That illustrious astronomer, the late Sir John Herschel, threw out a bold suggestion on this subject, which in the light of recent discoveries with regard to the delicate adjustment between the internal and external forces affecting the earth's surface, is worthy of careful consideration. His idea was that the mere weight of a thick mass of sediment resting on any portion of the earth's crust might cause a certain amount of sinking; and that this would cause portions on either side to swell up. It is certain that as great deposits of sedimentary materials accumulate on the floor of an ocean, that floor slowly sinks, otherwise the sea would become choked up, and dry land would take its place. Now, it is found that every great mountain-chain consists of many thousands of feet of strata thus formed; and more than this: it turns out that a greater thickness of such materials has been formed in regions where we now see mountain-chains than in those continental regions that lie farther away from them. This is an important fact, which was not known in Sir John Herschel's time. One striking example may be mentioned here. In the complicated region of the Appalachian chain the strata are estimated to have a total thickness of eight miles; while in Indiana, where the same strata are nearly horizontal, they are less than one mile thick. Hence it is not impossible that in the mere accumulation, through long periods of time, of vast masses of strata many thousands of feet thick, we may find a potent cause of earth-movements. The marginal regions of oceans, where most deposition takes place, seem to undergo slow subsidence, while the continents seem in most places to be as slowly rising. Modern geologists are inclined to think that as denudation wears down a continental surface, removing from it a great quantity of solid rocky matter (see chap. v., pp. 161-163), the pressure below is somewhat lessened, or in other words, so much weight is taken off; but that, on the other hand, as this extra amount of material accumulates on the bed of a neighbouring ocean the pressure is increased by a corresponding amount, and so the balance between internal and external forces is upset, and movements consequently take place. We have already seen that the external parts of the earth are much more subject to movements than might have been expected; and for our part, we are willing to believe that in this simple way upheaving forces might be called into play sufficient to account for even the elevation of mountain-chains. For suppose a great mass of strata to continue sinking as they were formed, for long periods of time; what seems to follow? The downward movement would go on until a time would come when the strata, in endeavouring to settle down at a lower level, would (as by the contraction theory above explained) be forced to fold themselves into ridges, and in this way long strips of them might even be elevated into mountain-ranges.