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CONDITIONS OF SEDIMENTARY DEPOSITION.
505

case the creature still inhabited the shell and preserved the essential parts of its house; in the latter case the decomposition of the fleshy parts may have assisted the solution of the calcareous skeletons. To this last point Murray calls attention:[1]

"It is probable, however, that carbonic acid does play an important part in the solution of shells of animals sinking through the water. The organic matter of the animal on being oxidized produces carbonic acid, which, being itself liquid at all depths over 200 fathoms, will form a locally concentrated acid solution inside the shell, which it will attack with vigor."

The shells which were corroded while still inhabited were also exposed to unusually active solvent influences since they lay upon the bottom, of which Agassiz writes:[2]

"The pelagic animals derive a large part of their food supply from the swarms of large and small pelagic algæ covering the surface of the sea in all oceans. On dying, both surface animals and plants drop to the bottom, and still retain an amount of nutritive matter sufficient to serve as food for the carnivorous animals living on the bottom. A sort of broth, as it has been called by Carpenter, collects on the bottom of the ocean, and probably remains serviceable for quite a period of time; the decomposition of the organic material which has found its way to the bottom takes place gradually, and its putrefaction must be very slow." Thus these more or less corroded shells, dredged from the deep sea, bear witness to the solvent evolved in a bottom layer of decomposing organic matter.

A more direct line of evidence as to the solvent action of the sea-water itself is afforded by observations on the depths to which calcareous skeletons will sink undissolved. The pelagic pteropods and foraminifera, living at the surface, sink on dying and are slowly dissolved; if the water be too deep they never reach bottom. The limits below which they are not found are about 1500 fathoms for pteropods, thin shells exposing large

  1. ↑ Narrative of the Cruise of the Challenger, Vol. I, Second part, p. 981.
  2. ↑ Three Cruises of the Blake, Vol. I, p. 313.
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