TME FORMATION OP MAGNESIAN LIMESTONES. 185 



The observations of Coquand and Delaoue have led them to con- 

 clude that dolomites have, in many cases at least, been formed by 

 the precipitation of carbonate of magnesia mingled with carbonate 

 of lime. It was, however, still necessary to the solution of the 

 problem of the origin of dolomites to show, first, the conditions 

 under which carbonate of magnesia could be found and deposited ', 

 and, secondly, how it could be made to unite chemically with carbo- 

 nate of lime to form the double salt dolomite. 



In the Eeport of the Geological Survey of Canada, for 1857 (see 

 also Am. Jour, of Science (2), vol. xxxvi. 110), I have shown two 

 processes by which sediments of magnesian carbonate may be formed. 

 First, by the action of solutions of bicarbonate of soda on basins of 

 sea-water, which precipitate all the lime as carbonate, and then give 

 rise to a soluble bicarbonate of magnesia ; and, secondly, the action 

 of bicarbonate of lime on solutions containing sulphate of magnesia. 

 I have found that the presence of this salt greatly increases the 

 solubility of bicarbonate of lime in water — bicarbonate of magnesia 

 and sulphate of lime being formed by double decomposition. By 

 adding alcohol to such a solution, or by evaporating it at a gentle 

 heat, gypsum is deposited, leaving the more soluble bicarbonate of 

 magnesia in solution. 



In the same way, alcohol separates gypsum from a mixed solution 

 of l)icarbonate of lime and sulphate of soda — an alkaline bicarbo- 

 nate remaining dissolved. 



The subsequent evaporation in shallow lakes, or basins, of solutions 

 of bicarbonate of magnesia, formed by either of the above mentioned 

 processes, must give rise to deposits of hydrated carbonate of mag- 

 nesia more frequently mingled with carbonate of lime, supplied by 

 springs containing either bicarbonate of lime or chloride of calcium. 

 The hydrated carbonate of magnesia, at 160* C. (and perhaps at a 

 lower temperature), under pressure to prevent the loss of carbonic 

 acid, is converted into magnesite or anhydrous carbonate of mag- 

 nesia ; but if carbonate of lime be present, the two combine to form 

 a double carbonate, which is dolomite, and may be separated from 

 intermixed carbonate of lime by the action of dilute acetic acid, at 

 32° P., which readily dissolves the latter, but attacks the dolomite 

 more slowly. 



I have found this union of the two carbonates to take place alike 

 in the presence of earthy and alkaline chlorides, sulphates, and ear- 



