MAGNESIA. '255 



Other specimens have fibres of asbestus running through them, and they sometimes have a 

 bluish tint, and contain crystals of bronze-yellow iron pyrites. These specimens resemble 

 the picrolitc. When dissolved in muriatic acid, there is usually a portion of silica unacted 

 on, probably from the serpentine which is mechanically mixed with it. 



Richmond County. A mineral closely resembling the gurhofite is found in a stratum at 

 the Quarantine. It is white, compact, has a flat conchoidal fracture, and is so hard as some- 

 times to strike fire with steel. It is exceedingly tough and difficult of solution, except wlien 

 m very fine powder. Specific gravity 2.712. An analysis of a specimen obtained near the 

 Pavilion at the Quarantine, gave the following results, viz : 



Carbonate of lime, .52.75 



Carbonate of magnesia, 42. 25 



Insoluble matter, chiefly silica, 5. 00 



Oxide of iron, traces. 



It differs from the hard carbonate of magnesia found at Hobokcn in New-Jersey. 



Dolomite, in the form of imperfect and variously aggregated rhombohedral crystals, is also 

 found near the locality of the preceding variety. It is of a bluish white colour, and is found 

 in a vein associated with a dark coloured argillaceous substance apparently the result of de- 

 composition. 



St. Lawrence County. Four miles from Hammond, there is a ledge of dolomite, or 

 coarsely granular magnesian limestone. It is greyish white, and contains small plates of yel- 

 low mica and particles of graphite.* 



Brown spar of a yellowish brown colour, and closely resembling that from Hobokeii, N. J., 

 is found in nodules in limestone near Gouverneur. 



At the Parish ore bed there are found curved crystals of brown or pearl spar, which pass 

 uUo carbonate of iron. They are usually small, and arc associated with calcareous and heavy 

 spar. 



Warren County. At Glen's-Falls, rhomb spar is found in well defined glistening crystals, 

 in the dark coloured limestone. Pearl spar also occurs in minute white and curved crystals, 

 with calcareous spar and crystallized quartz, in the calciferous sandstone on Diamond island, 

 and at Diamond point, Lake George. The compound form Fig. 128, is quite common. The 

 geodes which contain these crystals are abundant, but seldom large. 



Westchester County. The granular dolomite, commonly called irhile 7narbJe, is abun- 

 dantly diflused. Quarries have been opened at Hastings, Sing-Sing, Eastchcster, &c., and 

 have long been profitably worked. For a more detailed description of these marbles, the 

 reader is referred to page 71. 



The rounded masses called septaria, and which are found in the counties of Schoharie, 



♦ J. Finch. Ammam Journal of Science. XIX. 220. 



