CONDITIONS OF SEDIMENTARY DEPOSITION. 501 



are apparently opposed. On the one hand, the scientists who 

 have described material obtained by soundings on modern lime- 

 stone deposits have recognized only organic remains. The 

 Challenger in the open oceans, remote from great rivers, the 

 Coast Survey vessels in the Caribbean, 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 cal- 

 careous 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 micro- 

 scopic size. It is known that carbonates of lime and magnesia 

 are to a greater or less extent soluble in waters containing car- 

 bonic acid, and that the proportion of these carbonates dissolved 

 in ocean waters is small. According to Dittmar the salts in solu- 

 tion 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, accord- 

 ing to this view, impossible, or, if it occurs, is followed b}^ 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 holo- 

 crystalline 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. 



