126 



MINERALS AND GEOLOGY 



appears to occur sparingly amongst the Lake Superior traps ; and 

 occasionally in stalactitic coatings on the sides of cracks in some of 

 our limestone rocks, as in the township of Tring, and elsewhere, but 

 no very distinct or crystallized examples have as yet been found. 



90. Dolomite (Pearl Spar, Bitter Spar) : White, grey, brownish, 

 <fcc. Crystallization Hemi-Hexagonal, the crystals being, mostly, 

 rhombohedrons, the faces of which are often more or less curved. 

 Occurs also in lamellar cleavable masses, with cleavage angles of 

 106 15' and 73 45', and in granular and rock masses. H = 3.5 

 4.0 ; sp. gr. 2.8 2.95. BB, infusible, but becomes caustic. Slowly 

 soluble in cold acids, but rapidly dissolved witli strong effervescence 

 if the acid be gently heated. Essential composition : carbonic acid, 

 lime, and magnesia, forming carbonate of lime 54.35, carbonate of 

 magnesia 45.65, but small portions of the lime and magnesia are very 

 generally replaced by protoxide of iron and protoxide of manganese, 

 by which the cleavage angle is slightly altered. The various rhombo- 

 hedral carbonates, Calcite, Dolomite, Magnesite, Siderite, Rhodochro- 

 site, &c., merge, in fact, into each other by intermediate transitional 

 forms, to some of which distinct names have been given. The ferru- 

 ginous and manganesian dolomites become brown by weathering. 



Crystals and crystalline varieties of dolomite occur in many of the 

 metalliferous veins of Lake Superior and Lake Huron, and occasion- 

 ally in those of the Eastern Townships and other parts of Canada. 

 Groups of small rhombohedrons of more ov less pearly aspect, have 

 been obtained, more especially from the Wellington Mines on Lake 

 Huron. Small rhombohedral crystals occur also in cavities and on 

 the sides of cracks, &c., in many limestone strata : as in the dolo- 

 mitic limestones of the Calciferous formation near Prescott on the 

 St. Lawrence, and Rigaud on the Ottawa ; and also in the dolomitic 

 beds of the Niagara Formation in the vicinity of the Falls and else- 

 where. 



In the form or rock-masses, dolomite is of very common occur- 

 rence in many parts of Canada. A white fine-granular crystalline 

 variety, or dolomite marble, occurs in Laurention strata at Lake 

 Mazinaw, in the Township of Barrie, Frontenac County ; and many 

 of the marbles from the altered strata of the Eastern Townships are 

 more or less magnesian or dolomitic. In the unaltered Silurian 

 series, beds of dolomite, of a more or less sub-crystalline texture, 



