the continents are old, which implies that the parts of the earth
which they occupy have long been the seats of tolerably continuous
erosion. Now and then they have swung down partly beneath the sea, and
during their submersion they received a share of sediments. But, on
the whole, all parts of the lands except strips next the coast may be
reckoned as having been subjected to an excess of wearing action far
exceeding the depositional work. Therefore, as we readily see,
underneath such land areas there has been no blanketing process going
on which has served to increase the heat in the deep underlying rocks.
On the contrary, it would be easy to show, and the reader may see it
himself, that the progressive cooling of the earth has probably
brought about a lowering of the temperature in all the section from
the surface to very great depths, so that not only is the rock water
unaffected by increase of heat, but may be actually losing
temperature. In other words, the conditions which we assume bring
about volcanic action do not exist beneath the old land.

Beneath the seas, except in their very greatest depths, and perhaps
even there, the process of forming strata is continually going on.
Next the shores, sometimes for a hundred or two miles away to seaward,
the principal contribution may be the sediment worn from the lands by
the waves and the rivers. Farther away it is to a large extent made up
of the remains of animals and plants, which when dying give their
skeletons to form the strata. Much of the materials laid down--perhaps
in all more than half--consist of volcanic dust, ashes, and pumice,
which drifts very long times before it finds its way to the bottom. We
have as yet no data of a precise kind for determining the average rate
of accumulation of sediments upon the sea floor, but from what is
known of the wearing of the lands, and the amount of volcanic waste
which finds its way to the seas, it is probably not less than about a