Mincralogy and Petrography. "965 
- 1887) 
_ botryogen, from Fahlun, in Sweden, has been examined by 
= Hockauft The mean of two analyses yielded : 
So, FeO, FeO MnO ‘CaO MgO. H,O 
36.94 16.38 2.23 1.93 0.90 7.63 33-99» 
corresponding to the formula FeSo, + [Fe,(So,); + (FeO),So,]. 
botryogen of commerce was also analyzed, and found hot 
_ to have the composition of the genuine mineral. 
Crystallographic News.—In an article on hemimorphic py- 
rargyrite twins from Andreasberg, Schuster? discusses the classi- 
_ fication of twinned crystals, and defines them as symmetrical or 
_ unsymmetrical. The symmetrical ones he subdivides into those 
hemitropically developed and those which show no hemitropism. 
The hemitropic symmetrical twins include those of holohedral 
_ and of certain hemihedral and hemimorphic minerals. Theun- 
Symmetrical and the non-hemitropic symmetrical classes embrace 
the twins of those minerals which crystallize in the inclined and 
‘rapezohedral-hemihedral divisions of the different systems, and 
also certain hemimorphic minerals. Examples are cited to show 
_ tile application of the terms of this classification to the descrip- 
ee of complicated twins. As the result of new investigations 
; sai the Bertrand? microscope and lenses, Des Cloizeaux* finds 
ws the mineral which bears his name crystallizes in the ortho- — 
system. Cathrein’ describes crystals of orthoclase 
anite at Predazzo, which have their largest 
Ekoa i 
Met ee 2 Ib., p. 117. 
L£ keso "iin. de France, 1885, pp. 29, 377, and 426. 7. 
ASE, xii. p. 178. s Ib., xii. p. 34. ê Ib., xi. p.608. 7 Ib., xi. p- ay 
