x EARTH CHANGES & EVOLUTION 181 



lowered by the weight forcing the crust downwards, we have 

 a real and efficient cause for continuous subsidence and the 

 formation of parallel strata of enormous thicknesses. It 

 remains to account for the subsequent upheaval of these 

 areas, their tilting up at various angles, and in many cases 

 their being fractured, curved, or contorted often to an 

 enormous extent and in a most fantastic manner. 



Effects of a Cooling and Contracting Earth 



It is universally admitted that the earth is a cooling 

 and therefore a contracting body. The cooling, however, 

 does not take place by conduction from the heated interior 

 through the solid crust, the temperature of which at and 

 near the surface is due wholly to sun-heat, but by the escape of 

 heated matter to the surface through innumerable hot or warm 

 springs ; by a continuous flow of heated gases from volcanic 

 areas ; and frequent outbursts of red-hot ashes or liquid lavas 

 from volcanoes. 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 

 temperature determined by solar heat, and also because the 

 temperature at which the rocks become liquid is tolerably 

 uniform, 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 interior, 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 unequal strength and tenacity, the 

 actual results will be exceedingly unequal, and the in- 



