THE FOSSILIFEROUS ROCKS. I/ 



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 con-tain 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. 



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- 



