374 



THE GEOLOGIST. 



nection with the surveys for the previous year. These are cliielly analyses of 

 various dolomites and magnesian limestones, and experiments to serve to ex- 

 plain the conditions and mode of their formation. 



The term dolomite is employed to designate a mineral which, in its purest 

 state, is composed of equivalent weights of carbonate of lime and carbonate of 

 magnesia, these being in the proportion of 50 to 43, or in 100 parts of 54-35 

 carbonate of lime, and 45*65 of carbonate of magiiesia. This compound is dis- 

 tinguished from carbonate of lime by its superior density (3-85 to 3*90), by its 

 somewhat superior hardness, and by its being much less readily attacked by 

 acids than carbonate of lime. At ordinary temperatures 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 Hme, but slakes slowly and 

 with little evolution of heat. 



A portion of the magnesia in dolomite is often replaced by protoxyd of iron, 

 and more rarely by oxyd of manganese ; those dolomites containing carbonate 

 of iron being generally yellowish or reddish on their weathered surfaces from a 

 change of a portion of the iron into hydrated peroxyd, and those containing 

 carbonate of manganese become brownish-black on the exterior from a similar 

 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 rhomboliedrons 

 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 have in some cases a total thickness of 300 feet. These are im- 

 mediately overlaid by gypseous marls, to which succeed the limestones, gypsum, 

 and rock-salt of the Triassic series. Similar magnesian limestones occur in the 

 Devonian and Carboniferous formations in England and Russia ; 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 the Niagara 

 group, there occurs a remarkable deposit of dolomite. Dolomites also occur 

 interstratified with pure limestones in 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 hydraulic cement, also contains about 

 20 per cent, of magnesian carbonate. 



Beneath the oldest known fossiliferous 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 siliceous minerals. 



Ascending from the Permian, we find the Jurassic formation of the European 

 Alps containing immense masses of dolomite ; and the same occurs in the like 

 deposits of Erance and Germany. 



In Gascony, and in the Paris basin, 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 Erance. This latter, forming irregular 

 beds or masses several feet in thickness, reposes upon nummulitic limestone, 

 and is overlaid by the calcaire grossier. Its condition is that of an incoherent 

 sand, which consists, according to Damour, of nearly pui-e crystalline dolo- 

 mite, with a little bitumen and some quartzose sand. Between it and the 

 overlying fossiliferous limestone is a thin layer of yellowisli tufaceous cellular 

 limestone, which does not contain a trace of' magnesia. 



