30 THE PAST CONDITION 



it in its downward course the pebbles and lighter matters 

 from its banks, it crushes and pounds down the rocks 

 and earths in precisely the same way as the wearing action 

 of the sen waves. The matters forming the deposit are 

 torn from the mountain-side and whirled impetuously into 

 the valley, more slowly over the plain, thence into the 

 estuary, and from the estuary they are swept into the sea. 

 The coarser and heavier fragments are obviously deposited 

 first, that is, as soon as the current begins to lose its force 

 by becoming amalgamated with the stiller depths of the 

 ocean, but the finer and lighter particles are carried further 

 on, and eventually deposited in a deeper and stiller portion 

 of the ocean. 



It clearly follows from this that mud gives us a 

 chronology ; for it is evident that supposing this, which 

 I now sketch, to be the sea bottom, and supposing this to 

 be a coast-line ; from the washing action of the 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 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 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. 



Thr 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 



