376 



THE GEOLOGIST. 



with the dolomite. Such examples occur in the Muschelkalk of Thuringia ; in 

 the bituminous magnesian rock of the Salzberger Alps ; in the brown-coal 

 deposits of Giessen ; in the Calciferous sand-rock of Lake Superior ; in the 

 variegated marls of the Keuper of Germany; and in a dark-grey rock of the 

 same formation at Tubingen, and at Solothurn, in the analysis of some of 

 which we perceive the transition from dolomites to a ferriferous magnesite. 



The question of the origin and formation of dolomites and magnesian lime- 

 stones has long been regarded as one of extreme difficulty, and among the 

 many solutions hitherto proposed, none appear to be satisfactory. We will, 

 however, review them briefly. 



It is a well known fact that carbonate of magnesia occurs in but very small 

 quantities in calcareous tufas and travertine. The same thing is true in the 

 case of limestones of organic origin, which are generally pure carbonate of lime. 

 Such limestones are made up for the greater part of the remains, often finely 

 comminuted, of corals and molluscs ; the living species of these are in 

 general nearly pure carbonate of lime, and recent corals usually contain less 

 than one per cent, of magnesian carbonate. Millepores are in like manner 

 in the greater proportion, constituted of carbonate of lime ; in some, however, 

 the carbonate of magnesia attams from 16-0 to 19"0 per cent, of the inorganic 

 portion. These millepores are often very abundant, and a non-magnesian species 

 forms beds on the northern shores of Erance that are worked for burning into 

 lime ; while a species, containing a large proportion of magnesia is very abund- 

 ant on the coast of Algiers. M. Damour has called attention to the part 

 these mellipores may play in the production of magnesian limestones in the 

 "Annales de Chimie et de Physique" (3rd series, vol. xxxii., p. 362). He, 

 however, describes them as dissolving readily in acetic acid, and which would 

 seem to indicate the absence of dolomite. 



The carbonates of lime and magnesia are both much more soluble in carbon- 

 ated water than the double carbonate, which, according to Bischoff, yields little 

 or no magnesia to a solution of carbonic acid. Grand] ean and, after liim, Sand- 

 berger, supposes that certain dolomites may have been formed from limestones 

 containing an admixture of carbonate of magnesia by the action of carbonated 

 waters, wliich might give rise to dolomite and a soluble bi-carbonate of lime ; 

 the iron and other metallic oxyd, being thus concentrated in the residue, their 

 presence in some dolomites would be explained (Liebig and Kopp, " Jahres- 

 berischt," 1848, English edition, vol. ii., p. 501). 



Eorchammer, in attempting to illustrate by experiment the formation of 

 dolomite, found that when a solution of bicarbonate of lime is mingled with 

 sea-water at a boiling heat, the precipitated carbonate of Hme carried down 

 with it 12-33 per cent, of carbonate of magnesia ; while, if carbonate of soda 

 be mixed with the solution of bi-carbonate, the proportion of magnesian car- 

 bonate in the precipitate may rise to 27'93 per cent. The amount of magnesia 

 according to his statements appearing to augment in proportion with the in- 

 crease of temperature. 



Haidinger long since endeavoured to explain the formation of dolomite and 

 its frequent association with gypsum, by supposing that a re-action between 

 carbonate of lime and sulphate of magnesia might give rise to sulphate of 

 lime and carbonate of magnesia. At ordinary temperatures, however, the 

 inverse affinities prevail. Mitscherhch found that a solution of gypsum 

 was completely decomposed after fourteen days contact with carbonate of 

 magnesia into sulphate of magnesia and carbonate of lime ; and the same de- 

 composition takes place when a solution of gypsum is filtered through dolomite. 

 Haidinger, however, conjectured that at an elevated temperature these affinities 

 might be reversed, and this has been confirmed by Morlot, who found that 

 when a mixture of one equivalent of crystaUized sulphate of magnesia, and two 



