GEOGRAPHICAL EVOLUTION 353 



Yet even in the midst of such apparent chaos it is not 

 impossible to trace the fundamental law and order by which 

 it is underlaid. The prime fact to be noted is the universal 

 plication and crumpling of rocks which were at first nearly 

 horizontal. From the gentle undulations of the strata 

 beneath the plains to their violent contortion and inversion 

 among the mountains, there is that insensible gradation 

 which connects the whole of these disturbances as parts of 

 one common process. They cannot be accounted for by 

 any mere local movements, though such movements no 

 doubt took place abundantly. The existence of a mountain 

 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 con- 

 tinents, and has affected plains as well as high grounds. 



The only cause which, so far as our present knowledge 

 goes, could have produced such widespread 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 sunk 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 plication and fracture. 

 Though the analogy is not a very exact one, we may liken 

 our globe to a shrivelled 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 ex- 

 terior 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 



HC VOL. xxx L 



