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—lI 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 
