126 Dr. A. Morley Davies — On Isostasy. 



may estimate to have a density of 3. As to the latter, the average 

 density of sedimentary material (making no allowance for porosity) 

 is 2*7. Porosity to the extent of 20 per cent brings the density of 

 unconsolidated dri/ sediment to about 2-16 (Indian geologists give 

 2*2 as the average for the Siwalik rocks, so 2-16 is not too low for 

 quite unconsolidated material). But we have to deal with sediment 

 saturated with sea-water to the extent of the 20 per cent of its 

 volume allowed for porosity ; this brings the density up to 2'36. 

 The sediment, however, displaces sea-water as it accumulates, and 

 though it thereby raises the sea-level, that rise, being distributed 

 over the whole ocean, is negligible. The effective density for the 

 •calculation of isostatic overloading is therefore 1'36. 



A mass of sediment on the sea-bottom, then, would depress the 



latter to the extent of — ^— , or about nine-twentieths of its own 

 3 



thickness, if the isostatic adjustment is perfect and immediate. Thus 



.... 3 



at whatever depth deposition begins a thickness equal to ' 



or about 1'83 times that depth, could accumulate before the sea was 

 completely silted up. Taking the 100-fathom line as the limit 

 between deep and shallow water, shallow-water deposits could 

 accumulate, under conditions of the most delicate isostatic adjust- 

 ment, to a thickness of only about 1,100 feet before accumulation 

 became inter-tidal or subaerial in character, and in that thickness 

 there would be a gradual transition from deposits of 100-fathom type 

 at the bottom to littoral deposits at the top. If we suppose the 

 isostatic adjustment to be spasmodic instead of continuous, there will 

 be an alternating character in the sediments instead of a gradual 

 transition, but the total thickness will be, if anything, diminished, 

 since the adjustment will be less perfect. 



It may be objected that 3 is too high a figure for the density of 

 the supporting column. If we take it at so improbably low a figure 

 as 2-7, the maximum possible thickness is only increased from 1,100 

 to just over 1,200 feet. 



But what right have we to assume such delicate isostatic adjustment 

 as these calculations imply? The theory of isostasy originated in 

 America, where the careful investigations of Hayford and the United 

 States Coast and Geodetic Survey showed that there is an approxima- 

 tion to isostatic equilibrium — or, at least, that there is such on a 

 certain assumption as to the depth of compensation. When Crosfield 

 investigated India on the same assumption he found that country to 

 diverge considerably from isostatic equilibrium. This was naturally 

 explained by the immense crustal disturbances in that region, which, 

 reaching a maximum in the Miocene period, have not yet entirely 

 died out. If these great disturbances have not been capable of full 

 isostatic adjustment in the long period of time that has elapsed 

 since the Miocene period, can we justifiably assume that the gentle 

 accumulation of sediment is continually and immediately adjusted ? 



Without venturing into any discussion of the complicated subject 

 of the rigidity of the earth's crust, I may call .attention to the view 

 of Professor Barrell that the strength of the crust is " twenty, fifty, 



