60 THE INORGANIC CONSTITUENTS OF MARINE INVERTEBRATES. 



which may or may not be sustained, is that the dolomitic zone at Funafuti represents an old 

 reef upon which the present reef is superimposed. A simihir basing of new reefs upon okler 

 ones has already been pointed out by T. Wayland Vaughan.'^ 



On many other islands in the South Pacific dolomitized limestones which were originally 

 reefs are found at elevations hundreds of feet above sea level. A number of these rocks have 

 been described by E. W. Skeats," who made good series of chemical analyses of them and also 

 discussed their origin. Organic and especially algal remains were common in the rocks and 

 were clearly recognizable in spite of the fact that in many places they had been much altered 

 by the process of dolomitization. In these islands an elevation of the land had clearly taken 

 place. Similar limestones from the Fiji, Tonga, Tuamotu, and Ladrone islands have been 

 studied by R. L. Sherlock,'^ who found them to be composed in great part of algal and forami- 

 niferal remains. Among 47 thin sections which he examined, " Lifhothamnion" was found 

 in 35, Polytrema in 21, echinoderm fragments in 17, and corals in 15. In short, all the evidence 

 goes to prove the importance of the algm as limestone builders and the subordinate character 

 of the corals. This importance is now fully recognized by students of marine limestones and 

 by paleontologists generally. 



It is not our purpose to discuss tue origin of dolomite in general, for probably the rock 

 originates in more than one way.'^ At Funafuti, however, and at other sinular localities marine 

 organisms have much to do with its oiigin, and that phase of the dolomite problem may appro- 

 priately be considered here. The first step in the process, concentration by leaching, has already 

 been described, but that is only a beginning. The living organisms, plant or animal, contain 

 much less magnesia than is required to form dolomite, and its cjuantity must be increased 

 from some outer source. The source, or at least the only source wliich we can discover, is 

 found in ocean water, in which magnesium is much more abundant than calcium. This source 

 has been recognized by many authorities, and it is generally assumed that an exchange may 

 occur between the magnesium of the water and the calcium of the limestones, the one replacing 

 the other. This assumption is due to J. D. Dana,'" who sought to explain the dolomitization 

 of a limestone on thu island of Makatea. 



That an enrichment in magnesia from sea water is possible is shown by some experiments 

 made by C. Klement," who found that a concentrated solution of magnesium sulphate and 

 sodium chloride, at 90° C, attacked aragonite and corals strongly, yielding a mixture of car- 

 bonates containing as high as 41.9 per cent of magnesium carbonate. Calcite, on the other 

 hand, was but slightly altered. In the light afforded by these experiments the nonmagnesian 

 aragonitic organisms assume new importance and are perhaps more influential in the produc- 

 tion of dolomite than the distinctly magnesian species. Their solubility is evident in the 

 first stage of magnesian concentration; their alterability is effective in the second. 



Klement's experiments, however, were performed with concentrated solutions and at a 

 rather high temperature. Under natural conditions, with less concentration and at lower tem- 

 peratures, the same reaction may take place very slowly but be equally complete in time. Years, 

 or even decades, may be needed to effect such changes as were produced in the laboratory 

 within 48 hours. In the study of geochemical processes the time factor must always be taken 

 into account. 



So far, according to Klement, a mixture of the two carbonates has been formed. To 

 convert them into the double carbonate, dolomite, another step must be taken, and here again 

 time may be important. A porous rock has been produced, which is saturated u-ith water and 

 buried at a depth which subjects it to considerable pressure. If the two carbonates of calcium 

 and magnesium were perfectly dry, pressure alone would probably not effect their union, but 



" Vaughan, T. W., Papers from the Tortiigas laboratory of the Carnegie Institution of Washington, vol. 5, p. 66, 1914. 

 " .Skeats. E. W., Harvard Coll. Mus. Comp. Zoology Bull., vol. 42, p. 51, 1903. 

 " Sherlock, R. L., idem., vol. 3S, p. 349, 1903. 



« For a rather full summary of the subject see Clarke, F. W., The data of geochemistry, 4th ed., U.S. Geol. Survey Bull. 695, pp. 557-572, 1920. 

 'S Dana, J. D., Corals and coral islands, 3d. cd., p. 393, IS90. 



" Element, C, Min. pet. Mitt., vol. 14, p. 526, 1894. See also PfaCf, F. W., Centralbl. Mineralogie, 1903, p. 059, and Neues Jahrb., Beilage B^nd 9, 

 p. 4S5, 1894. Two separate papers. 



