336 THE CAUSES OF THE xi 



hardening, other mud which is coming from the 

 same source will, of course, be carried to the same 

 place ; and, as it is quite impossible 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 constantly 

 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 different 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 sand- 

 stone, 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 after- 

 wards. 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 

 iixing of the time which it has taken to form this 



