596 THE POPULAR SCIENCE MONTHLY. 



the whole of these disturbances as parts of one common process. They 

 can not be accounted for by any mere local movements, though such 

 movements no doubt took place abundantly. The existence of a moun- 

 tain-chain is not to be explained by a special upheaval or series of 

 upheavals caused by an expansive force acting from below. . Manifestly 

 the elevation is only one phase of a vast terrestrial movement which has 

 extended over whole continents, and has affected plains as well as high 

 grounds. 



The only cause which, so far as our present knowledge goes, could 

 have produced such wide-spread changes is a general contraction of the 

 earth's mass. There can be no doubt that at one time our planet existed 

 in a gaseous, then in a liquid condition. Since these early periods it 

 has continued to lose heat, and consequently to contract and to grow 

 more and more solid, until, as the physicists insist, it has now become 

 practically as rigid as a globe of glass or of steel. But in the course of 

 the contraction, after the solid external crust was formed, the inner hot 

 nucleus has lost heat more rapidly than the crust, and has tended to 

 shrink inward from it. As a consequence of this internal movement, 

 the outer solid shell has been obliged to sink down upon the retreating 

 nucleus. In so doing, it has of course had to accommodate itself to a 

 diminished area, and this it could only accomplish by undergoing pli- 

 cation and crumpling. Though the analogy is not n very exact one, 

 we may liken our globe to a shriveled apple. The skin of the apple 

 does not contract equally. As the internal moisture passes off, and the 

 bulk of the fruit is reduced, the once smooth exterior becomes here 

 and there corrugated and dimpled. 



Without entering into this difficult problem in physical geology, it 

 may suffice if we carry with us the idea that our globe must once have 

 had a greater diameter than it now possesses, and that the crumpling 

 of its outer layers, whether due to mere contraction or, as has been 

 suggested, to the escape also of subterranean vapors, affords evidence 

 of this diminution. A little reflection suffices to show us that, even 

 without any knowledge of the actual history of the contraction, we 

 might anticipate that the effects would neither be continuous nor 

 everywhere uniform. The solid crust would not, we may be sure, sub- 

 side as fast as the mass inside. It would, for a time at least, cohere 

 and support itself, until at last, gravitation proving too much for its 

 strength, it would sink down. And the areas and amount of descent 

 would be greatly regulated by the varying thickness and structure of 

 the crust. Subsidence would not take place everywhere ; for, as a 

 consequence of the narrower space into which the crust sank, some 

 regions would necessarily be pushed up. These conditions appear to 

 have been fulfilled in the past history of the earth. There is evidence 

 that the terrestrial disturbance has been renewed again and again, 

 after long pauses, and that, while the ocean-basins have on the whole 

 been the great areas of depression, the continents have been the lines 



