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that those shells have undergone enormous pressure beneath
an ocean, when they were surrounded with mud.* But previous
to such pressure, the shells must have grown naturally
somewhere:—certainly not at the bottom of an ocean; because
they are shells of a compai'atively delicate structure which are
* On this subject, the pressure of an ocean, Mr. Lyell remarks,
(Elements of Geology, 1838, pp. 7, 8,9.) “ When sand and mud sink to
the bottom of a deep sea the particles are not pressed down by the
enormous weight of the incumbent ocean; for the water, which becomes
mingled with the sand and mud, resists pressure with a force equal to
that of the column of fluid above.” “ Nevertheless if the materials of
a stratum remain in a yielding state, and do not set or solidify, they will
be gradually squeezed down by the weight of other materials successively
heaped upon them, just as soft clay or loose sand on which a house is
built may give way. By such downward pressure particles of clay, sand,
and marl may become packed into a smaller space, and he made to cohere
together permanently.”
“ But the action of heat at various depths is probably the most powerful
of all causes in hardening sedimentary strata.”
In reflecting upon these passages it appears to me that Mr. Lyell has
supposed what may not always take place in a deep sea, namely—that
sand and mud sink to the bottom.
Whenever particles of sand and mud are at the bottom, they must be
lower than contiguous particles of water, or they could not be at the
bottom ; therefore those particles of sand and mud have water above, while
resting upon some other substance below. Pressure there can be none,
excepting of some earthy particles upon others, while the specific gravity
of the sand and mud exceeds that of the displaced fluid. But, if the
depth of water he increased, and its specific gravity at the bottom
augmented, the sand and mud at the bottom must rise, if they do not
cohere together, and to the surface on which they lie ; in which case
the increasing weight and density of water would tend to compress and
make them cohere still more.
The smaller kinds of sea shells are very little heavier than sea water.
This would prevent their heing carried by the action of the sea to great
depths, even if it were possible for them to be so rolled over rocks, sand,
or mud, in which they would stick, or he buried, before they had heen
moved many miles from the place where they grew. These two considerations
may help to account for the fact that seamen do not find impressions
of shells, on the « arming’ of the lead, when sounding in very
deep water, at a considerable distance from any shore where they grow,
Sea-shells, I need hardly remark, grow only in comparatively shallow
water.
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usually found within a few feet of low water ; some at least
of the number being identical with living species.
I f the square miles of solid land in which those myriads of
shells are now embedded, had been upheaved (as geologists say),
either gradually, or rapidly, shells could not be found there in
their present confused and compressed state. Had the land
sunk down many thousand feet with shells upon it, they
might have been covered with mud, and on being afterwards
upheaved again they would have appeared embedded regularly
where they grew, in a matrix which, with the pressure of a
superincumbent ocean, might have flattened and penetrated
them : but they would not have been torn away from their
roots, rolled, broken, mashed, and mixed in endless confusion,
similarly to those now in my possession.
There is also another consideration : geologists who contend
for the central heat of the earth assert that substances
subjected to great pressure under the sea become altered:
hence, in conformity with their theory, these shells could not
have been long buried under a deep ocean, and afterwards
raised in their pristine state. So little changed are these shells,
except in form, that they appear as if they had been heaped
water. The specific gravity of oyster shell, when dead, is about twice
that of sea-water (2092,1028). Most other shells are much lighter, and
but few at all heavier than the oyster.
Before ending this note I must remark that the horizontal movement
of water near the bottom, though gentle, may tend to press together and
smooth any loose sand, mud that sinks, oazy clay, or fragments of shells,
before many of their particles travel far. Water in rapid motion is
known to hold sand as well as mud in suspension, but not shells, unless
the current is very strong. To such a constant agitation of the sea,
oscillating gently with each tide, we may perhaps ascribe the comparatively
level and smoothened state of the bed of the ocean, where
it has hitherto been sounded. Excepting near irregular, rocky land,
one finds, generally speaking, no ravines, no vallies, no abrupt transitions
in the bottom of the sea. For miles together there is an almost equal
or gradually altering depth of water : and little similarity can be traced
between the contour of the bed of a sea and the neighbouring dry land,
until you are near the shore, where the sea acts differently, and irregular
bottom is as frequent as it is usually dangerous for shipping.