SILICA. 307 



Westchester County. Fine specimens of tremolite, of considerable size, are found in 

 veins in the dolomitic limestone. Thus it occurs at the Eastcliestcr quarries, and at Hastings 

 in the northern part of the county. It is often associated with white; pyroxene, and e.xhibits 

 the peculiarities of the same mineral in Putnam and New-York counties. 



The same variety is found at the marble quarries at Sing-Sing ; and indeed, wdierever the 

 white limestone occurs, this is one of the imbedded minerals. 



In the white limestone near Yonkers, the veins of tremolite are sometimes glassy, while it 

 is mixed with pyroxene of a dull white, and the whole is every where penetrated by the dolo- 

 mite, as is evident from the effervescence which is exhibited when the specimens are sub- 

 jected to the action of acids. Sometimes these veins traverse a soft serpentine or soapstone. 



Near West-Farms, the glassy variety is found in quartz. Tremolite also occurs in the ser- 

 pentine on the peninsula east of the village of New-Rochelle, and at Rye. At the preceding 

 locality, there are large bowlders, containing abundance of long six-sided prisms of black horn- 

 blende, without regular terminations. These have a high lustre. Small crystals, usually 

 imperfect, are found with black mica in quartz on the Crolon aqueduct, two and a half miles 

 north of the village of Yonkjis. 



APPENDIX. 



Hornblende j^seudomorphs. In the town of Warwick, in Orange county, there occur, in 

 magnesian limestone, crystals having the form and cleavage of hornblende, but which are 

 peculiar in having a soapy feel like steatite, and are often so soft as to be easily cut with a 

 knife. They are usually in the form of six-sided prisms with two terminal planes, resembling 

 Fig. 243, or in that of Fig. 247 ; but they are sometimes bent and distorted, as if they had 

 been subjected to fusion. The measurements sometimes vary slightly from those of horn- 

 blende, but the general form is always the same. Before the blowpipe, they melt with some 

 difficulty into a white glass. 



An analysis of one of these crystals gave the following results in 100 parts, viz : 



Silica, 35 . 00 



Alumiiia, 32 . 33 



Lime, 1 . 80 



Magnesia, 20 . 70 



The principal difference in chemical composition between this mineral and several varie- 

 ties of hornblende, is in the larger amount of alumina which has in part replaced the silica. 

 In a specimen of white tremolite, Bonsdorf found silica 60, magnesia 24, alumina nearly 14. 

 The large proportion of alumina in this and other cases is ascribed by Bcudant to tiic sub- 

 stances which arc associated with the hornblende, as spinelle, wcrncrile, etc. 



The limestone in which this soft hornblende occurs, is not, I believe, remarkable for the 

 abundance of spinelle ; but a variety of mica, the silvery white, which contains from 34 to 37 

 per cent, of alumina, is disseminated through it. 



