296 



DESCRIPTIVE MINERALOGY. 



As this mineral closely resembles what has been termed hypersthene, I submitted it to a 

 chemical examination, chiefly for the purpose of determining the proportion of lime wliich it 

 contains ; this, so far as composition is concerned, being the chief difference between pyro- 

 xene and hypersthene. Tlie composition of the Lake George mineral is as follows : 



Silica, alumina and oxide of iron, 56.94 



Lime, 24.06 



Magnesia, 29.00 



The large proportion of lime places this mineral under pyroxene. The proportions of the other 

 constituents also agree with those found in some varieties of this species. 



Westchester County. Pyroxene, in various forms, is often met with in the dolomitic 

 beds of this county. The crystallized white variety, usually in six-sided tables, is found in 

 the quarries at Sing-Sing and elsewhere. The common variety has been found at Sparta. 

 The dolomite, moreover, often contains a substance with the cleavage of pyroxene, but which 

 is soft, and has a considerable admixture of carbonate of lime and magnesia. The external 

 characters strongly resemble those of white lamellar pyroxene ; but the effervescence which 

 is exliibited when the mineral is acted on by a dilute acid, shows that it is largely mixed with 

 one or botli the carbonates above mentioned. 



Dr. MacCulIoch long ago remarked that in Tirey (Scotland), crystals of tremolite and 

 sahlite are so confounded, that a single crystal sometimes contains both substances, as if 

 there was a transition between the two minerals.* I have observed the same thing in the 

 dolomitic limestones of Westchester and Putnam counties ; tremolite and pyroxene occurring 

 in immediate contact with, and indeed running into, each other. Veins of glassy tremolite 

 every where run through a dull white mineral, which has the cleavage of pyroxene. 



Pyroxene is also found near New-Rochelle ; but it is so closely associated with hornblende 

 and serpentine, that it is not easy to identify it. 



Fig- 222. Fig. 223. J ^ill only add, that Fig. 222 represents a crystal 



obtained by Dr. Emmons from Warren county. M or 

 M' on o 145° 9'. Fig. 223 is the form of crystalhzed 

 pyroxene from Bytown in Canada. The latter figure 

 is introduced on the authority of Prof. Shepard. M or 

 M' on X 134° 17' ; a; on a; 131° 6' ; r on t 106° 6' ; 

 M on M 131° 8'; a; on r 126° 36'. 



In Massachusetts, crystals of diopside are found in 

 limestone at Bolton, where the massive sahlite also oc- 

 curs. In Connecticut, also, there are several localities 

 of this mineral, as at Canaan and New-Milford. 

 The variety .Teffersonitc is found at the Franklin furnace, and elsewhere, in New- Jersey. 



M 



M 



Ml 



' Braester's Edinburgh Journal of Science. I. 225. 



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