CASSITERITE CRYSTALS. 67 



The various members of all the twin groups observed 

 tend, as far as may be judged from their development as 

 twins, to assume the form shown in the single crystal (figs. 

 1, 2), Thus there is but one crystal type present, of which 

 ■the single crystal is typical. 



This type crystal is the only untwinned one observed; it 

 is very small, measuring approximately *75 X *75 x # 6 mm. 

 in the direction of the a, b and c axes respectively. It has 

 a stout prismatic habit, and is terminated by a large basal 

 pinacoid. All the prisms present with the exception of 

 the a face, are strongly striated, the striations, running 

 parallel to the c axis, being due to oscillatory combination 

 between the various forms present, the first order prism 

 m (110) being the predominating form. The a face is a 

 well-marked smooth form giving excellent reflections. The 

 prisms present are a (100), m (110), B (570), r (230), h (120) 

 and (540) — the last mentioned one being listed by Borg- 

 strom as a doubtful form. The h face is a remarkably 

 narrow and frequently occurring form. Owing to the 

 position and similarity in size of the two prominent prisms 

 a (100) and m (110), the crystal assumes a roughly octagonal 

 shape as seen in the orthographic drawing (fig. 1). This 

 is still more emphasized by the presence of the large c face 

 regularly bounded by the s and e faces. Other forms 

 represented are the primary pyramid s (111), a prominent 

 face with a bright smooth surface and yielding good reflec- 

 tions; the second order pyramid e (101), also a smooth face 

 giving good reflections and rectangular in shape; z (231) a 

 remarkably small form giving only faint signals, and c (001) 

 the basal pinacoid, a large pitted and lustreless face, yield- 

 ing very distorted reflections. 



The main features of this crystal — striated strongly 

 developed prism zone; smooth a face, giving excellent reflec- 

 tions (which are of great assistance in mounting the 



