12 INTRODUCTION. 



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 con- 

 tributed 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, 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 at their mouths much of the material which they bring 

 down, forming in this way their " deltas." Whenever such a delta 

 is cut through, either by man or by some channel of the river alter- 

 ing its course, we find that it is composed of a succession of hori- 

 zontal 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, 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. 



Whenever, then, we find anywhere in the interior of the land any 

 series of beds having these characters — composed, that is, of distinct 

 layers, the particles of which, both large and small, show distinct 

 traces of the wearing action of water — whenever and wherever we 

 find such rocks, we are justified in assuming that they have been 

 deposited by water in the manner above mentioned. Either they 

 were laid down in some former lake by the combined action of the 

 streams which flowed into it ; or they were deposited in some por- 

 tion of the course of an ancient river ; or they were laid down at 

 the bottom of the ocean. In the first two cases, any fossils which 

 the beds might contain would be the remains of fresh-water or ter- 

 restrial organisms. In the last case, the majority, at any rate, of the 

 fossils would be the remains of marine animals. 



The term " formation " is often employed by geologists in a loose 



