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 which 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 
ol 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 justly 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 building 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. 
