R. A. Daly — Limeless, Ocean of Pre- Cambrian Time. 109 



The observations of Murray and Irvine on mud-waters sug- 

 gest that, at the present day, there may be a slow addition of 

 magnesium carbonate to the deposits of pure calcium carbonate 

 shells or skeletons. A fairly pure calcareous ooze or shell- 

 bank or a porous coral reef may be charged with decaying 

 animal matter. Within the myriad interstices of the deposit 

 there is sea-water into which ammonium carbonate is being 

 passed. This alkali precipitates all the calcium salts in the 

 quasi-imprisoned water. Thereafter will follow a slow but 

 steady precipitation of magnesium carbonate within the ooze 

 or sand ; if the amount of alkaline carbonate suffices, the 

 magnesium salt may be added to the deposit in large amounts. 



Again, the law of mass-action may also have sway. After 

 the calcium salts have been precipitated from sea-water, the 

 relatively abundant magnesium sulphate may possibly react on 

 the already precipitated calcium carbonate so as to replace a 

 considerable portion of it by magnesium carbonate.* The 

 writer is not familiar with experiments bearing on this point 

 but it is noted as a possible mode of the formation of dolomite, 

 which is a real though indirect result of the decay of animal 

 matter on the sea-bottom. So far as this method operated, it 

 too was most potent in Eozoic times. 



There are yet other ways in which magnesium carbonate 

 may be elaborated from sea-water — through certain algae and 

 a few animals known to secrete magnesium carbonate along 

 with the dominant calcium carbonate of their hard structures, 

 or, finally, through the local evaporation of sea-water. How- 

 ever, the quantitative value of all these sources just mentioned 

 may well be suspected to be but subsidiary to a more general 

 cause of dolomite formation. Most of the world's magnesian 

 limestones and dolomites seem to owe their origin neither to 

 the secretions of special organisms nor to evaporation. The 

 special organisms are too rare in one case ; evaporation must 

 be too local for the other case. 



The scope of the present paper does not permit of a critical 

 discussion of the many published theories concerning the dolo- 

 mites. It may only be stated that, if we accept the leaching 

 hypothesis or the hypothesis that dolomite is the result of 

 metamorphic processes by which magnesium comes to replace 

 calcium in ordinary limestone, we meet with very grave diffi- 

 culties, long ago stated and never overcome. The rapid alter- 

 nation of clean-cut beds of pure or nearly pure calcium 

 carbonate with other clean-cut beds of magnesian limestone or 



* An experiment by Liebe showed that this mass effect does not take place 

 where chalk was immersed a year and a half in variously concentrated solu- 

 tions of magnesium chloride. Zeitschrif t der Deutschen Geologischen Gres- 

 ellschaft, vol. vii, p. 431, 1855. 



Am. Jour. Sci. — Fourth Series, Vol. XXIII, No. 134.— February, 1907. 



