munities, do we find
animals which can make such head against the action of the waves that
they can build great deposits in their realm.

It should be noted that a part of the advantage which is afforded to
organic life by the shore belt is due to the fact that the waters are
there subjected to a constant process of aeration by the whipping into
foam and spray which occurs where the waves overturn.

It will be interesting to the student to note the great number of
mechanical contrivances which have been devised to give security to
animals and plants which face these difficult conditions arising from
successive violent blows of falling water. Among these may be briefly
noted those of the limpets--mollusks which dwell in a conical shell,
which faces the water with a domelike outside, and which at the moment
of the stroke is drawn down upon the rock by the strong muscle which
fastens the creature to its foundation. The barnacles, which with
their wedge-shaped prows cut the water at the moment of the stroke,
but open in the pauses between the waves, so that the creature may
with its branching arms grasp at the food which floats about it; the
nullipores, forms of seaweed which are framed of limestone and cling
firmly to the rock--afford yet other instances of protective
adaptations contrived to insure the safety of creatures which dwell in
the field of abundant food supply.

* * * * *

The facts above presented will show the reader that the marine
sediments are formed under conditions which permit a great variety in
the nature of the materials of which they are composed. As soon as the
deposits are built into rocks and covered by later accumulations,
their materials enter the laboratory of the under earth, where they
are subjected to progressive changes. Even before they have attained a
great depth, through the laying down of later deposits upon them,
changes begin which serve to alter their structure. The fragments of a
soluble kind begin to be dissolved, and are redeposited, so that the
mass commonly becomes much more solid, passing from the state of
detritus to that of more or less solid rock. When yet more deeply
buried, and thereby brought into a realm of greater warmth, or perhaps
when penetrated by dikes and thereby heated, these changes go yet
further. More of the material is commonly rearranged by solution and
redeposition, so that limestone may be converted into crystal