MAGNESIA. 
253 
Pearl Spar—Chaux Carbonatee Ferro-magnesifere of Hairy. Occurs in obtuse rhombo. 
hedrons with curved faces. It is white, reddish white, red, brown and pearl grey ; has gene¬ 
rally a pearly lustre ; is translucent, and rather harder than calcareous spar. 
Brown Spar. This name has sometimes been applied to those varieties which have a red¬ 
dish brown colour, and do not exhibit curved faces. 
Dolomite. This occurs massive, and sometimes has a slaty structure. It is made up of 
small granular concretions, which are sometimes so loosely aggregated as to separate by the 
mere pressure of the fingers. The colour is usually white, but it sometimes has a tinge of 
blue or grey. The magnesian limestone of England is a dolomite with yellow or yellowish 
brown colours. 
Gurhojian, or Gurhofite. A name which was given to a variety found near Gurhof in Lower 
Austria. It has a snow-white colour, and is very compact; the fragments, which are sharp, 
are translucent on the edges. Fracture Hat, conchoidal. 
Composition. This mineral is composed cf the carbonates of lime and magnesia, with 
minute quantities of oxide of iron, manganese, etc. The proportions of the principal ingre¬ 
dients, however, are liable to some variation. In eight analyses by Berthier, the proportion 
of lime is from 10.7 to 44.0 per cent.; that of magnesia, from 7.7 to 22.4; of carbonic 
acid, from 18.6 to 47.0. The protoxides of iron and manganese, when present, do not exceed 
1.8 per cent. 
Compact Dolomite —Carbonate of lime 59.40, carbonate of magnesia 39.70, peroxide of 
iron 0.9 ( Beudant). 
Rhombohedral Dolomite —Carbonate of lime 68.00, carbonate of magnesia 25.50, carbonate 
of iron 1.00, water 2.00, argil 2.00 {Klaproth). 
Gurhofite —Carbonate of lime 70.50, carbonate of magnesia 29.50 {Klaproth). 
Geological Situation. The white granular or saccharoid variety is found in beds in pri¬ 
mary rocks ; the crystallized and cleavable varieties occur in transition and secondary lime¬ 
stones. In New-York, the latter are principally met with in calciferous sandstone, in the 
Lockport limestone and in the Hudson river slate. 
LOCALITIES. 
Columbia County. The variety called Brown Spar, occurs somewhat abundantly in a 
narrow range in this county. It is associated with quartz in the slate. It seems here to con¬ 
tain an unusually large proportion of oxide of manganese ; and by its decomposition, it gives 
rise, as Mr. Mather thinks, to the occurrence of Wad, which is here quite common.* 
Dutchess County. The variety dolomite is found in vast beds in the towns of Amema, 
Dover, Pawlings, Beekman and Fishkill (see page 68). 
K ' New- York Geological Reports, 18S9. 
