40 ON CHANGES IN THE DISPOSITION 



further examples or varieties, it remains to enquire into 

 the causes which have been assigned for it. The or- 

 dinary action of running water on the surface is the 

 first and most obvious of these. So great, it is said, 

 have been the changes of the surface since rivers began 

 to flow, that we can scarcely say where they might not 

 have flowed, or what they might not have destroyed. 

 Yet I know not by what possible system of waters, in 

 any state of our island which can be imagined, the 

 sandstone mountains of Rossshire could have been cut 

 out of a solid stratum. It is equally impossible, on 

 this principle, to account for the peculiar denudation 

 of the east side of Scotland, where the present rivers, 

 under any imaginable former bulk, or under any 

 changes of place, seem insufficient for this purpose. It 

 is not within my plan to, examine visionary causes. 

 Appulses of comets, or whatever else, must be sought 

 where they are to be found, by those who seek what I 

 do not : the present pursuit is Knowledge. I do not 

 however see how some of the appearances in question 

 can be explained by aught but currents more extensive 

 and powerful than any rivers could have produced : 

 while, as to other cases, it must be left to the observer 

 himself to apply this solution to those in which he may 

 think it adequate, as it probably is in that of Pittsburgh, 

 just quoted. But, of such currents, I can say no 

 more than I have done already. I can discover no 

 reasonable cause for them but the elevations of strata ; 

 while those accumulations of alluvia which are inex- 

 plicable under any conceivable system of rivers, may 

 often indicate the direction of such a current. Yet 

 even this gives us little assistance^ unless we can ex- 

 plain how a current of this nature, even aided by stones 

 in motion, could effect, on whole mountains, in a very 

 limited time, what a mountain torrent, surely not lcb.s 



