180 THE WORLD OF LIFE CHAP. 



Now, it has been ascertained that the various rocks of 

 the crust are of less specific gravity in the solid state than 

 when they are liquefied, so that the crust may be looked 

 upon as actually floating upon the liquid interior, very much 

 as the polar ice-sheets float upon the ocean. A curious 

 confirmation of this has been given by measurements of the 

 force of gravity, which show that near all great mountain 

 masses gravity is diminished, not only by the amount due to 

 the mass of the mountain itself, but to about double that 

 amount This is so universally the case that it has been 

 concluded that the weight of the mountain mass is supported 

 by a corresponding mass forced down into the fluid magma, 

 and hence termed the " roots of the mountains " ; just as 

 every lofty iceberg must have a mass of submerged ice 

 about nine times as great to support it in the water. This, 

 of course, proves that the crust is flexible, and that just as 

 any portion of it is upheaved or made thicker by additions 

 above, a corresponding increase in thickness must occur below 

 to keep it in equilibrium. 



Thus are explained the very frequent phenomena of 

 horizontal strata occurring in similar beds for thousands of 

 feet thick, while each successive bed must have been formed 

 at or near the surface. Such are the deposits recently 

 formed in the deltas of great rivers, in many of which borings 

 have been made from 350 to 640 feet deep, with indications 

 that each successive layer was formed near the surface, and 

 that during the entire process of deposition the whole 

 area must have been sinking at a very regular rate. This 

 can best be explained by the weight of the matter deposited 

 causing the slow subsidence. Exactly similar phenomena 

 occur through the whole series of the geological formations 

 to the most ancient ; in some cases strata eight miles in 

 thickness showing proofs that the very lowest beds were not 

 deposited in a deep ocean, but in quite shallow water near 

 shore. 1 



Now, as we have seen that, over many areas not far from 

 shore, deposition may occur 100 or even 1000 times as 

 fast as denudation, and that this same area is continuously 



1 In chapter iii. of vol. i. of my Studies Scientific and Social I have given 

 details of these phenomena on the highest geological authority. 



