266 Sir David Brewster on the Optical Phenomena of 



to the laminae of the mica. I have found, however, a few cases 

 in which the flat summit of the hexagonal prism is parallel to 

 the laminse. The crystallizations of quartz have also the axis of 

 the prism, or its hexagonal faces parallel to the laminse. 



The other crystals of tourmaline which I have discovered in 

 mica have a very different character; they have been formed 

 subsequently to the crystallization of the mica, and exist only 

 between its laminse. I have not been able to discover any cavi- 

 ties in mica containing fluids or gases, but I have found thou- 

 sands from which the fluids and gases have escaped, — the one 

 crystallizing into hexagonal plates of tourmaline, and the other 

 separating the laminse, or running between them, and carrying 

 along with it minute portions of crystallizable matter. 



The hexagonal crystals thus formed have their faces perpen- 

 dicular to the axis of double refraction, which is the axis of the 

 prism ; and what is peculiarly interesting, the fluid from which 

 they were formed has insinuated itself between several of the 

 laminse, and the different plates of tourmaline which they formed 

 have, of course, the sides of the hexagon incoincident. Sometimes 

 these crystals extend to different distances from the centre of the 

 original cavity, and are occasionally formed round it in a circular 

 group. See Plate III. fig. 1. 



The centre of the cavity from which these crystals have been 

 projected is occupied by a spherical group of granular or capil- 

 lary crystals, which is generally very opake, though such groups 

 sometimes exhibit, in particular spots, double refraction, and a 

 speck of light is occasionally seen through the centre of the 

 group. In some cases I have observed these very thin hexagonal 

 plates without this opake centre, and they have probably been 

 formed by a portion of the fluid projected to a distance between 

 faces of easy cleavage. The black spherical group already men- 

 tioned has its outward surface bristled with points, which are 

 the extremities of the crystals radiating from its centre ; and in 

 one fine specimen to be further described, it is surrounded with 

 a ring of less opacity than the nucleus, and analogous to what 

 is common in circular crystals. See fig. 1 . 



The thin plates thus formed between the laminse, whether 

 hexagonal or prismatic, are always of a faint brownish-yellow, 

 which at an increased thickness becomes green ; and so exceed- 

 ingly thin are these plates, especially those furthest from the 

 nucleus, that with a power of 400 it is often very difficult to see 

 their terminal lines. 



In order to convey an idea of these phsenomena, I have given 

 a drawing in fig. 1 of a very interesting one, where the prismatic 

 crystal nearest the black central group is a bright green in all 

 azimuths with polarized light, surrounded with three or four 



