Deposition of sediments and subsidence.  The stratified rocks of the land show in many places ancient sediments which reach a thickness which is measured in miles, and which are yet the product of well-nigh continuous deposition.  Such strata may prove by their fossils and by their composition and structure that they were all laid offshore in shallow water.  We must infer that, during the vast length of time recorded by the enormous pile, the floor of the sea along the coast was slowly sinking, and that the trough was constantly being filled, foot by foot, as fast as it was depressed.  Such gradual, quiet movements of the earth’s crust not only modify the outline of coasts, as we have seen, but are of far greater geological importance in that they permit the making of immense deposits of stratified rock.

A slow subsidence continued during long time is recorded also in the succession of the various kinds of rock that come to be deposited in the same area.  As the sea transgresses the land, i.e. encroaches upon it, any given part of the sea bottom is brought farther and farther from the shore.  The basal conglomerate formed by bowlder and pebble beaches comes to be covered with sheets of sand, and these with layers of mud as the sea becomes deeper and the shore more remote; while deposits of limestone are made when at last no waste is brought to the place from the now distant land, and the water is left clear for the growth of mollusks and other lime-secreting organisms.

Rate of deposition.  As deposition in the sea corresponds to denudation on the land, we are able to make a general estimate of the rate at which the former process is going on.  Leaving out of account the soluble matter removed, the Mississippi is lowering its basin at the rate of one foot in five thousand years, and we may assume this as the average rate at which the earth’s land surface of fifty-seven million square miles is now being denuded by the removal of its mechanical waste.  But sediments from the land are spread within a zone but two or three hundred miles in width along the margin of the continents, a line one hundred thousand miles long.  As the area of deposition—­about twenty-five million square miles—­is about one half the area of denudation, the average rate of deposition must be twice the average rate of denudation, i.e. about one foot in twenty-five hundred years.  If some deposits are made much more rapidly than this, others are made much more slowly.  If they were laid no faster than the present average rate, the strata of ancient sea deposits exposed in a quarry fifty feet deep represent a lapse of at least one hundred and twenty-five thousand years, and those of a formation five hundred feet thick required for their accumulation one million two hundred and fifty thousand years.

The sedimentary record and the denudation cycle.  We have seen that the successive stages in a cycle of denudation, such as that by which a land mass of lofty mountains is worn to low plains, are marked each by its own peculiar land forms, and that the forms of the earlier stages are more or less completely effaced as the cycle draws toward an end.  Far more lasting records of each stage are left in the sedimentary deposits of the continental delta.