AGENTS THAT FORM ROCKS. 155 



water is instrumental in forming, must be derived 

 from the destruction of rocks. 



Rivers are the chief agents in the transportation 

 of these materials ; and there are none vv^hich do 

 not contain more or less earthy and sandy particles, 

 originally derived from the rocks and banks skirting 

 these streams or their tributary branches. As these 

 materials possess a specific gravity greater than that 

 oi the water in which they are transported, they 

 must consequently be deposited either in the bed 

 of the rivers, or in the lakes or seas into which their 

 waters flow. 



Mr. Lyell very iustly observes, that " the transport 

 of sediment and pebbles to form a new deposite ne- 

 cessarily implies that there has been somewhere 

 else a grinding down of rock into rounded frag- 

 ments, sand, or mud, equal in quantity to the new 

 strata. The gain at one point has merely been suf- 

 ficient to balance the loss at some other. When 

 we see a stone building, we know that somewhere, 

 far or near, a quarry has been opened. The cour- 

 ses of stone in the bdilding may be compared to 

 successive strata ; the quarry to a ravine or valley 

 which has suffered denudation. As the strata, like 

 the courses of hewn stone, have been laid one upon 

 another gradually, so the excavation, both of the 

 valley and quarry, have been gradual. To pursue 

 the comparison still farther, the superficial heaps of 

 mud, sand, and gravel, usually called alluvium, may 

 be likened to the rubbish of a quarry, which has been 

 rejected as useless by the workmen, or has fallen 

 upon the road between the quarry and the building, 

 so as to be scattered at random over the ground." 



Quantity of Sediment in River -water. — We can 

 easily calculate* how much solid matter is con- 



♦ The first step in such calculations is to ascertain the aver- 

 age volume of water passing annually down the channel of a 

 river. It is easy to ascertain this at any one particular time, by 

 getting the mean depth, breadth, and velocity of the stream. 



