mestone, which would otherwise form, we have only a claylike
residuum, such as is obtained when we dissolve lime rocks in acids.
This process of solution, by which the limy matter deposited on the
bottom is taken back into the water, goes on everywhere, but at a rate
which increases with the depth. This increase is due in part to the
augmentation of pressure, and in part to the larger share of carbonic
dioxide which the water at great depths holds. The result is, that
explorations with the dredge seem to indicate that on certain parts of
the deeper sea floors the rocks are undergoing a process of
dissolution comparable to that which takes place in limestone caverns.
So considerable is the solvent work that a large part of the inorganic
waste appears to be taken up by the waters, so as to leave the bottom
essentially without sedimentary accumulations. The sea, in a word,
appears to be eating into rocks which it laid down before the
depression attained its present great depth.

We should here note something of the conditions which determine the
supply of food which the marine animals obtain. First of all, we may
recur to the point that the ocean waters appear to contain something
of all the earth materials which do not readily decompose when they
are taken into the state of solution. These mineral substances,
including the metals, are obtained in part from the lands, through the
action of the rain water and the waves, but perhaps in larger share
from the volcanic matter which, in the form of floating lava, pumice,
or dust, is plentifully delivered to the sea. Except doubtfully, and
at most in a very small way, this chemical store of the sea water can
not be directly taken into the structures of animals; it can only be
immediately appropriated by the marine plants. These forms can only
develop in that superficial realm of the seas which is penetrated by
the sunlight, or say within the depth of five hundred feet, mostly
within one hundred feet of the surface, about one thirtieth of the
average, and about one fiftieth of the maximum ocean depth. On this
marine plant life, and in a small measure on the vegetable matter
derived from the land, the marine animals primarily depend for their
provender. Through the conditions which bring about the formation of
_Sargassum_ seas, those areas of the ocean where seaweeds grow afloat,
as well as by the water-logging and weighting down of other vegetable
matter, some part of the plant re