“Mountains, mountain ranges, and valleys equivalent to mountains exist generally in virtue of the rigidity of the earth’s crust; continents, continental plateaus, and ocean basins exist in virtue of isostatic equilibrium in a crust heterogeneous as to density.”—G. E. Gilbert, 1890.
The relation between rigidity and equilibrium in the lithosphere was thus stated by Gilbert thirty-odd years ago, and seems to me still to have been most philosophically and accurately stated. Opposed to this view, or rather supplementing it by refinements of extraordinary precision, is the conclusion reached by Hayford and Bowie, to the effect that complete isostatic compensation exists generally within the depth of 71 miles and for areas of a square degree of extent or even less. I have elsewhere2 stated my reasons for not being able to accept the conclusions based on the mathematical analysis of gravity determinations, and will here . . .