MINERALS AND GEOLOGY OF CANADA. 155 
§ 3. EFFERVESCING IN HEATED Hyproontoric AcID, BUT NOT 
AT ALL, OR ONLY FEEBLY, IN CoLD ACIDS. 
Dolomite.—White, grey, brown, &c., in lamellar and granular 
masses, and in rhombohedrons, closely resembling cale spar. H. 
3°5-4°0 ; sp. gr. 2°8-2'95. Infusible, but becoming caustic after 
ignition. LEffervesces feebly in cold, but vigorously io heated acids. 
Composition: carbonic acid, lime, and magnesia; or, carbonate of 
lime 54°35, carbonate of magnesia 45°65 ; a certain portion of the 
lime and magnesia being, however, generally replaced by protoxide 
of iron or manganese. Dolomite occurs (in small groups of rhombo- 
hedrons) amongst the copper ores of Lake Huron, and also in fissures 
and cavities in many of our limestone rocks, as at Niagara Falls and 
elsewhere. Many of our so-called limestones indeed, consist, in 
themselves, of dolomite, pure, or nearly so. Those of Galt, Guelph, 
&c., in Canada West, may be cited as examples. Others are dolo- 
mitic limestones, or mixtures of limestone and dolomite. Very few 
are wholly destitute of magnesia. Crystalline dolomite and dolomi- 
tic limestone, again, exactly resembling the ordinary crystalline 
limestones, occur in beds amongst the gneissoid rocks of the Lau- 
rentian Series, as at Lake Mazinaw, &c. These rocks come pro- 
perly under discussion in Part V. 
Magnesite —White, grey, &c., in granular-crystalline masses and 
in rhombohedrons, much like those of cale spar and dolomite.* H. 
35-45; sp. gr. 2°8-3:0. Infusible, but becoming’ caustic after 
strong ignition. Composition: carbonic acid 52°5, magnesia 47.5 ; 
but most specimens contain a small amount of carbonate of iron, 
lime, &c. Magnesia does not efferversce in cold hydrochloric or 
nitric acid, and dissolves but slowly in these acids under the aid of 
heat. In Canada, this mineral occurs in beds amongst the altered 
Silurian strata of Bolton and Sutton townships, in Canada East. 
(See analyses by T. Sterry Hunt in the Geological Report for 1856.) 
§ 4. FuSsIBLE. 
Fluor Spar.—Chiefly in cubes, either simple, or modified onthe 
edges and angles (Fig. 48, a to c). These cubical crystals break 
readily at the corners, owing to their strongly-pronounced octahedral 
* In calc spar, the cleavage rhombohedron measures 105° 5' over a polar edge; in dolomite, 
106° 15’; and in magnesite 107° 29’. In carbonate of iron (a mineral also belonging, with 
carbonate of manganese, carbonate of zinc, &c., to the natural group of Rhombohedral 
Carbonates), the same angle equals 107°. 
