OF THE EARTH S SURFACE. 



179 



has flowed over it, capable of- overwhelming and disregarding 

 objects by which the Nile or the Ganges would have been 

 turned out of their course. 



But the testimonies in favour of this hypothesis are not de- 

 rived from these large features alone ; and it is not concei- 

 vable, that such agents could have been at work, without leav- 

 ing behind them indications of their influence still more un- 

 equivocal. These occur in the very places indicated by the 

 theory, and exhibit remarkable instances of abrasion. In 

 order to investigate them with success, we must have recourse 

 once more to the effects produced by one of our common 



streams. 



Wliere a firm rock of any kind has been exposed to the action 

 of a rapid river, its surface acquires in consequence of that abra- 

 sion a peculiar character, which every one recognizes at a 

 glance, but which it is difficult to describe in words. The 

 most obvious and universal effect of such an action, is the 

 rounding of all the original angles of the rock ; not only the pro- 

 minent, but also the entering angles. For where an original 

 hollow has occurred, coinciding at all with the course of the 

 stream, the water has undergone an acceleration along that 

 hollow, and has excavated for itself a waving groove more or 

 less longitudinal. The whole has thus acquired a pecu- 

 liar character, by an assemblage of flowing lines, which re- 

 calls the water-worn state of the rock. Another set of 

 forms also present themselves in all such cases, which could 

 not easily have been foreseen, and whose existence we 

 learn only from observation of the fact: we observe, that 

 the surface is in many parts excavated by shallow depres- 

 sions of various sizes, which I shall distinguish by the name of 

 scoopings, as resembling the effect which would be produced 

 upon a soft; body by the oblique blow of a spoon or scoop. I 



Z 2 conceive 



