OF ORGANIC NATURE. 33 



sea upon the rock, wearing and grinding it down 

 into a sediment of mud, the mud will be carried 

 down, and at length, deposited in the deeper parts of 

 this sea bottom, where it will form a layer ; and 

 then, while that first layer is hardening, other mud 

 which is coming from the same source will, of course, 

 be carried to the same place ; and, as it is quite im- 

 possible for it to get beneath the layer already there, 

 it deposits itself above it, and forms another layer, and 

 in that way you gradually have layers of mud con- 

 stantly forming and hardening one above the other, 

 and conveying a record of time. 



It is a necessary result of the operation of the law of 

 gravitation that the uppermost layer shall be the 

 youngest and the lowest the oldest, and that the differ- 

 ent beds shall be older at any particular point or spot 

 in exactly the ratio of their depth from the surface. 

 So that if they were upheaved afterwards, and you had 

 a series of these different layers of mud, converted into 

 sandstone, or limestone, as the case might be, you might 

 be sure that the bottom layer was deposited first, and 

 that the upper layers were formed afterwards. Here, 

 you see, is the first step in the history — these layers 

 of mud give us an idea of time. 



The whole surface of the earth, — I speak broadly, 

 and leave out minor qualifications, — is made up of such 

 layers of mud, so hard, the majority of them, that we 

 call them rock, whether limestone or sandstone, or other 

 varieties of rock. And, seeing that every part of the 

 crust of the earth is made up in this way, you might 

 think that the determination of the chronology, the 

 fixing of the time which it has taken to form this crust 



