PSEUDOMORPHOU3 MINERALS OF NEAV YORK. 247 



er than we might suppose them to be from theu' composition as 

 above stated, it is quite probable that during the process of sub- 

 stitution the cohesion of all the particles has been impaired so 

 as to bring about the apparent softness of the crystals, while the 

 particles are really so hard as they prove to be when subjected 

 to the action of the pestle. 



Changes in Pyroxenic Minerals. 



The changes about to be noticed are of an equally striking 

 character with those already described, but they often occur upon 

 a much larger scale, so much so that they are exhibited in rocky 

 deposits of considerable extent. 



The first example to which I shall here refer is the mineral 

 substance, or rock, which has been called by Dr. Emmons Rens- 

 selaerite, but which, for reasons to be presently given, I denomi- 

 nate steatitic-pyroxene. 



The description given by Dr. Emmons is as follows : 

 Hardness^3.5 to 4.0. Specific gravity, 2.874. Form, oblique 

 rhombic prism M on M=94° and 86° ; P on M=106' 30': (these 

 are not, however, by any means constant.) Color various, some- 

 times white or yeUowish-white, but often dark. Fracture un- 

 even. Before the blow-pipe it fuses with difficulty into a white 

 enamel. 



According to my analysis, the composition of a specimen from 

 Canton, in St. Lawrence county, furnished me by Dr. Emmons, 

 is as follows, namely : 



Silica, ......... o9.7o 



Magnesia, 32.90 



Lime, 1.00 



Peroxide of iron, ....... 3.40 



Water, 2.85 



The above results differ from those afforded by most of the 

 pyroxenes in the greater proportion of magnesia, which replaces 

 fime, and is isomorphous with it, and in a little excess of silica. 

 They are, how^ever, quite similar to those obtained from the 

 steatitic pyroxenes of Sahla, noficed by Beudant. These com- 

 pounds are described as containing variable proportions of for- 



