PLEISTOCENE LOAMY DEPOSITS. 153 



becomes more or less argillaceous, and even passes into a regular 

 brick-earth. Or it may consist of a succession of alternate 

 layers of brick-earth and calcareous loam or loss properly so 

 called. In the valleys of the Seine, the Somme, and other 

 streams in the north, it overlies those ossiferous and implement- 

 bearing gravels, which are known to Trench geologists as dilu- 

 vium gris and diluvium rouge. The gray calcareous diluvium or 

 gravel, as we may call it, from its prevailing character, differs 

 from the overlying red non-calcareous diluvium chiefly in colour. 

 In point of fact the red diluvium is often only the discoloured 

 upper portion of the gray gravel. It is also certain that the so- 

 called " red diluvium " which is found resting directly upon the 

 chalk over wide areas in Northern France is not of fiuviatile 

 origin at all, but simply the insoluble residue of red earth and 

 flint which has resulted from the long -continued action of 

 acidulated rain-water upon the chalk. This "red diluvium" 

 may be followed through extensive districts in every country 

 where Cretaceous strata are well developed. But the reddish- 

 coloured gravel and earth that overlie the gray diluvium of the 

 valleys and valley-slopes are unquestionably fiuviatile — their 

 colour and present condition being simply the results of 

 chemical changes, which have influenced the calcareous gray 

 diluvium in the same way as they have acted upon the Creta- 

 ceous strata. Sometimes, indeed, we may observe a similar dis- 

 coloration in the upper part of the loss, which in these cases 

 appears to be overlaid by a later deposit of red earth. This 

 appearance, however, is deceptive, and like the others is due to 

 the chemical action of acidulated water soaking into the loss 

 from the surface. The line between the red earth and the yellow 

 loss is generally very uneven, but occasionally it may approach 

 horizontality, when the acid-water has been stopped in its descent 

 by some lamina or layer of impermeable argillaceous matter. 1 

 Loss or loam may be said to cloak all the plains or plateaux of 



1 On the origin of the red-coloured gravel or diluvium and loss of Northern 

 France, see papers by MM. Meugy {Bull. Soc. Geol. France, 3 e Ser. t. v. p. 226) 

 and Vanden Broeck, Op. cit., pp. 298, 326. 



