we know, be conveyed
for any distance across the seas. Mingled with this sediment of an
inorganic origin we almost invariably find a share of organic waste,
derived not from creatures which dwelt upon the bottom, but from those
which inhabited the higher-lying waters. If, now, we take a portion of
the limestone layer which lies above or below the shale parting, and
carefully dissolve out with acids the limy matter which it contains,
we obtain a residuum which in general character, except so far as the
particles may have been affected by the acid, is exactly like the
material which forms the claylike partition. We are thus readily led
to the conclusion that on the floors of the deeper seas there is
constantly descending, in the form of a very slow shower, a mass of
mineral detritus. Where organic life belonging to the species which
secrete hard shells or skeletons is absent, this accumulation,
proceeding with exceeding slowness, gradually accumulates layers,
which take on a shaly character. Where limestone-making animals
abound, they so increase the rate of deposition that the proportion of
the mineral material in the growing strata is very much reduced; it
may, indeed, become as small as one per cent of the mass. In this case
we may say that the deposit of limestone grew a hundred times as fast
as the intervening beds of shale.

The foregoing considerations make it tolerably clear that the sea
floor is in receipt of two diverse classes of sediment--those of a
mineral and those of an organic origin. The mineral, or inorganic,
materials predominate along the shores. They gradually diminish in
quantity toward the open sea, where the supply is mainly dependent on
the substances thrown forth from volcanoes, on pumice in its massive
or its comminuted form--i.e., volcanic dust, states of lava in which
the material, because of the vesicles which it contains, can float for
ages before it comes to rest on the sea bottom. Variations in the
volcanic waste contributed to the sea floor may somewhat affect the
quantity of the inorganic sediments, but, as a whole, the downfalling
of these fragments is probably at a singularly uniform rate. It is
otherwise with the contributions of sediment arising from organic
forms. This varies in a surprising measure. On the coral reefs, such
as form in the mid oceans, the proportion of matter which has not come
into the accumulation through the bodies of animals and plants may be
as small as one t