Determination of Diallage. 135 



HAUY quotes among the peculiarities of certain joints surnu- 

 tnerctires, that they contain films of heterogeneous minerals, a 

 property in which they likewise agree with the above-mentioned 

 faces of composition. Thus the black Hornblende from the Kien- 

 nerud mine, contains lamina? of Talc (prismatic Talc-mica) ; many 

 varieties of the green contain laminae of paratomous Augite- 

 spar. 



If we try to determine the specific gravity of two varieties of 

 Diallage of a different colour, the grass-green and those of a dark- 

 er shade, which, at the same time, are found to occur in one iden- 

 tical specimen, we meet very often with a difference amounting 

 to, or even exceeding, one-tenth. Thus, irr one instance, where 

 both the varieties were remarkably distinct, I found the specific 

 gravity of the green, at a temperature of 15 centigr. (59 Fahr.) to 

 be 3.129 (agreeing with SAUSSURE'S statement of 3.140), whilst 

 that of the greenish-black was only zi 3.007, a difference, the 

 cause of which I was naturally led to inquire into more close- 

 ly. When examining the darker parts, and comparing them 

 with the green, I found that in the latter there were thin la- 

 minae of another mineral, interposed between the particles 

 of hornblende, the face in which they meet being that of 

 P r -j- oo . These lamina? are not at all cleavable in the direction 

 of the faces of (Pr -f oo ) 3 = 124 13', but they may be cleaved 

 in other directions, intersecting the axis of that prism under ob- 

 lique angles, the latter faces of cleavage being much inferior to 

 the former, both in lustre and continuity. In several of the va- 

 rieties of this mineral from the Bacher, the interposed lamina? 

 have increased in thickness, so far that the hemiprismatic Au- 

 gite-spar, entirely disappears from the composition, and we are 

 led to a mineral, which, with all its peculiarities, except the co- 

 lour of grass-green, we must acknowledge to be a variety of pa- 

 ratomous Augite-spar. Now, we find the specific gravity of this 



