MALLETS THEORY OF VOLCANOES. 155 



has indicated. The shrivelled skin of an apple affords no 

 inapt representation of the corrugated surface of our earth, 

 and according to the new theory, the shrivelling of such 

 a skin is precisely analogous to the processes at work upon 

 the earth when mountain-ranges were being formed. Again, 

 there are few students of geology who have not found a 

 source of perplexity in the foldings and overlappings of 

 strata in mountainous regions. No forces of upheaval seem 

 competent to produce this arrangement But by the new 

 theory this feature of the earth's surface is at once ex- 

 plained ; indeed, no other arrangement could be looked for. 

 It is worthy of notice that Mr. Mallet's theory of Vol- 

 canic energy is completely opposed to ordinary ideas 

 respecting earthquakes and volcanoes. We have been 

 accustomed vaguely to regard these phenomena as due to 

 the eruptive outbursting power of the earth's interior; we 

 shall now have to consider them as due to the subsidence 

 and shrinkage of the earth's exterior. Mountains have not 

 been upheaved, but valleys have sunk down. And in 

 another respect the new theory tends to modify views which 

 have been generally entertained in recent times. Our most 

 eminent geologists have taught that . the earth's internal 

 forces may be as active now as in the epochs when the 

 mountain-ranges were formed. But Mr. Mallet's theory 

 tends to show that the volcanic energy of the earth is a 

 declining force. Its chief action had already been exerted 

 when mountains began to be formed ; what remains now 

 is but the minutest fraction of the volcanic energy of the 

 mountain-forming era ; and each year, as the earth parts 

 with more and more of its internal heat, the sources of her 

 subterranean energy are more and more exhausted. The 

 thought once entertained by astronomers that the earth 

 might explode like a bomb, her scattered fragments pro- 

 ducing a ring of bodies resembling the zone of asteroids, 

 seems further than ever from probability ; if ever there was 

 any danger of such a catastrophe, the danger has long since 

 passed away. 



