58 THE INORGANIC CONSTITUENTS OF MARINE INVERTEBRATES. 



Of these genera, so far as they have been studied chemically, all in the aragonite column 

 are almost completely noninagnesian. The trifling amounts of magnesia which they contain 

 may be due to impurity or to alteration. Two in the calcite column, Terebratula and Balanus, 

 are also nonmagnesian, and the others are all rich in magnesium carbonate. Meigen also tested 

 twentj' zoantharian corals, all aragonitic and nonmagnesian, and a considerable number of 

 mollusks. Some of the molluscan shells were aragonitic and some were calcitic, but all except 

 Argonauta were nearly free from magnesia. One echinoderm in Meigen's list, of a genus not 

 represented in our series of analyses, was calcitic, and so too were ours. All the echinoderms, 

 so far as we know, are distinctly magnesian. In short, it seems probable, in the light of existing 

 evidence, that all aragonitic organisms are essentially nonmagnesian; and that those char- 

 acterized by the presence of much magnesia are calcitic. Many calcitic forms, however, are 

 practically free from magnesia. The general relation thus brought out is very suggestive. 

 Magnesium carbonate associates itself only with calcite, with which it is isomorphous, rather 

 than with aragonite, of different crystalline form, but why some organisms should secrete calcite 

 and others aragonite in building their shells or skeletons is as yet unexplained, although the 

 difference may be of physiologic origin, and may perhaps be correlated with differences of 

 structure. 



The considerations presented in the preceding pages bear directly upon the problem of 

 the origin of marine dolomite. We now know what classes of organisms supply magnesia to 

 the limestones and something also of what may be called their mineralogic nature. The dolo- 

 mite ratio between the two carbonates is, however, never directly reached; there is always at 

 first a large excess of calcium over magnesium, and a mixture is formed instead of the true 

 double salt. To produce dolomite the original limestone must either be enriched by magnesia 

 derived from sea water or else concentrated by leaching away of lime; furthermore, its two 

 component carbonates must be somehow forced to combine. These processes may be operative 

 simultaneously, but it is more probable that the change from magnesian limestone or dolomite 

 is brought about by a series of steps, taken one at a time. 



In this connection the report on Funafuti, already cited, is remarkably suggestive. On 

 that atoll or island a committee of the Royal Society put down a boring 1,114 feet, in lime- 

 stone all the waJ^ The rock throughout contained organic remains and was studied both 

 microscopically and chemically. At a depth of 4 feet from the surface it contained 4.23 per 

 cent of magnesium carbonate, and at 15 feet 16.4 per cent. To this point a concentration 

 by leaching is indicated, even if not absolutely proved, and it is probable that the relatively 

 soluble nonmagnesian aragonitic structures had been in part at least dissolved away. The 

 unstable aragonite is more easily dissolved than calcite, a relation so well established that it does 

 not need to be discussed here."' The fact that many sections of the core are described as "cav- 

 ernous" in structure is additional evidence that solution had occurred. Solution is also aided 

 by the carbonic acid generated during the decomposition of the organic matter of the organisms, 

 and through its agency calcite would be dissolved also. Magnesium carbonate is much less 

 readily removed. 



At a depth of 25 feet the core contained 16 per cent of magnesium carbonate, but the 

 specimen examined consisted largely of Lithothamnion remains, which accounts for its compo- 

 sition. On the other hand, the core at 40 feet carried only 5.85 per cent of magnesium carbon- 

 ate and was in great part composed of Heliopora and Millepora, both originally nonmagnesian. 

 The core sections evidently varied in composition according to the variations in the organisms 

 from which they were formed. A sample taken only a few feet away from the boring might 

 have had a different composition. Some fluctuations in the series of analyses may be accounted 

 for in this way. 



Below 40 feet the magnesian content of the rock diminished rapidly, falling at one 

 point to 0.79 per cent of magnesium carbonate. Between 40 and 637 feet the composition of 

 the rock was about that of an ordinary limestone, but at the latter depth crystals of dolomite 



« This leaching at Funafuti has already been iiointed out by J. W. Judd in The atoll of Funafuti, p. 3S4. 



