46 Field Columbian Museum — Geology, Vol. III. 



The magnesian character of so recently formed a limestone of 

 organic origin is somewhat unexpected; even though analyses of 

 reef rock, coral limestone, and coquina invariably show magnesia in 

 similar quantity. The composition of these nodules is essentially 

 that of the Bahama reefs and of other limestones of comparatively 

 recent organic origin.* There are ancient crystalline marbles (e.g. 

 Vermont) which are shown by analysis to have a similar constitution 

 as regards magnesia. 



While the source of the magnesia is undoubtedly the magnesian 

 salts in sea water, the modus operandi of the transfer from the sea 

 salt to the nodule appears doubtful. There are three possible methods: 

 i. Formation of the nodules by direct chemical precipitation of 

 the two carbonates; 2. Metasomatic replacement of calcium by 

 magnesium; 3. Secretion of magnesium carbonate with the lime by 

 organisms. The present tendency of geological belief is towards the 

 replacement hypothesis, although there are yet those who believe the 

 older dolomites are direct chemical precipitates. The application 

 of the theory of replacement of lime by magnesia to the present case 

 meets serious objections. 



Experimental studies of the replacement of calcium by magnesium 

 in carbonates indicate that under certain abnormal conditions of 

 pressure and temperature such replacements readily occur, f Also 

 a co-precipitation of carbonates of lime and magnesia may be 

 produced under conditions of concentration of the mother liquor 

 which cause it to differ widely from sea water in character. On the 

 other hand, experiments by Bischof,^ and others have indicated that 

 under normal conditions either such replacement does not occur or 

 takes place so slowly that an experiment of several years' duration 

 yields no perceptible result. So eminent an authority as Mendeleef, 

 however, states that such replacement can occur and will proceed until 

 a condition of equilibrium dependent upon concentration and temper- 

 ature is attained. § Such an origin of dolomitic limestones necessarily 

 postulates that they are formed under two sets of widely variant condi- 

 tions, under one of which the equilibrium is reached at from one to ten 

 per cent magnesium carbonate, and under the other the equilibrium is 

 reached when the magnesium carbonate in the dolomite attains a 

 proportion not greatly below 45.65%, which corresponds to the 

 double salt MgC0 3 .CaCO s . Limestones with magnesian content 



* U. S. G. S. Bull. 228. 



t Fouque et Levy : Synthese des Mineraux, p. 204. 



J Bischof: Chemical and Ph ysical Geology, vol. Ill, p. 167. 



§Mendel£eff: Principles of Chemistry, vol. I, ch. 14, footnote 11. 



