MAGNESIA. 253 



Pearl Spar — Chaux Carhonatce Fcrro-viagncsifi're of Haiiy. Occurs in obtuse rhombo. 

 liedrons with curved faces. It is while, 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 magncsian limestone of England is a dolomite with yellow or yellowish 

 brown colours. 



Gurhofiaii, or Gurliofite. A name whicli 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 flat, 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 1 8 . G 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 

 u'on 0.9 (Beudunt.). 



Rhombohcdral Dolomite — Carbonate of lime 68.00, carbonate of magnesia 25.50, carbonate 

 of iron 1 .00, water 2.00, argil 2.00 {Klaproth). 



Gurhojite — 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 soiiievvhat 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 Amenia, 

 Dover, Pawlings, Beekman and Fishkill (see page 68). 



^ ycw-Yojli Geological Reports^ 18^41. 



