382 On some Reactions of the Salts of Lime and Magnesia. 
tion Bischof, Chem. Geology, i. 428,) and those deposits which 
like travertines have been formed from subterranean springs.. 
In neither case however, should they be confounded with the 
tufaceous limestones mentioned in § 68. 
72. The union of the mingled carbonates of lime and magne- 
sia to form dolomite, is attended with contraction, which in case 
the sediment was already somewhat consolidated, would give rise 
to fissures and cavities in the mass. Should the dolomitic strata 
be afterwards exposed to the action of infiltrating carbonated 
waters, the excess of carbonate of lime and any calcareous fossils 
cavities both in magnesian and in pure limestones, not less than 
its deposition in veins and druses, indicates that dolomite 1s 
under certain conditions soluble. t ; 
e lowest temperature at which hydrous magnesian sedi- . 
ments may be transformed into magnesite and dolomite has yet 
to be determined. The requisite heat has however doubtless 
been attained by the accumulation of overlying sediments, in 
virtue of that law which causes the temperature to increase aS 
we penetrate the earth’s crust. This increase we may suppose 
with Mr. Hopkins to have been much more rapid im former 
epochs than at present.—(Geol. Journal, viii, 59, also Phillips 
| Manual of Geology, 609.) 
Conclusions. 
1. The action of solutions of bicarbonate of soda upon sea 
water separates in the first place the whole of the lime in the 
form of carbonate, and then gives rise to a solution of bicarbon- 
ate of magnesia, which by evaporation deposits hydrous mag- 
nesian carbonate. 
2. The addition of solutions of bicarbonate of lime to sulphate 
of soda or sulphate of magnesia gives rise to bicarbonates of 
these together with sulphate of lime, which latter may be 
thrown down by alcohol. By the evaporation of a solution con- 
taining bicarbonate of magnesia and sulphate of lime, either 
with or without sea salt, gypsum and hydrous carbonate of mag- 
nesia are successively deposited. 
3. When the hydrous carbonate of magnesia is heated alone 
under pressure it is converted into magnesite, but if carbonate 
