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Prof. J. Prestwich. 



as suggested by Darwin, that great coral areas of the Indian and 

 Pacific Oceans have long been areas of subsidence,* while adjoining 

 volcanic areas have been areas of elevation. In some cases areas, 

 once areas of depression, have become areas of elevation, as in the 

 instances of some coral islands, which, though formed during periods 

 of depression, have been since raised above the waters to heights of 

 from 200 to 300 feet or more. 



In conclusion, I may point to the imposing spectacle afforded by 

 the slow secular upheaval of the vast tracts of Arctic lands on the 

 shores of North America and Asia — an area of elevation so extensive 

 that it embraces almost all the land bordering the Polar Seas. This 

 elevation has in comparatively modern times raised the land from 

 100 to 400 feet above the present sea-level, and is still, in our own 

 times, in visible action over a superficial area extending in some 

 directions, for thousands of miles. 



§ 7. Primary Cause of Volcanic Action. 



If water only plays a secondary part in volcanic action, and the 

 presence of the vapour of water in the volcanic foci be not the 

 primary cause of the expulsion of the lava, to what other cause is 

 it to be attributed ? I see none but a modification of the old hypothesis, 

 namely, that of the contraction of solid crust upon a yielding and 

 hot nucleus. The objection to this hypothesis rested mainly on the 

 fact that if, as was at that time assumed, the whole nucleus beneath 

 the solid crust consisted of a molten fluid, it would be subject to tides 

 that would lessen or neutralise the surface tides. I might also suggest 

 as a further reason that there could be no local deformations on a 

 continental scale : — the pressure caused by local squeezings would be 

 dissipated through the entire mass and lost. Sir William Thomsou 

 and the late Mr. Hopkins have clearly proved that the earth pos- 

 sesses a rigidity perfectly incompatible with a fluid nucleus. At the 

 same time, objections have been taken by other physicists to the 

 hypothesis of an entirely solid globef on the grounds, amongst others, 

 that the question has been dealt with on the assumption of a perfect 

 fluid, and that not sufficient allowance has been made for friction. 



Nor are those views so incompatible as might at first sight appear. 

 The questions connected with the surface tides are not yet settled. 

 Admitting an extreme rigidity of the crust, it has not yet been 

 proved that notwithstanding this rigidity, the crust is quite free 



* This has been contested. There are no doubt instances of reefs formed without 

 subsidence, but for the depths of the Pacific Darwin's hypothesis best answers all 

 the conditions. 



f See the various papers on this subject by Hennessy, Haughton, Delaunay, and 

 others. 



