182 THE WORLD OF LIFE CHAP. 



equalities will be most manifested along or near to certain 

 lines of weakness caused by earlier shrinkage due to the 

 same cause. 



As the crust will be of greater extent than the contracted 

 liquid core it has finally to rest upon, and as the chief effects 

 of contraction are limited to certain directions and to com- 

 paratively small areas, and if the less fractured and more 

 rigid portions settle down almost undisturbed upon the 

 contracted interior, then considerable areas along, or parallel 

 to, the lines of weakness must be crumpled, fractured, and 

 forced upward, and thus produce great elevations on the 

 surface, though small in proportion to the whole dimensions 

 of the earth. Now, the ocean floors are enormous plains, 

 except that they have, here and there, volcanic islands rising 

 out of them. The water which covers them preserves uniform 

 temperature, which, at the bottom, is not much above the 

 freezing point of sea-water. We may conclude, therefore, 

 that they are very nearly stable. Pendulum experiments 

 show that the crust below these oceans is more dense than 

 the subaerial crust, due, probably, to the uniform pressure 

 and temperature they have been subject to for geologic 

 periods. We may assume, therefore, that they do not be- 

 come crumpled or distorted by the contraction of the liquid 

 earth beneath them. The great plains of Russia, mostly of 

 Triassic and Jurassic age, consist of nearly horizontal strata 

 while the Alps of Central Europe are greatly upheaved and 

 contorted ; and the same difference between adjacent areas 

 is found in the United States, and most probably in all the 

 great continents. 



Mathematical physicists have calculated the possible 

 upheavals that could be produced by a shrinking crust at 

 probable rates of contraction, and have declared them to be 

 too small to account for the elevation of the existing land- 

 masses above the ocean floors, that is, for the whole differ- 

 ences of height of the land surfaces. But if, as the Rev. O. 

 Fisher suggests, the oceanic basins were formed at an early 

 stage of the earth's consolidation, by the separation from it 

 of the moon in the way described by Sir George Darwin 

 and accepted by Sir Robert Ball ; and if the whole wrinkling 

 effect of contraction is concentrated on a few lines or areas 



