244 PSEUDOMORPIIOUS MINERALS OF NEW YORK. 



Island is little different in composition from true serpentine. 

 The mineral from Stony Point, and which has been referred 

 to kerolite, dilfers from serpentine in containing ten or twelve 

 per cent, of oxide of iron, a circmnstance which I attribute to 

 its being associated with a hornblendic rock. In the case of 

 Dr. Thomson's retinalite, the replacement of the soda for mag- 

 nesia, may on examination prove to be refenible to some accom- 

 panying mineral. 



It is perhaps almost unnecessary to state, that the hydi'atc 

 of magnesia is constantly undergoing change in consequence of 

 its combination with carbonic acid. The most striking exhibition 

 of this fact that I have observed, is in the town of Rye, in the 

 county of Westchester, where veins of hydrate of magnesia are 

 frequently invested with a white crust or powder of the carbon- 

 ate. Indeed, a similar change takes place in the cabinet. 



Changes in Hornblendic IVIaterials. 



In the town of Warwick, in Orange county, there occur in 

 magnesian limestone crystals of a gray color, having the form 

 and cleavage of hornblende. These crystals are sometimes bent 

 as if they had been fused, but they are sometimes very perfect, 

 and admit of measurement by the common goniometer. They 

 are peculiar, however, in having a soapy feel lilce steatite, and 

 they are often so soft that they can be easily cut with a knife. 



An analysis of one of these crystals gave the following results, 

 in one hundred parts, namely : 



Silica, 35.00 



Alumina, ........ 32.33 



LiiTie, 10.80 



Magnesia, 20.70 



Water, 1.17 



The principal difference in chemical composition between 

 this mineral and the several varieties of hornblende, is in the 

 larger amount of alumina, which has, in part, replaced the 

 silica. In a specimen of white tremolite, Bondsdorf found sLxty 

 parts of silica, twenty-four of magnesia, and nearly fourteen of 

 alumina. The larger proportion of alumina in tliis and other 



