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 cal- 

 careous skeletons. To this last point Murray calls attention: 1 



"It is probable, however, that carbonic acid does play an im- 

 portant 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 algae 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. 



