 7, p. 107. X See above, p. 136. 



