Note 10 — Chapter XXVIII · A VERY FEW REMARKS WITH REFERENCE TO THE DELUGE.
Reading edition
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 be 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 be 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 being 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 be buried, before they had been 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. 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.