ALLUVIA. 625 



is only a defect which, on other occasions, it has 

 been found impossible to avoid. 



The substances here considered as alluvial 

 rocks, are, either extensive calcareous deposits 

 from existing waters, or loose alluvia which have 

 been consolidated by one or other of the follow- 

 ing causes ; namely, the deposition of carbonat 

 of lime from water, or the agglutinating power 

 of that compound of water and carbonat of iron, 

 called rust, to which no chemical term has yet 

 been applied. It is not improbable that, in some 

 more rare instances, there may be added to these, 

 the recent solution of silica, and an adhesion of 

 certain mixed earths which, although it appears 

 to be merely the result of pressure and repose, is 

 probably owing to the same circumstance in a 

 degree less perceptible. 



The power of carbonat of lime, it must also 

 be added, may be called into action without the 

 actual infiltration of foreign solutions; and thus, 

 fragments of shells, or mixtures of these, or of 

 other calcareous matters, with sand, are some- 

 times consolidated; the very substances them- 



s s 



