£36 ACCOUNT OF THE 



spread over the surface of our hills and valleys in an over- 

 lying position. This appears to be the same that in the 

 north of Europe is denominated geest.,* and which Mr. De 

 Luc considers as the last deposit made by the sea before its 

 final retreat. Sir Humphry Davy regards the soil or fria- 

 ble argillaceous suiface of Great Britain at least, as having 

 resulted from the disintegration of the rocky strata under- 

 neath. f There are doubtless but few rocks that would not 

 be mouldered by exposure to the action of the atmosphere, 

 and where the waters, before their final retreat, made 

 no protecting layer, the argillaceous surface may be fairly 

 referred to decomposition. But there is much reason for 

 believing that the greater part of the geest of this country 

 has subsided from fresh water, and is a true alluvion. It varies 

 considerably, 1 acknowledge, in different places, but its essen- 

 tial consituents, clay, sand, and yellow oxide of iron, combined 

 in varying proportions, seem still to be present. It is indeed 

 destitute of tiie remains of aquatic animals, and might not 

 therefore be supposed to have had the origin here intimated ; 

 but the old alluvial beds which have manifestly been accu- 

 mulated by water, either salt or fresh, are almost equally de- 

 stitute of aquatic exuvise. On the other hand, it exhibits, in 

 some places, an obscure stratification, it does not abound 

 particularly in the undecomposed fragn»ents of the rocks 

 over which it is spread, its colour and density are nearly the 

 same from bottom to top, and it envelopes, as we shall pre- 

 sently see, along with more or less gravel, large masses of 

 adventitious rocks, conditions that sufficiently characterise it 

 as a distinct and ve2i\ formation. It has been washed by the 

 rains into the lower paits of the valleys, where beconiing 

 blended with the recent alluvion of the streams, it encloses 

 the bones of the arctic elephant, great mastodon, and other 

 extinct quadrupeds. On the south side of the Ohio, it is 

 only from four to eight or twelve feet deep ; but to the 



» L' Histoire de la Terre et de 1' Homme, tome v. 

 t See his Elements of Agricultural Chemistry. 



