31 THE PAST CONDITION 



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 impossible for it to get 

 beneath the layer already there, it deposits itself above 

 it, and forms another layer, and in that way you grad- 

 ually have layers of mud constantly forming and hard- 

 ening 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 is a comparatively simple matter. To take a broad 

 average, ascertain how fast the mud is deposited upon 

 the bottom of the sea, or in the estuary of rivers ; take 

 it to be an inch, or two, or three inches a year, or what- 

 ever you may roughly estimate it at; then take the 



