ORIGIN OF DOLOMITE. 799 



at some past time secreted magnesium in greater quantities than at present, 

 have held that magnesium limestones and dolomites as such are original 

 chemical precipitates. While the sea is not now saturated with calcium 

 and magnesium salts, it is held that this might have been the case before 

 life became abundant, and that consequently magnesium limestone and 

 dolomite formed directly. This is again a pure, unverified assumption, 

 which seems to be negatived by the following considerations: (a) Since the 

 living animals extract so little magnesium, on this hypothesis it is difficult 

 to explain why the sea is not now saturated or nearly saturated with 

 magnesium salts; (b) Bischof" found by experiment that when solutions 

 become saturated with carbonates of magnesium and calcium the calcium 

 carbonate is largely precipitated before the magnesium carbonate begins to 

 be thrown down in appreciable quantity. Hence, by chemical precipitation, 

 there would be produced separate layers composed mainly of carbonate of 

 calcium and carbonate of magnesium, rather than calcium-magnesium 

 carbonate. According with Bischof 's experiment, the alkaline earth car- 

 bonates precipitated in Lakes Lahontan and Bonneville are essentially 

 calcium carbonate, containing not more than 2.14 per cent of magnesium. 6 

 The deposits of inland lakes in which the salts accumulate until precipita- 

 tion occurs should be analogous to those of the ocean in ancient times, on 

 the supposition that before life existed abundantly chemical percipitates 

 formed. The fact that tufas deposited in such lakes contain so little 

 magnesia seems to bear against the hypothesis that the dolomites are 

 original chemical precipitates. 



(3) But perhaps the most decisive of the various lines of evidence in 

 favor of secondary dolomitization are observed facts of occurrence which 

 seem to be explicable only upon the basis of replacement. While, as 

 already noted, coral is nearly pure calcium carbonate, Dana found that the 

 limestone of the elevated coral island Metia is heavily magnesian, one 

 specimen containing as much as 38 per cent of magnesium carbonate." 

 Prestwich states that in the Carboniferous limestones of Kilkenny and Cork 

 the upper surface and parts of the rock along the bedding and joint planes 



a Bischof, Gustav, Elements of chemical and physical geology, trans, by B. H. Paul: Cavendish 

 Society, London, vol. 3, 1859, pp. 169-170. 



&Kussell. I. C, Geological history of Lake Lahontan: Mon. U. S. Geol. Survey, vol. 11, 1885, 

 p. 203. Gilbert, G. K., Lake Bonneville: Mon. IT. S. Geol. Survey, vol. 1, 1890, p. 168. 



"Dana, Jas. D., Corals and Coral Islands, 3d ed., Dodd, Mead & Co., New York, 1890, pp. 393-394. 



