WATER CARBONIC ACID OXYGEN. 69 



heaps ; and could a thousand years be as one day, 

 we should probably see many of them swept away 

 and levelled to the ground before our feet. 



It will now be interesting to inquire by what 

 processes these destructive, wearing-down opera- 

 tions are accomplished. The apparently feeble 

 and contemptible powers are Water, Carbonic 

 Acid, and Oxygen. Water acts in two ways, 

 first simply as a mechanical agent, or as a solvent 

 of various matters; and secondly as a medium 

 by which carbonic acid and oxygen in a dis- 

 solved state are applied to the substances under- 

 going the change. The sacred philosopher long 

 since wrote : " The dropping of water weareth 

 away the stones," and undoubtedly in moun- 

 tainous regions where the force of running 

 water is very great, or in any other place 

 where a large mass of water sweeps along the 

 earth, the mechanically-destructive powers of 

 water are very great. At the Falls of Niagara, 

 for example, geologists are considered to have 

 proved that in the course of time the river 

 has cut its way back through several miles 

 of rock, and is still gradually receding, though 

 with extreme slowness, at the rate, it is said, of 

 a foot a year.* In so doing it is difficult even 



* It will be understood that while admitting this fact, no 

 assent is thereby given to the argument as to the assumed age 

 of the earth, which is conceived to be supported by this phe- 



