336 EDWARD STEIDTMANN 



mitization in any given case may be influenced slightly by the 

 original composition of the calcareous secretions: witness the 

 difference in the content of magnesium carbonate in secretions 

 from Lithothamnium and other members of this genus as compared 

 with those of the corals. Certain calcareous secretions have been 

 found to consist of calcite while others like those of the corals 

 consist largely of aragonite. Aragonite is much less stable than 

 calcite, hence the aragonite secretions are probably much more 

 subject to dolomitization through leaching and replacement than 

 the more stable form of calcium carbonate, calcite. Furthermore, 

 it is highly probable that the composition of the sea is an important 

 factor in dolomitization. If sea water is high in magnesium and 

 low in calcium, it is probable that the conditions are more favorable 

 for the dolomitization of calcareous deposits than if the reverse 

 condition prevailed, as may be inferred from the following experi- 

 ments. If a crystal of calcite is placed in water, solution will 

 take place until equilibrium is reached between the calcite and the 

 solution. Obviously, replacement, in the absence of magnesium 

 salts, cannot take place. On the other hand, Sorby found that a 

 crystal of calcite placed in a concentrated solution of magnesium 

 chloride became slowly incrusted with magnesium carbonate, 

 and Pfaff developed a dolomitic material by subjecting calcium 

 carbonate to the action of a solution of magnesium chloride or 

 sulphate and common salt, under pressure comparable to the deep 

 sea, in which case the speed of reaction was proportional to the 

 concentration of the solution. 



It is obvious that 60 pounds of calcium carbonate could not be 

 replaced by a solution containing only 10 pounds of magnesium 

 salt in anything like the proportions required by the dolomite 

 ratio. It seems equally probable, a priori, that if the river waters 

 carried calcium and magnesium to the sea in the ratio of 1:1, the 

 chances for dolomitization through leaching, secondary replacement, 

 or possibly primary deposition would be better, all other things 

 being equal, than when the proportion of river-borne calcium to 

 magnesium is 6:1, which is the approximate ratio at the present 

 time according to the best estimate. 1 



1 See F. W. Clarke, "A Preliminary Study of Chemical Denudation," Smithsonian 

 Mis. Coll., LVI, No. 5, p. 8. 



