Mineralogical Journey. 295 



breadth ; in general, however they do not occur in pieces above half 

 of this superficial size, and with a thickness of about one inch, — the 

 laminae composing them being straight and closely aggregated. 

 When held between the eye and the light, with the prismatic axis 

 towards the eye, the light transmitted is faint, and of a rich, red- 

 dish brown color : but on giving the crystal a revolution through 

 half a circle, more and more light is transmitted, until it is in 

 a position nearly perpendicular to the axis, when the light penetrates 

 the crystal most freely, and this, notwithstanding the quantity of mat- 

 ter through which it is obliged to pass, in the latter position, having 

 become considerably augmented, — the light continually changing 

 in color as well as in intensity, and finally becoming of a greenish yel- 

 low tinge. 



This Mica, although interesting on its own account, is still more so, 

 on account of the Tourmalines which it embraces ; and which are dis- 

 posed in long acicular crystals between its laminae. The largest of 

 these are about a quarter of an inch in thickness, and three or four 

 inches in length. They are, for the most part, of a leek green color and 

 transparent. They are rarely isolated ; but much more generally, 

 variously grouped. In a few instances, I have noticed two prisms 

 crossing each other (with mutual penetration) at right angles. The 

 most common composition, however, is that of several crystals di- 

 verging from a common point, all situated in the same plane ; and 

 that plane parallel with the cleavages of the Mica. Bundles of this 

 sort, forming 60°, 90°, and in some cases, 180° of a circle, are fre- 

 quent, — the latter aggregation forcibly reminding one of the repre- 

 sentation in a picture of the diverging rays of the rising sun, when 

 half above the horizon : especially, when the mass of Mica is held 

 between the eye and the light, owing to the greater freedom with 

 which the light flows through the crystals of Tourmaline than the Mica. 

 Indeed, we occasionally observe an exceedingly thin film of Cleave- 

 landite feldspar filling up the spaces intermediate between the fibres 

 of Tourmaline, and thus increasing the opacity in those directions. 



No gangue could be equally favorable for bringing into view these 

 delicate compositions of Tourmaline, as that of the Mica, since by 

 holding a mass of it between the eye and a strong hght, the crystals 

 may very easily be detected even when far below the surface ; and 

 nothing is easier, than to remove the superfluous laminae without in 

 the least disturbing their arrangement. 



