196 THE WORLD OF LIFE 



canoes. The springs bring up from great depths a quantity 

 of matter in solution, and the whole of the above-mentioned 

 agencies result not only in a very considerable loss of heat, but 

 also in a very great outflow of solid matter, which, in the course 

 of ages, must leave extensive cavities at various depths, and 

 thus produce lines or areas of weakness which almost certainly 

 determine the mode in which contraction will produce its chief 

 effects. 



As the outer crust for a considerable depth has its tem- 

 perature determined by solar heat, and also because the tem- 

 perature at which the rocks become liquid is tolerably uni- 

 form, the loss of heat, causing shrinkage of the globe as a 

 whole, must occur in the liquid interior; and, as this becomes 

 reduced in size, however slowly, it tends to shrink away from 

 the crust. Hence the crust must readjust itself to the in- 

 terior, and it can only do so by a process of crumpling up, 

 owing to each successive concentric layer having a less area 

 than that above it. This shrinkage has been compared with 

 that of the rind of a drying-up apple. But the earth's crust 

 having been for ages subject to ever-varying compressions and 

 upheavals, and being formed of materials which are of un- 

 equal strength and tenacity, the actual results will be exceed- 

 ingly unequal, and the inequalities 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. IN'ow, the 

 ocean floors are enormous plains, except that they have, here 

 and there, volcanic islands rising out of them. The water which 



