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

ON THE DELUGE. 665 usually found within a few feet of low water; some at least of the number being identical with living species. If 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 con- tend 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 oj'ster. 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 com- paratively 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.