414 S. V. Wood — Ongin of the Loess. 



biefieux, and the yellow purer limon as all one in their mode of 

 origin, and the process giving rise to them to have been thus, viz. : 

 ^7 ^The subsoil having become permanently frozen to a considerable 

 depth, and all vertical percolation thus arrested, the annual alterna- 

 tion of frost and thaw in the upper layer of this shattered the flints 

 there, and so produced the angular fragments of flint (eclats) and 

 grains of silex, of which, the " bief a silex " is made up. Upon 

 this material, as well as upon the chalk with flints beneath (as, 

 by the sliding of the material, it became exposed to the alterna- 

 tion of thaw and frost), and upon the resulting limon as well, the 

 shattering agency continued to operate ; and by this the flint became 

 eventually reduced to the state of fragments, varying from the 

 " eclats " to the minutest grains, of which the limon is made up. 

 Having only the two or three feet of thawing surface layer to 

 saturate, the water resulting from the thawing of the snow and the 

 rain during summer took up by degrees all the chalk in this layer, 

 and filtering through the mass of silex grains and eclats escaped 

 with it, spring-wise, along the surface of the permanently frozen 

 ground beneath ("ravining" this surface as it did so); so that 

 the calcareous part of the parent soil, the chalk, was removed, 

 and the siliceous part left; while the slide of this surface layer 

 during a long period caused the resulting material to accumulate 

 thickly in places favourable to such accumulation. 



Thus, it seems to me, the lower portion of the limon, which was 

 formed when the chalk itself was the main source, should, as M. de 

 Mercey says it does, contain the larger fragments most abundantly, 

 and have at its base a material, the " bief a silex," formed princi- 

 pally of the coarser grains, and inclosing flints shattered into eclats, 

 but held together by the limon in the form of an entire flint ; while, 

 where the accumulation is thick, the upper portion of it should be^ 

 as he says it is, more free from eclats by reason of the surface from 

 which this slid, and on which the process was now operating, 

 having become to a great extent limon instead of chalk. 



Where, as upon the higher {i.e. the Mwsubmerged) parts of the 

 plateaux, this process went on during the long duration of the 

 major glaciation, and was renewed during the (probably much 

 shorter) duration of the minor, there has thus accumulated, ac- 

 cording to M. de Mercej', limon wholly made up of disintegrated 

 flint, which in places is 30 feet and more in thickness. The valleys 

 having been below the sea during the major glaciation, the limon 

 upon, and at the foot of their slopes is that of the minor glaciation 

 only, which occurred when England, having for the most part 

 recovered from the great submergence of the major glaciation, and 

 again undergone the small depression and limited submergence 

 which accompanied the Cyrena formation, was recovering from this 

 also, and become nearly all land. It is therefore thin on these 

 slopes, and over the Cyrena gravel of the Somme Valley, which 

 occupies those slopes ; though in hollows at their feet, and in the 

 cavity afforded by the sea cliff of this time, which is exposed at 

 Sangatte, it has accumulated more thickly. 



