1 8 PRINCIPLES OF PALAEONTOLOGY. 



posits by its own action. Upon every coast-line the sea is 

 constantly eating back into the land and reducing its com- 

 ponent 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 at the mouth of some ancient river, 

 forming its delta ; 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 terres- 

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

 the fossils would be the remains of marine animals. 



The term " formation " is employed by geologists to express 

 "any group of rocks which have some character in common, 

 whether of origin, age, or composition " (Lyell) ; so that we 

 may speak of stratified and unstratified formations, aqueous 

 or igneous formations, fresh-water or marine formations, and 

 so on. 



CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 



The Aqueous Rocks may be divided into two great sections, 

 the Mechanically-formed and the Chemically-formed, includ- 

 ing under the last head all rocks which owe their origin to 

 vital action, as well as those produced by ordinary chemical 

 agencies. 



A. MECHANICALLY-FORMED ROCKS. These are all those 

 Aqueous Rocks of which we can obtain proofs that their 

 particles have been mechanically transported to their present 

 situation. Thus, if we examine a piece of conglomerate or 

 puddingstone, we find it to be composed of a number of 

 rounded pebbles embedded in an enveloping matrix or paste, 

 which is usually of a sandy nature, but may be composed of 

 carbonate of lime (when the rock is said to be a " calcareous 

 conglomerate "). The pebbles in all conglomerates are worn 

 and rounded by the action of water in motion, and thus show 



