250 DESCRIPTIVE MINERALOGY. 



Composition. Hydrocarbonate of Magnesia from Hobokei, N. J. — Carbonic acid 36.82, 

 magnesia 42.41, oxide of iron 0.27, water 18.53, silica 0.57 {Wachtmeister). 



Giobertite from. Baumgarten — Carbonic acid 50.75, magnesia 47.63, oxide of manganese 

 0.81, water I AQ i^Stromeyer). 



The Magnesian Marble of Nuttall, referred to this species by Dr. Thomson, belongs to 

 Dolomite ; and the Magnesite, often also placed with it, is a silicate of magnesia, and is 

 more nearly allied to serpentine than to carbonate of magnesia. The water, oxide of iron, 

 oxide of manganese and silica in the preceding analyses, are probably accidental ingredients. 



Geological Sitdation. This mineral, I believe, invariably accompanies magnesian rocks, 

 and in our localities it is always associated with some of the varieties of serpentine. 



localities. 



Richmond County. Crystallized carbonate of magnesia has been found in veins and in 

 cavities in serpentine or steatite. The crystals are delicately acicular, diverging or radiating 

 in groups, white and possessing a satiny lustre. It is also found in flesh-coloured crusts not 

 more than two lines thick, having a fohated or sparry structure. It is totally soluble in sul- 

 phuric acid. 



The pulverulent variety is also found in various parts of this serpentine range, although it 

 is seldom so abundant as at Hoboken in New-Jersey, where similar magnesian minerals occur. 

 In the purest specimens of this powder which I have obtained, there were portions of silica, 

 alumina, and a little oxide of iron. The proportion of water is very variable. It is probably 

 mixed with serpentine or magnesite. 



The occurrence of carbonate of magnesia on Staten island, was first noticed by James 

 Pierce, Esq.* 



Westchester County. Carbonate of magnesia, both compact and imperfectly crystallized, 

 is found associated with serpentine on the peninsula east of New-Rochelle. 



Two miles north of Rye, it is in the form of a white powder, and in concreted masses in 

 veins associated with magnesite or serpentine. In this case, the carbonate is probably the 

 result of the action of air upon the hydrate of magnesia, which is also found with the above 

 named minerals. 



* American Journal of Scienc*. I. 142. 



