HI LECTURES ON EVOLUTION 59 



which is of so plain and so simple a character 

 that it is impossible in any way to escape from 

 the conclusions which it forces upon us. 



You are, doubtless, all aware that the outer 

 substance of the earth, which alone is accessible 

 to direct observation, is not of a homogeneous 

 character, but that it is made up of a number of 

 layers or strata, the titles of the principal groups 

 of which are placed upon the accompanying 

 diagram. Each of these groups represents a 

 number of beds of sand, of stone, of clay, of slate, 

 and of various other materials. 



On careful examination, it is found that the 

 materials of which each of these layers of more 

 or less hard rock are composed are, for the most 

 part, of the same nature as those which are at 

 present being formed under known conditions on 

 the surface of the earth. For example, the chalk, 

 which constitutes a great part of the Cretaceous 

 formation in some parts of the world, is prac- 

 tically identical in its physical and chemical 

 characters with a substance which is now being 

 formed at the bottom of the Atlantic Ocean, and 

 covers an enormous area ; other beds of rock are 

 comparable with the sands which are being 

 formed upon sea-shores, packed together, and so 

 on. Thus, omitting rocks of igneous origin, it is 

 demonstrable that all these beds of stone, of 

 which a total of not less than seventy thousand 

 feet is known, have been formed by natural 



