GENERAL DISCUSSION. 61 



under the influence of moisture, with slight sohition going on at the surfaces of the solid par- 

 ticles, there would be a degree of molecular mobility wliich might bring about combination. 

 This is probable, although so far as we know it has not been actually proved. The establish- 

 ment of the facts ought not to be beyond the range of experimental investigation. Views 

 similar to ours relative to the formation of dolomite have already been advanced by General 

 E. Jourdy, who especially recognizes the importance of the alga^, of aragonite, and of time. 

 Mucli remains to be done, however, before the problem of dolomitization can be completely 

 solved.'* 



On the subject of phosphate rock we have little to say. We have shown that several 

 groups of organisms are rich in phosphates, but the extent of their contributions to the sedi- 

 ments is uncertain. At best they can at lii'st form beds of only moderately phosphatic lime- 

 stone, which may perhaps be concentrated by the leaching away of the excess of carbonates. 

 Vertebrate skeletons are also phosphatic and may possibly be more important additions to the 

 sediments than invertebrate remains. In some localities limestones have been phosphatizcd 

 by percolations from beds of guano," but that process is not one which needs to be considered 

 here. It has no relation to the present research. 



In the com-se of our investigation we have made one very curious discovery, to which 

 we have repeatedly called attention, namely, the fact that in certain groups of organisms the 

 proportion of magnesium carbonate is dependent upon or determined by temperature. The 

 crinoids and alcyonarians show tliis I'elation very clearly, and it is also suggested by our analyses 

 of foraminifera, crustaceans, and algje. As a rule the organisms from warm waters are much 

 richer in magnesia than those from cold waters, and the observed differences are often strikingly 

 conspicuous. Tliis rule, or rather tendency, we are inclined to believe is general, although we 

 must admit that there are probably exceptions to it. Whether the increase in magnesia in 

 passing from cold to warm regions is absolutely regular or not we do not venture to say, but 

 apparent iri-egularities may be due to any one of several different causes. Slight analytical 

 errors, uncertainty as to exact temperatures, impurities in the specimens analyzed, and differ- 

 ences in the concentration of sea water may all help to produce irregularities, which, however, 

 are not likely to be large. The salinity of ocean water is very variable; it is 3.5 per cent in 

 the great ocean, 4 per cent on the southern shores of the Mediterranean, and less than 1 per 

 cent in the Baltic, differences that are great enough to exert some influence upon the vital 

 processes of marine animals. Although the ratio between calcium and magnesium is prac- 

 tically constant in all oceanic waters, a concentrated water would contain more magnesium, 

 volume for volume, than a water that was more dilute. Whether an organism living in a con- 

 centrated water would assimilate more magnesia because of its enriched environment no one 

 can say, but conceivably it might do so. The influence of temperature might in that way be 

 slightly modified. This is only a suggestion, not a statement of established fact. That warmth 

 favors the assimilation of magnesia by marine invertebrates seems to be reasonably certain, 

 but why it should be so is not clear. The relation is definite but as yet unexplained. We 

 hope it is not inexplicable. 



Attempts will probably be made to use our data in studies of climatology, but are such 

 attempts likely to be fruitful? The question is not easy to answer. At a first glance it would 

 seem as if warm regions should be more favorable to the formation of magnesian limestones 

 than cold regions, but the evidence is by no means conclusive. A dense population of cold- 

 water organisms might add more magnesia to the sediments than a sparse population of warm- 

 water foi-ms. The massiveness of the animals must also be considered. The alcyonarian 

 Paragorgia arborea, as its specific name indicates, grows to treelike dimensions, but its skeleton 

 contains only about 9 per cent of magnesium carbonate. Tropical alcyonarians are much richer 

 individually in magnesia, but they are not so large. One Paragorgia would therefore count 



" Jourdy, E., Soc. g^ol. France Bull., 4tli ser., vol. 14, p. 279, 1914. 



"Clarke, F. W., The data of geochemistry: U. S. Geol. Survey Bull. 695, pp. 515-526, 1920; gives a summary of our knowledge of phosphate rock. 



