i8 PRINCIPLES OF PALAEONTOLOGY. 



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 

 receptacle 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 inhabited the lake at the time these beds 

 were being deposited. 



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. " 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 suc- 

 cession of horizontal layers or strata of sand or mud, varying in 

 mineral composition, in structure, or in grain, according to the 

 nature of the material 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. 



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, however, lost, but are 

 ultimately deposited elsewhere in the form of new stratified 

 accumulations, in which are buried the remains of animals 

 inhabiting the sea at the time. 



