EVOLUTION OF LIMESTONE AND DOLOMITE 331 



lime and magnesia in solution. The rapid corrosion of marine 

 shells is partly ascribed to this agency, by the authors of the 

 Challenger deep-sea report. Thomson's 1 observations that the 

 local abundance of carbonic acid on the ocean bottom is associated 

 with an abundance of organisms is suggestive. Corrosion of 

 calcareous remains by the direct action of sea water, but pre- 

 dominantly by the action of carbonic acid resulting from organic 

 decay, is regarded by Murray as sufficiently powerful to prevent 

 the accumulation of calcareous ooze at a mean depth of 2,730 

 fathoms, over 51,500,000 square miles of ocean bottom, the Red 

 Clay area. 



It therefore seems to be an open question whether the net 

 chemical effect resulting from organic decay in the ocean is pre- 

 cipitation or corrosion. The scavengers of the present ocean do 

 not arrest the accumulation of calcareous oozes over millions of 

 square miles of the ocean bottom. The evidence that direct pre- 

 cipitation of calcium carbonate is taking place on those vast areas 

 of organic decay is doubtful. Philippi, 2 viewing direct chemical 

 precipitation as an important process in the origin of limestones, 

 after citing the data offered by various investigators for direct 

 chemical precipitation from the present ocean, concludes that 

 certain incrustations and the cements of some oozes are probably 

 chemical precipitates. He believes that warm seas, teeming with 

 life, where decay on the bottom is rapid are favorable localities 

 for the precipitation of calcium carbonate. 



The acquisition of the lime-secreting habit by marine organisms 

 in response to an accumulation of lime in the ocean postulated as 

 a result of the post-Huronian erosion cycle, and the increase in 

 scavengers may be possible. Paleontologists believe that at least 

 nine-tenths of the organic evolution into main stocks was accom- 

 plished before the Cambrian. The Cambrian record contains 

 evidence for the existence of all phyla excepting the vertebrates, 

 and the unsuccessful search for vertebrate fossils in the Cambrian 

 cannot be regarded as conclusive evidence for their absence. The 

 burden of proof, therefore, rests heavily on any theory based on 



1 C. Wyville Thomson, Depths of the Sea, 502-11. 



2 Philippi, Neues Jahrb. (1907), 444. 



