196 ON THE REVOLUTIONS 



their action upon the rock would be most powerful, provided 

 they were driven forward by sufficient force ; the horizontal 

 motion thus urging each stone against the object to be ground, 

 (by an action similar to that which impels forward the chisel of 

 a mason, dressing the surface of a stone, which acts in the 

 direction of the axis of the chisel, forming an angle with the 

 surface upon which it acts), whereas, if the declivity lay the 

 opposite way, that same horizontal force would tend imme- 

 diately to draw away the grinder, and prevent its action. We 

 observe, accordingly, that though furrows or dressings do occur 

 at a little distance to the eastward of the base, upon elevations 

 presenting a western aspect, or lying flat, none such are any 

 where to be found on a surface of rock inclining to the east. 



It is further remarkable, how little the direction of this ac- 

 tion has been disturbed by local circumstances. Thus, the 

 furrows in No. 6. follow the medium course right up the hill;, 

 though close to the ravine on the south, and though, upon the 

 other hand, at the distance of about two or three hundred 

 vards from the abrupt termination of the hill itself. This 

 seems to prove, not only that the stream flowed over Corstor- 

 phine Hill, but that it flowed over it with a great depth ; for 

 had the stream done no more than just cover the summit, its 

 course must have locally obeyed the inclination of these sur- 

 faces. To pass along and disregard them entirely, seems to 

 denote a great superiority of depth ; and in calling it double, 

 we probably do not exceed what is necessary. We have seen 

 enough, however, to justify my assertion, that the stream must 

 have been incomparably superior in magnitude to the Nile or 

 the Ganges, since even the Corstorphine Hill, occurring in the 

 course of these mighty streams, must have completely deran- 

 ged their course. 



This 



