THE FOSSILIFEROUS ROCKS. 17 
mains of fresh-water shells or plants or other organisms which 
inhabited the lake at the time these beds were being de- 
posited. 
In the same way large rivers—such as the Ganges or 
Mississippi—deposit all the materials which they bring down 
at their mouths, forming in this way their ‘deltas.’ When- 
ever such a delta is cut through, either by man or by some 
channel of the river altering its course, we find that it is com- 
posed of a succession of horizontal layers or strata of sand or 
mud, varying in mineral composition, in structure, or in grain, 
according to the nature of the materials brought down by the 
river at different periods. Such deltas, also, will contain the 
remains of animals which inhabit the river, with fragments of 
the plants which grew on its banks, or bones of the animals 
which lived in its basin. 
Nor is this action confined, of course, to large rivers only, 
though naturally most conspicuous in the greatest bodies of 
water. On the contrary, all streams, of whatever size, are 
engaged in the work of wearing down the dry land, and of 
transporting the materials thus derived from higher to lower 
levels, never resting in this work till they reach the sea. 
Yili, 
Fig. 5.—Diagram to illustrate the formation of sedimentary deposits at the point 
where a river debouches into the sea. 
Lastly, the sea itself—irrespective of the materials delivered 
into it by rivers—is constantly preparing fresh stratified de- 
B 
