136 AGENTS THAT DESTROY ROCKS. 



greater quantity of soil than it would do if the rain 

 had been distributed over the plains and mountains 

 equally, in proportion to their relative areas. The 

 rocks, also, in the higher regions are more exposed 

 to atmospheric influences, such as frost, rain, and 

 vapour, and to great alternations* of heat and cold. 



Besides, when earthy matter has once been inter- 

 mixed with running water, anew mechanical power 

 is obtained by the attrition of sand and pebbles, 

 borne along with violence by a storm. Running wa- 

 ter, charged with foreign ingredients, being thrown 

 against a rock, excavates it by mechanical force, sap- 

 ping and undermining till the superincumbent por- 

 tion is at length precipitated into the stream. 



In estimating the transporting power of water, it 

 is necessary to bear in mind that the specific grav- 

 ity of many rocks is not more than twice that of 

 ■water, and rarely more than thrice, so that almost 

 all the fragments propelled by a stream have lost 

 from a third to one half of their weight. This may 

 be easily shown by lifting a stone in air, and then in 

 water. It is also well to know that the velocity at 

 the bottom of a running stream of water is every- 

 where less than in any part above it, and greatest 

 of all at the surface. Accordingly, the superficial 

 particles in the middle of a stream move swifter 

 than those at the sides. This retardation of the 

 lowest and lateral currents is produced by friction ; 

 when the velocity is sufficiently great, the soil com- 

 posing the sides and bottoms gives way. A veloci- 

 ty of three inches per second at the bottom is as- 

 certained to be sufficient to bear up fine clay ; six 

 inches per second, fine sand ; twelve inchesf per 

 second, fine gravel ; and three feet per second, stones 

 of the size of an egg. 



A few examples will show the immense power 



* LyelPs Geology. 



t Encyc. Brit., Art. Rivers. 



