THE GEOLOGIST. 
with the dolomite. Sucli examples occur in the Muschelkalk of Tliuriiigia ; in 
tlie bituminous magncsian rocK of the Salzberger Alps ; in the browu-coal 
deposits of Giessen ; in tlie Calciferous sand-rock of Lake Superior ; iu the 
variegated marls of the Keuper of Germany ; and in a dark-grey rock of the 
same formation at Tubingen, and at Solothurn, in tlic analysis of some of 
wliich we perceive the transition from dolomites to a ferriferous magnesite. 
The question of the origin and formation of dolomites and magnesiau 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 tliat carbonate of magnesia occiirs in but very small 
quantities in calcareo\is tufas and travertine. The same thing is true in tlie 
case of limestones of organic origin, which are generally pure carbonate of lime. 
Sucli 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 Kmc, and recent corals usually contain less 
than one per cent, of magnesiau carbonate. MUlepores are in like manner 
in the greater proportion, constituted of carbonate of lime ; in some, however, 
the carbonate of magnesia attains from 16 0 to 19 0 per cent, of the inorganic 
portion. These mdlepores are often very abundant, and a non-magnesian species 
forms beds on the northern shores of France that are worked for burning into 
bine ; Wilde a species, contauimg 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 magncsian limestones in the 
"Annales de Chimie et de Physique" (3rd series, vol. xxxii., p. 362). He, 
however, describes them as dissolving readUy 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 carl)oiiate, whicli, according to Bischoff, yields little 
or no magnesia to a solution of carbonic acid. Grandjean and, after hmi, Sand- 
berger, supposes that certain dolomites may have been formed from limestones 
contauiing an admixture of carbonate of magnesia by the action of carbonated 
waters, which might give rise to dolomite and a soluble bi-carbonate of lime ; 
the iron and other metallic oxyd, bemg thus concentrated in the residue, their 
Eresence iu some dolomites woidd be explained (Liebig and Kopp, " Jahres- 
erischt," 1848, EngHsh edition, vol. ii., p. 501). 
Forchammer, 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 boding heat, the precipitated carbonate of lime carried down 
with it 12 33 per cent, of carbonate of magnesia; wlulc, if carbonate of soda 
be mixed with the solution of bi-carbouate, the proportion of magncsian 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 tne 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 prevad. Mitscherlich 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 aflimties 
might be reversed, and this has been confirmed by Morlot, who found that 
when a mixture of one equivalent of crystallized sulphate of magnesia, and two 
