igoS.J THE PHYSICS OF THE EARTH. 251 



earth has been ascertained to amount to 11.3 kilometers per second, the 

 compressibiHty of that region must be 31 times less than that of quartz, 

 that is, eight times less than that of solid steel, according to Voigt. This 

 is a figure of precisely that order of magnitude which was to be expected. 

 We may well believe that at depths of more than 1,000 kilometers the com- 

 pressibility of gaseous iron sinks down to some ten times less than that of 

 steel. 



" The interior of the earth, therefore, with the exception of a solid 

 crust about 40 kilometers thick, consists of a molten magma 100 or 200 

 kilometers in depth which shades continuously inward into a gaseous center. 

 The liquids and gases in the interior possess a viscosity and incompressi- 

 bility such as permit them to be regarded as solid bodies. From these, 

 however, they are distinguished in the first place by the fact that differentia- 

 tions are possible to a considerable degree, the effects of which may long 

 endure. In the second place, long continued pressures, when acting on a 

 large enough scale, may produce great deformations. Further, the liquids 

 must possess the property of great expansion on a diminution of the high 

 pressure, thereby readily becoming fluid. The process must thus differ but 

 little from a normal melting with increase of volume, and especially of 

 fluidity, as well as with absorption of heat. And yet the condition of aggre- 

 gation is not thereby altered." 



Geikie remarks that the theory of Arrheniits accords well with 



geological requirements : 



" With reference to the crust of the earth, it meets the constanth- re- 

 peated objections of the geologists to whom the existence of a comparatively 

 thin crust has always seemed an essential condition for the production of 

 that crumpled and fractured structure which the rocks of the land so uni- 

 versally present. If the solid crust of the earth is allowed to be about 25 

 miles thick, we must conceive that in the lower four fifths of its mass 

 the rocks are in a condition of latent plasticity. They lie much beyond the 

 crushing strength which they exhibit at the surafec. They are not crushed 

 into powder as they would be under a similar strain above ground, but they 

 are ready to yield to the deformations that may arise consequent upon ad- 

 justments of the gigantic pressure to which they are subjected. Hence the 

 solid crust down as far as its structure has been disclosed abounds in proofs 

 that it has undergone colossal plication and fracture, and that higher por- 

 tions of it many square miles in extent have been thrust bodily over each 

 other for many miles." 



The last view here expressed by Geikie as to how the crust 



becomes thrust over itself for many miles is not, we think, well 



founded, because it is shown in this paper that all this folding and 



overlapping of the crust arises in the trenches dug out in the sea 



bottom by earthquakes. This crumpling and overthrusting of the 



crust certainly would not arise except for earthquakes produced by 



