THE FOSSILIFEROUS ROCKS. 11 



mud, sand, and rounded pebbles, derived from the waste of 

 the rocks which form its bed and banks. When these 

 materials cease to be impelled by the force of the moving- 

 water they sink to the bottom, the heaviest pebbles, of 

 course, sinking first, the smaller pebbles and sand next, 

 and the finest mud last. Ultimately, therefore, as might 

 have been inferred upon theoretical grounds, and as is 

 proved by practical experience, every lake becomes a recep- 

 tacle for a series of stratified rocks produced by the streams 

 flowing into it. These deposits may vary in different parts 

 of the lake, according as one stream brought down one kind 

 of material and another stream contributed another material; 

 but in all cases the materials will bear ample evidence that 

 they were produced, sorted, and deposited by running water. 

 The finer beds of clay or sand will all be arranged in thicker 

 or thinner layers or laminae ; and if there are any beds of 

 pebbles these will all be rounded or smooth, just like the 

 water-worn pebbles of any brook-course. In all probability, 

 also, we should find in some of the beds the remains of 

 fresh-water shells or plants or other organisms which inhab- 

 ited the lake at the time these beds were being deposited. 



In the same way large rivers such as the Ganges or 

 Mississippi deposit much of the material which they bring 

 down at their mouths, forming in this way their " deltas." 

 Whenever such a delta is cut through, either by man or by 

 some channel of the river altering its course, we find that 

 it is composed 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. 



Lastly, the sea itself irrespective of the materials 

 delivered into it by rivers is constantly preparing fresh 

 stratified deposits by its own action. Upon every coast-line 

 the sea is constantly eating back into the land and reducing 

 its component rocks to form the shingle and sand which we 

 see upon every shore. The materials thus produced are not, 



