bib THE POPULAR SCIENCE MONTHLY. 



his " Student's Elements of Geology," begun his book with the upper- 

 most, that is, newest strata, or layers ; and has gone regularly down- 

 ward in the course of the book to the lowest or earliest strata ; and I 

 shall follow his plan. 



I must ask you meanwhile to remember one law or rule, which 

 seems to me founded on common-sense, namely, that the uppermost 

 strata are really almost always the newest ; that when two or more 

 layers, whether of rock or earth or indeed two stones in the street, 

 or two sheets on a bed, or two books on a table any two or more 

 lifeless things, in fact, lie one on the other, then the lower one was 

 most probably put there first, and the upper one laid down on the 

 lower. Does that seem to you a truism ? Do I seem almost imperti- 

 nent in asking you to remember it ? So much the better. I shall be 

 saved unnecessary trouble hereafter. 



But some one may say, and will have a right to say, " Stop the 

 lower thing may have been thrust under the upper one." Quite true : 

 and therefore I said only that the lower one was most probably put 

 there first. And I said " most probably," because it is most probable 

 that in Nature we should find things done by the method which costs 

 least force, just as you do them. I will warrant that, when you want 

 to hide a thing, you lay something down on it ten times for once that 

 you thrust it under something else. You may say : " What ? When 

 I want to hide a paper, say, under the sofa-cover, do I not thrust it 

 under ? " No, you lift up the cover, and slip the paper in, and let the 

 cover fall on it again. And so, even in that case, the paper has got 

 into its first place. 



Now, why is this ? Simply because in laying one thing on another 

 you only move weight. In thrusting one thing under another, you 

 have not only to move weight, but to overcome friction. That is why 

 you do it, though you are hardly aware of it : simply because so you 

 employ less force, and take less trouble. 



And so do clays and sands and stones. They are laid down on 

 each other, and not thrust under each other, because thus less force is 

 expended in getting them into place. 



There are exceptions. There are cases in which Nature does try 

 to thrust one rock under another. But to do that she requires a force 

 so enormous, compared with what is employed in laying one rock on 

 another, that (so to speak) she continually fails ; and, instead of pro- 

 ducing a volcanic eruption, produces only an earthquake. Of that I 

 may speak hereafter, and may tell you, in good time, how to distin- 

 guish rocks which have been thrust in from beneath, from rocks which 

 have been laid down from above, as every rock between London and 

 Birmingham or Exeter has been laid down. That I only assert now. 

 But I do not wish you to take it on trust from me. I wish to prove it 

 to you as I go on, or, to do what is far better for you, to put you in 

 the way of proving it for yourselves, by using your common-sense. 



