46 Fietp CoLtumB1AN Musreum—GEo oey, Vot. 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: 
1. 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.t 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 Mendeléef, 
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 MgCO,.CaCO,. Limestones with magnesian content 
*U.S.G.S. Bull. 228. 
+ Fouque et Levy: Synthese des Mineraux, p. 204. 
+ Bischof: Chemical and Physical Geology, vol. III, p. 167. 
§ Mendeléeff: Principles of Chemistry, vol. I, ch. 14, footnote 11. ; 
