280 Characters of the Cyanite and Sillimanite. 
The Sillimanite occurs in lengthened rhombic prisms, 
never terminated with regularity, or modified upon their 
edges: It does not admit of cleavage parallel with the sides 
of this prism, but has a terminal cleavage, thus presenting us 
with an oblique rhombie prism, for its primary form, whose 
anglesare, —S—- 
M on M’ - - 108° 
M or MW on P - 344 
planes of crystallised mica 
 Froma ison of these statements, it is almost unne- 
-eessary to remark that the crystalline structure of the Sil- 
limanite is wholly incompatible with that of the Cyanite. 
The measurements which I have given above of the Silli- 
manite, it will be observed, differ slightly from those obtained 
by Mr. Bowen. He found the inclination of the lateral 
planes to be about 106° 30’ and 73° 70’, and that of the 
base to the axis of the prism 113°. My results, however, I 
flatter myself, will be found to be not far from the truth, 25 
they are the f numerous trials. For, it may be re- 
