284 [Proc. B.^N. F. C, 



I, in common with an increasing number of geologists, am a 

 firm believer in the theory that there exists somewhere between 

 the earth's crust, which has become solid from loss of heat, and 

 the interior, which is solid through density and pressure, a 

 plastic zone. Many rocks are metamorphosed or melted through 

 heat, and this heat has been generated through pressure, for 

 metamorphosed rocks occur chiefly in mountain areas where 

 they have been squeezed in the process of elevation. On the 

 other hand, the interior of the earth has over and over again 

 been proved to be rigid. There is no escape from the inference 

 that it is kept solid at a temperature above its melting point 

 through excess of pressure. This nucleus cannot pass into a 

 crust whose solidity is due to its low temperature without passing 

 through that intermediate stage of temperature and pressure at 

 which rocks are molten, and a continuous layer of molten matter 

 must therefore underlie the solid outer crust and follow to a 

 great extent the earth's contour. 



Seismologists have demonstrated the extreme sensitiveness 

 of this crust, and its perpetual state of unrest, so that we now 

 know that there is nothing more urjstable than solid ground. 

 Geologists have called attention to the fact, reiterated in every 

 formation and in every country, how invariably the great 

 sedimentary strata have formed in areas in which subsidence has 

 kept pace with their accumulation, so that no matter what their 

 thickness, they always remained at or about the sea-level. I 

 discussed these facts in " Nature" only last August, with a view 

 to uphold the conclusion that the earth's crust was sensitive 

 under the increase of weight on any given area, and sufficiently 

 flexible to yield under it and cause subsidence. Passing over all 

 lesser examples of areas in which weight might be added, I 

 endeavoured to show that the great depths of the oceans must 

 ever be the areas over which the greatest and a constantly 

 increasing pressure is exerted. 



If ocean basins are permanent, sediment must have been 

 slowly accumulating in them since Eozooic times, and attained 

 an aggregate vertical thickness that must be colossal. The 

 pressure of water alone will equal in a depth of 400 fathoms 



