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Tllli (iKOLOGIST. 
ncction witli Uie survey, I'or tlie previous year. These arc chiefly analyses of 
various dolomites aud inngncsian liiuestoucs, aud experiments to serve to ex- 
plain the conditions and mode of their formation. 
The term dolomite is emjiloyerl to designate a mineral which, m its purest 
state, is composed of equivalent weights of carl)onate of lime and carbonate of 
magnesia, these bemg in the proportion of 50 to 42, or in 100 parts of 54'35 
carbonate of lime, and 45'05 of carbonate of magnesia. Tliis compound is dis- 
tinguished from carbonate of bmc l)y its superior density (2-8.5 to 2'90), by its 
somewhat sn]icrior hardness, and by its being nuich less readily attacked by 
acids than carbonate of lime. At ordinary tcm])eratures it does not perceptibly 
effervesce with nitric or muriatic acids unless reduced to powder. Calcined it 
is said to yield a stronger mortar than ordinary Ihne, but slakes slowly and 
with little evolution of heat. 
A portion of the magnesia in dolonjite is often replaced by protoxyd of iron, 
and more rarely by oxyd of manganese ; those dolomites coutainhig carboimte 
of iron bemg generally yellowish or reddish on their weathered surfaces from a 
change of a portion of the iron into hydratcd peroxyd, and those containing 
carbonate of manganese become brownish-black on the exterior from a simOar 
cause. 
Besides the crystallized dolomites which occur in veins and cavities in 
various rocks, and have received the names of hitter-spar and pearl-spar (the 
latter in allusion to the pearly lustre of the faces of the rhombohedrons 
which are generally curved), we find this double carbonate forming great beds 
of a rock which is also known by the name of magnesian limestone. The 
yellow magnesian limestones of the Permian system in England are those best 
known, and Iiave in some cases a total thickness of 300 feet. These are im- 
mediately overlaid by gypseous marls, to which succeed the lunestones, gypsum, 
and rock-salt of the Triassic series. Simdar magnesian limestones occur m the 
Devonian and Carboniferous formations in Enghuid and liussia ; and, descending 
in the geological series, we find in the Saliferous group of Western Canada and 
New York-beds of dolomite with gypsum. Immediately below, in tlic Niagara 
group, there occurs a remarkable dejjosit of dolomite. Dolomites also occur 
mterstratified with pure limestones m the Hudson River group ; while in 
Michigan, Iowa, and Minnesota, the calcareous strata overlying the Potsdam 
sandstone, and corresponding to the Calciferous sand-rock, are highly mag- 
nesian, often constituting true dolomites. 
Thin layers of dolomite are also met with among the limestones of the Chazy 
division in the island of Montreal. The argillaceous limestone from this for- 
mation at Hull (Canada), employed as an hyckauUc cement, also contains about 
20 per cent, of magnesian carbonate. 
Beneath the olclest known fossdiferous rocks of Canada, among the lime- 
stones of the Laurentian series, are great beds of dolomite, sometimes fer- 
riferous, and often containing serpentine and other sdieeous minerals. 
Ascending from the Permian, wc find the Jurassic formation of the European 
Alps containing immense masses of dolomite ; and the same occurs in the like 
deposits of France and Germany. 
In Gascony, and in the Paris liasin, dolomites occur in the Cretaceous for- 
mation ; and there is a deposit of dolomite in the Tertiary strata of Pont St. 
Maxence, in the valley of the Oise, in France. This latter, fonning irregular 
beds or masses several feet in thickness, reposes upon munmulitic limestone, 
and is overlaid by the culcaire grassier. Its condition is that of an incoherent 
sand, which consists, accordhig to Damour, of nearly pure crystalline dolo- 
mite, with a little bitumen and some quartzose sand. Between it and the 
overlying fossibfcrous limestone is a thin layer of yellowisli tufaceous cellvdar 
limestone, wliich does not contain a trace of magnesia. 
