Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.2): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

664 A FEW REMARKS that those shells have undergone enormous pressure beneath an ocean, when they were surrounded with mud.* But pre- vious to such pressure, the shells must have grown naturally somewhere -.^-certainly not at the bottom of an ocean ; because they are shells of a comparatively delicate structure which are * On this subject, the pressure of an ocean, Mr. Lyell remarks, (Elements of Geology, ] 838, 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 solidif)', 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 power- ful 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 con- siderations may help to account for the fact that seamen do not find im- pressions 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, 1 need hardly remark, grow onlv in comparatively shallow water.