of Crystallized Bodies. 63 



transverse section of the prisms does not exhibit the appear- 

 ance of a figure belonging to the pyramidal system, although 

 he has correctly ascertained this to be the class of forms to 

 which the crystals of peritomous Titanium-ore belong. The 

 composition is often repeated parallel to all the faces of the iso- 

 sceles four-sided pyramid ; a central individual, generally a 

 four-sided prism, thicker than the rest, is then surrounded by 

 numerous crystals respectively in parallel positions with each 

 other, branching off in four directions, each branch assuming 

 very often again the function of a central individual, which 

 supports new branches, fixed to it according to the same law. 

 The 11th Figure will convey a faint idea of an extremely ele- 

 gant group of this kind, of the same size as the sketch, in- 

 closed in rock crystal from Brazil, in the cabinet of Mr Allan. 

 It is impossible to give it the freshness and lustre which it de- 

 rives from the contrast of its almost metallic appearance, and 

 dark colour, with its transparent colourless matrix. A compo- 

 sition of acicular crystals of the species, disposed on a plane 

 surface, produces the well known reticulated appearance, in 

 allusion to which Saussure adopted the name of sagenite, from 

 eay^rt, a net, for this species, which formerly had been con- 

 sidered as a variety of Schorl.* 



The mineral most nearly allied to the two preceding species 

 in regard to its forms in general, but more particularly for its 

 regular compositions, is the pyramidal Manganese-ore of Pro- 

 fessor Mohs, a species but of late more accurately described 

 by mineralogists, though it seems to have been known at an 

 early period of mineralogical inquiry. According to Mohs,-}* 

 the fundamental form P (Fig. 1 2.) of it is an acute isosceles 

 four-sided pyramid of 105° 25', 117° 54', (by approximate 

 measurement.) This pyramid is cleavable with considerable fa- 

 cility in the direction of P — <x> , the plane perpendicular to the 

 axis, and likewise, though less distinctly parallel to Pand parallel 

 to P — 1 , another isosceles four-sided pyramid, which replaces 

 the terminal edges of P, and the angles of which are 114° 51' 

 and 99° 11'. Messrs Brooke and Phillips found the angles of 



* y°y a g e dant !es Alpes, tome vii. sect. 1894. 

 t Treatise on Mineralogy, transl. vol. ii. p. 4.1 G. 



