122 MINERALS AND GEOLOGY 



as lime, magnesia, and the like. In acids these compounds become 

 decomposed with strong effervescence, the latter effect being due to 

 the liberation of their carbonic acid, but in many cases the application 

 of heat is required to develop the phenomenon. j/"The substance, in 

 the form of a small particle or two, or in powder, may be conveniently 

 examined, with some diluted hydrochloric acid, in a test-tube or deep 

 watch-glass supported over a common spirit-lamp. (See under 

 " Action of Acids," in Part I.) The carbonates, also, when fused 

 with borax before the blowpipe, dissolve with marked effervescence, 

 their carbonic acid being driven off. Up to the present time, only 

 eight carbonates have been recognized amongst Canadian minerals, 

 and five of these are altogether unimportant. We arrange the whole, 

 therefore, simply under two groups : Anhydrous and Hydrous Car- 

 bonates, respectively.] 



(1) GROUP OF ANHYDROUS CARBONATES. 



[The anhydrous carbonates belong properly to several distinct 

 groups : more especially to a Rhombohedral Group, typified by calcite 

 or ordinary calc spar, and including dtlomite, magnesite, siderite, &c.; 

 and a Prismatic Group of Rhombic and Monoclinic species, typified 

 by Arragonite, and including carbonates of lead, baryta, strontia, &c. 

 But in Canada, the latter group is only represented, and that 

 obscurely, by arragonite or prismatic carbonate of lime.] 



88. Calcite or Calc Spar (Rhombohedral Carbonate of Lime) : 

 White, grey, reddish white, greenish-white, yellowish-white, red, 

 black, &c., but mostly colourless or lightly tinted. Hexagonal or 

 Hemi-Hexagonal in crystallization, with strongly pronounced rhorn- 

 bohedral cleavage. The crystals are chiefly obtuse and acute 

 rhombohedrous (Figs 72 and 74) ; combinations of a rhoinbohedroii 

 and hexagonal prism, the so-called "nail-headed" crystals (Fig. 73); 

 and more or 

 less acute scal- 

 enohed ro n s 

 (Fig. 75), the 

 mineral in the 

 latter form be- 

 ing often popu- 

 larly known as FlG . 72 . FIG. 73. FIG. 74. FIG. 75. 

 " dog-tooth spar." Calcite occurs also abundantly in lamellar, 



