364 ON THE ALLUVIAL DEPOSITS. 



the forms of the land itself have undergone. Thus, if 

 the preceding case be of this nature, the blocks in 

 question must have traversed', not only many small 

 vallies, but that of the Rhone, now 3,000 feet below 

 their places. To produce further illustrations would 

 be to repeat facts familiar to geologists : and after all 

 the attempts to explain particular cases, this continues 

 to be one of our difficulties ; whatever causes we may 

 assign for the changes in the forms of the land. 



The preceding suggestion respecting submarine al- 

 luvia offers a more probable solution, at least, than the 

 floating of ice, the excavation of such vallies as that of 

 the Rhone by its own river, or other conjectural causes ; 

 among which, however, the decomposition of conglo- 

 merates, in situ, will be considered hereafter. 



On these general principles also, which regulate the 

 deposition of solids from water in motion, we may 

 sometimes infer an antient deposit, merely from the 

 places which it occupies ; adding, of course, the nega- 

 tive proof from the absence of other causes. I do not 

 perceive that this reasoning has been applied as it 

 ought, to the obscure alluvia of our own country, or 

 of Europe at large ; while such an origin has often 

 been ignorantly assigned to cases proceeding from the 

 effects of decomposition, and of the descent of mate- 

 rials along declivities; not unfrequently also to the ac- 

 cumulation in the seats of antient lakes. All these 

 arc cases which require attention, but for which no 

 rules can be given, beyond those which may be de- 

 rived from the present and former remarks. 



As the rounded form of the materials is rather ini- 

 mical than favourable to the opinion that this has 

 been produced by temporary currents, since we know 

 that this process requires a long time, it can not be 

 held to prove an antient alluvium from such a cause, 



