THE STEATIFIED ROCKS 283 



carrying down to the sea immense quantities of sand and mud, 

 which will ultimately be deposited in the form of new strata ; 

 every cliff by the sea-shore is slowly or rapidly weathering away 

 under the influence of atmospheric agencies rain, frost, tides 

 and wind ; and far out, beyond the reach of the detritus derived 

 from the land, the sea-bottom is being covered by the slow 

 accumulation of ooze or mud derived almost entirely from the 

 remains of marine organisms, while the activities of corals and 

 other reef -building animals in their turn make no small contribu- 

 tion to the grand total. 



The sedimentary rocks naturally contain the remains of vast 

 numbers of organisms which flourished on the earth at more or 

 less remote periods of its history, and the study of these fossils 

 may justly be expected to yield results of the greatest importance 

 from the point of view of the theory of organic evolution. We 

 have here, in fact, the only really direct evidence of the course 

 which the evolution of the animal and vegetable kingdoms has 

 actually taken, and this evidence constitutes what is commonly 

 known as the geological record. 



The relative ages of the different sedimentary rocks and of 

 the different geological formations which they build up are of 

 course determined primarily by their position with regard to one 

 another, each layer or bed being necessarily of more recent date 

 than the one below it. 1 The series of strata met with in any 

 particular locality may, however, be very different from that 

 which occurs in other localities, for while mud may be accumu- 

 lating in one place, sandstone may be forming in another and 

 limestone in a third, and over other areas, which lie above 

 sea-level, rock formation will be replaced by destruction or 

 denudation. Nevertheless, geologists have found it possible to 

 compare the series of stratified rocks found in different parts of 

 the world one with another, and, by the aid of the fossils which 

 they contain, to identify a number of more or less well defined 

 epochs in the geological history of the earth. 



At the bottom of the stratified series lies an immense thickness 

 of rocks which seem to have been originally sedimentary, 

 but many of which have undergone profound changes due to 

 pressure and heat since they were laid down. These rocks 

 indicate a very long period of the earth's history, the 



1 Except in a few cases where contortion has been so extensive as to reverse the 

 relative positions. 



