ON THE ALLUVIAL DEPOSITS. 363 



ferred to them, especially by the supporters of a well- 

 known hypothesis. 



The judgment must be determined by their posi- 

 tions, their extent, and the nature of the materials ; 

 and, above all, negatively, by the absence of all other 

 possible causes. If the original directions of any cur- 

 rents which might have deposited them could be as- 

 certained, there would be little comparative difficulty; 

 but as that is hopeless, from the necessary intricacy of 

 such currents, this must be attempted in another 

 manner. Every torrent, be its extent what it may, 

 must have deposited its solid contents under one of 

 the following circumstances. Wherever it met with 

 an obstruction, they must have subsided on the face of 

 it; yet not extensively, unless it was wide and great. 

 Where it formed an eddy or a bifurcation, or under any 

 other circumstances of retardation, producing a lee 

 place, there also we should expect alluvial deposits, 

 extensive in proportion to the extent of the slack 

 water. And thus, any other kind of retardation, aris- 

 ing from its approaching cessation, the attainment of 

 an equilibrium, or the forms of the land, must have 

 caused the water to deposit its contents. Hence, though 

 we cannot, from the current, prove the deposit, we 

 may infer the direction of the former from the places 

 of the alluvia; a judgment aided by comparing the 

 materials with the distant rocks whence it is supposed 

 to have flowed. Thus, if the granite blocks on the 

 Righi and the Rossberg belong to the Alps, while 

 the fragments of those mountains must, on the con- 

 trary, be sought in the Canton of Zurich, we infer the 

 former existence of a current from the south-east. 



In all these attempts, if difficulties arise from the 

 intermixture of modern alluvia, and from subsequent 

 disturbances^ others are found in the changes winch 



