488 GEOLOGICAL REVOLUTIONS. CHAP. 



operation. Taking the upper Thames for our example, and allow- 

 ing, according to several analyses., twenty grains of salts dis- 

 solved in a gallon of the water, we find this to represent about 

 ^-sVo-tib. of the weight; and allowing the specific gravity of the 

 salts to be twice that of water, it will represent yQ^th of the 

 volume of water. The depth of river- water (about one-third of the 

 whole quantity of rain) being 9 inches d , we have for the annual 

 waste of the upper Thames area y^^ths of an inch ; so that 

 in less than 800 years our whole area will be lowered by surface - 

 waste one inch. Not wasted equally, nor wholly at the surface, 

 but on the average, and really wasted to that extent. And thus 

 hills and valleys are in course of continual modification, and thus 

 their peculiar features may have been produced e . 



We may next trace the operation of the sea on the same tract 

 of land A, B, C, D, on the supposition, which no one will now 

 dispute, of the gradual and successive uncovering of the rising 

 land. In this case the parts successively exposed are, each in 

 its turn, exposed to the action of the sea, rising and sinking, 

 rushing in currents and falling in breakers, and moistening with 

 foam. The lines of weakness are thus thoroughly explored by 

 the searching waves, and experience more than other parts the 

 wasting power of moving water, and the grinding effect of sands 

 and gravels which it drifts along its course. 



First at i we may conceive, then at 2, 3, and so on in succession, 

 all parts of the surface, beginning at what is now highest, must 

 yield more or less to the strong attack of the sea ; and yield, prob- 

 ably, so much the more as the attack followed instantly on each 

 change of position, when the disturbing effect of the upward 

 movement must be supposed the greatest. The result would be 

 valleys of the same general character as that already ascribed to 

 valleys of erosion by rain. In each case erosion by water. 



There would be a difference of some importance in the case of 

 calcareous rocks, for though the sea would act both chemically 

 and mechanically on limestone, this would not, probably, occasion 

 long underground channels and caverns ; though if such existed 



d See p. 46 for the evidence of this. 



I find by a communication of Mr. Clutterbuck, that he allowed 100 grains 

 of carbonate of lime in one cubic foot of water flowing from the chalk in the river 

 Gade, near Watford. 



