PKESENT KNOWLEDGE OF THE EAETH WIECHEET. 447 



what is it? Referred to water, 3,4. A better agreement than this 

 could not be wished, and through it we gain a further and very im- 

 portant support for the whole series of our conclusions regarding the 

 condition of the globe. 



(6) The earth'' s crust. — Vast differences in elevation are found 

 to exist on the surface of the earth. Some plateaus are situated 

 2,000 meters and more above sea level ; the highest mountain tops 

 rise to nearly 9,000 meters. The floor of the ocean lies on an average 

 about 3,500 meters below sea level; the greatest depths of the sea 

 attain about 9,000 meters. On an average, the land rises about 4,200 

 meters above the floor of the sea. In view of these facts it seems 

 natural to infer that the elevations indicate accumulations of mass 

 and the depressions deficiencies of mass, and in fact this was formerly 

 sujDposed to be the case. But if this were really so, gravitation on 

 the earth would have to be greater the higher the surface; but this 

 is by no means the case. On the contrary, the measurements of 

 gravitation have shown that while variations occur, indicating an 

 excess or a deficiency of mass at various points, on the whole the 

 mass of the earth is uniformly distributed over its surface. Thus 

 the elevations and depressions of the earth's crust in a general way 

 mean simply that in the former the crust is less dense, in the latter 

 denser, and that for this reason it rises higher at the former points, 

 less high at the latter. 



These conclusions assume a special significance for the reason that, 

 as shown by geologic investigation, rock strata many thousand meters 

 thick have in the course of past ages, mainly through water circula- 

 tion, been carried away from certain parts of the earth and deposited 

 at other points. How is it possible that nevertheless the mass is 

 to-day distributed with practical uniformity all over the earth? 

 To explain this there is no other way than by assuming that the 

 superficial accumulations of mass are compensated for by subterra- 

 nean removal. Therefore the solid crust must have a soft sub- 

 stratum, on which it floats, as it were. Geologists have for this 

 reason often been led to infer that the interior of the earth is liquid. 

 We have seen that the earth as a whole, in the presence of the tidal 

 forces, in the case of pole oscillations and of earthquake waves, 

 behaves as a solid and that it is even remarkably unyielding in the 

 matter of elasticity. Hence we can not conceive the interior of the 

 earth as being liquid throughout, in the current meaning of the 

 word. It is possible, however, that beneath the outer solid crust 

 there exists at no very great depth a molten layer, so thin in pro- 

 portion to the earth's body as not to cause any perceptible diminution 

 of its rigidity. Volcanoes might then be regarded as vents connect- 

 ing that layer with the surface. It might also be imagined that the 

 liquid layer at the present day no longer extends beneath the entire 



