r from their bodies in a yet more perfectly divided form. Hence
it comes about that the limestone beds, so commonly formed beneath the
open seas, are generally composed of materials which show but few and
very imperfect fossils. Studying any series of limestone beds, we
commonly find that each layer, in greater or less degree, is made up
of rather massive materials, which evidently came to their place in
the form of a limy mud. Very often this lime has crystallized, and
thus has lost all trace of its original organic structure.

One of the conspicuous features which may be observed in any
succession of limestone beds is the partings or divisions into layers
which occur with varied frequency. Sometimes at vertical intervals of
not more than one or two inches, again with spacings of a score of
feet, we find divisional planes, which indicate a sudden change in the
process of rock formation. The lime disappears, and in place of it we
have a thin layer of very fine detritus, which takes on the form of a
clay. Examining these partings with care, we observe that on the upper
surface on the limestone the remains of the animal which dwelt on the
ancient sea floor are remarkably well preserved, they having evidently
escaped the effect of the process which reduced their ancestors,
whose remains constitute the layer, to mud. Furthermore, we note that
the shaly layer is not only lacking in lime, but commonly contains no
trace of animals such as might have dwelt on the bottom. The fossils
it bears are usually of species which swam in the overlying water and
came to the bottom after death. Following up through the layer of
shale, we note that the ordinary bottom life gradually reappears, and
shortly becomes so plentiful that the deposit resumes the character
which it had before the interruption began. Often, however, we note
that the assemblage of species which dwelt on the given area of sea
floor has undergone a considerable change. Forms in existence in the
lower layer may be lacking in the upper, their place being taken by
new varieties.

So far the origin of these divisional planes in marine deposits has
received little attention from geologists; they have, indeed, assumed
that each of these alterations indicates some sudden disturbance of
the life of the sea floors. They have, however, generally assumed that
the change was due to alterations in the depth of the sea or in the
run of ocean currents. It seems to the writer, however