vi.] THE SLATES ON THE ROOF. 141 



That such a contraction and expansion goes on in 

 the crust of the earth is evident; for here are the 

 palpable effects of it. And the simplest general cause 

 which I can give for it is this : That things expand as 

 they are heated, and contract as they are cooled. 



Now I am not learned enough and were I, I have 

 not time to enter into the various theories which 

 philosophers have put forward, to account for these 

 grand phenomena. 



The most remarkable, perhaps, and the most 

 probable, is the theory of M. Elie de Beaumont, 

 which is, in a few words, this : 



That this earth, like all the planets, must have 

 been once in a state of intense heat throughout, as its 

 mass inside is probably now. 



That it must be cooling, and giving off its heat 

 into space. 



That, therefore, as it cools, its crust must contract. 

 That, therefore, in contracting, wrinkles (for the 

 loftiest mountain chains are nothing but tiny wrinkles, 

 compared with the whole mass of the earth), wrinkles, 

 I say, must form on its surface from time to time. 

 And that the mountain chains are these wrinkles. 



Be that as it may, we may safely say this. That 

 wherever the internal heat of the earth tends (as in 

 the case of volcanoes) towards a particular spot, that 

 spot must expand, and swell up, bulging the rocks 

 out, and probably cracking them, and inserting 

 melting lava into those cracks from below. On the 

 other hand, if the internal heat leaves that spot again, 

 and it cools, then it must contract more or less, in 

 falling inward toward the centre of the earth ; and so 

 the beds must be crumpled, and crushed, and shifted 



