1835.] and Porphyritic Greenstone Rocks. 447 



crystals of labradorite from Ajatskaja, are more easily 

 detached from their basis than usually happens. Their 

 specific gravity, Rose found, 2*730. When reduced to 

 powder they were decomposed with great difiiculty by 

 muriatic acid. When fused with barytes, Rose found in 

 these crystals, as in the other specimens of labradorite, 

 silica, alumina, some portions of iron, lime, and soda. 



The crystals of augite have the same form as when they 

 grow together ; they form four-sided vertical prisms of 88°, 

 with blunt and sharp lateral edges which have their extre- 

 mities terminated with an oblique four-sided prism of 120°. 

 They are cleavable in the direction of the faces of the 

 vertical prisms, and the truncatures of the lateral edges. 



The faces of cleavage are more distinct than those of the 

 combined augite crystals in basalt, but more indistinct than 

 in the crystals of hornblende. They are on the upper sur- 

 face partly smooth and splendent, partly indistinct and 

 vertically, streaked faintly. In the first case they are 

 strongly, in the last case feebly united with the basis, from 

 whence they fall out when the rock is fractured, and leave 

 behind an impression, from which the form of the crystals 

 can be distinguished. 



Their colour varies from grass-green to blackish-green ; 

 usually they are translucent. Before the blow-pipe small 

 fragments fuse on the edges with difficulty and frothing 

 into a green glass. In many instances, the crystals in the 

 combined augite porphyry have the form of augite, but 

 also two faces of cleavage which appear as sharp faces of 

 the sharp lateral edges of four-sided vertical prisms mea- 

 suring 88°, and cutting each other at an angle of 124° as 

 in hornblende. These are the crystals which Rose has 

 described under the name of uralite as occurring in UraL 

 He considers them as augite crystals, which retaining their 

 external form, have been converted into masses of horn- 

 blende. They possess a blackish-green colour ; the faces 

 of cleavage are faintly streaked vertically ; the surface of 

 the crystal is more strongly streaked and indistinct. Thin 

 fragments in the platinum forceps fuse before the blow- 

 pipe into a blackish-green glass, and more readily than 

 augite. They exist very distinctly in the augite porphyry 

 of Mostawaja, 35 versts to the north of Katharinenburg, 



