96 PHYSIOGRAPHY. CLASS II. 



synonymy of the present species with any degree of cer- 

 tainty, since in these the determination of the species does 

 not always rest upon sure characters, but very often upon 

 such uncertain ones as colour, composition, lustre, &c., 

 and upon the chemical mixture, which in the present spe- 

 cies does not seem to have yet been brought to sufficient 

 clearness. The massive varieties, of granular composition, 

 sometimes but slightly coherent, and of white colours, have 

 been called Dolomite. Rhomb-spar and Bitterspar, are the 

 names of crystallised or large grained and easily cleavable 

 varieties, chiefly of greenish colours. These, however, in- 

 clude in the older systems the varieties of brachytypous 

 Lime-haloide, though different from them not only in the 

 admeasurement of their angles, but also in hardness and 

 specific gravity. Brown-spar^ with its subdivisions, the 

 foliated and fibrous brown-spar,, comprehends those varieties, 

 in which the lustre approaches somewhat more to pearly, 

 and whose colours incline to red or brown. Yet prismatic 

 Lime-haloide in the latter, and macrotypous Parachrose- 

 bary te, or even rhombohedral Lime-haloide in the former, 

 have often been described as brown-spar, and are frequent- 

 ly confounded with each other in collections. In most cases 

 specific gravity will be found a decisive and convenient 

 character, for distinguishing these different species. 



2. It is equally difficult to judge properly of the chemical 

 composition of the present species. It contains carbonate of 

 lime and carbonate of magnesia, but the relative quantity 

 of the two seems not to be exactly the same in all varieties. 

 From several analyses by KLAPROTH, this proportion is 

 nearly as 54-18 : 45-82, upon which supposition the com- 

 position will be expressed by Ca C a + Mg C 2 , being one 

 atom of each, which corresponds to 30-56 of lime, 22-18 

 magnesia, and 47-26 carbonic-acid. Several analyses of 

 brown-spar give very similar results, others deviate more 

 or less from them. In general Brown-spar seems to con- 

 tain more oxide of iron and manganese than either Dolo- 

 mite or Rhomb-spar. The varieties of the present species 



