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CONDITIONS OF SEDIMENTARY DEPOSITION.
501

are apparently opposed. On the one hand, the scientists who have described material obtained by soundings on modern limestone deposits have recognized only organic remains. The Challenger in the open oceans, remote from great rivers, the Coast Survey vessels in the Carribbean, the Gulf of Mexico and off the Atlantic coast, the Norwegian expedition in the North Atlantic and English vessels in the Indian ocean have found calcareous oozes of various kinds and rocky limestone formations, but in every case the calcareous matter is described as composed wholly of the tests of pelagic organisms, many of them of microscopic size. It is known that carbonates of lime and magnesia are to a greater or less extent soluble in waters containing carbonic acid, and that the proportion of these carbonates dissolved in ocean waters is small. According to Dittmar the salts in solution in ocean waters contain 0.345 per cent of carbonate of lime and 3.600 per cent of sulphate of lime,[1] and the ocean is capable of dissolving all the lime poured into it by rivers.[2] This view being accepted, it follows that pelagic organisms, which possess the power of secreting solid carbonate of lime from solution, alone can cause lime deposits. Chemical precipitation is, according to this view, impossible, or, if it occurs, is followed by speedy re-solution, and all limestones deposited under conditions of the existing oceans are of organic origin. On the other hand, there are many limestones, deposited at different periods of geologic time, from Algonkian to the present, including some now forming, which consist of more or less clearly crystalline calcite, devoid of organic structure. If this calcite was originally built into organic forms they have been entirely obliterated. Such limestones do indeed contain fossils which sometimes exhibit more or less crystalline texture, but the occurrence of these organic forms in the holocrystalline matrix only raises the question: If the mass was originally all organic and has undergone secondary crystallization after lithifaction, why was the process so complete in the matrix

  1. ↑ Report on the Scientific Results of the Voyage of H. M. S. Challenger."Physics and Chemistry."Vol. 1, p. 204.
  2. ↑ Op. cit. p. 221.
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