366 ON THE ALLUVIAL DEPOSITS. 



mined in a former chapter, under the question of 

 transportation ; and I shall only further remark, that 

 when the presence of shells, when deposits of shells 

 lying in their natural positions under subsidence, and 

 when even such deposits, of the most tender shells, 

 uninjured, have been pointed out as instances or 

 proofs of a "diluvian" deposit, it is in vain to make 

 an answer to any one who can, at the same time be- 

 lieve that these deluges, or this Deluge, demolished 

 mountains, transported granite blocks, and rounded 

 the materials of such alluvia. 



Alluvia of Disintegration. 



Of the modern alluvia, the simplest are those which 

 arise from the disintegration of rocks; lying in the 

 places where they were produced. The general fact 

 and its causes were formerly described ; and they may 

 commonly be recognized by the following appearances. 



In examining the rocks below them, we find those, 

 at first, merely fissured, and then divided into fragments, 

 which, gradually diminishing in size, are at length 

 intermixed with clay and sand, or, finally, converted 

 into those snbstances. In some instances, the disinte- 

 gration is more perfect ; but the fragments are angu- 

 lar, or, if otherwise, bear marks, not of attrition, but 

 of decomposition. Gneiss, argillaceous schist, lime- 

 stones, and sandstones, present examples ; and the 

 consequences are easily traced in the resulting soils. 

 It is one of the great processes in nature for pro- 

 ducing these. In the unstratified rocks, and some- 

 times also in gneiss, decomposition is more common 

 than disintegration ; and hence the produce is more 

 frequently clay, or clay and gravel, than in the strati- 

 fied. But, how deep and extensive the former may 

 be, can be conjectured^ when I have traced them, in 



