Tourmaline, fyc. within Mica and other Minerals. 267 



larger prismatic yellowish plates, growing fainter both in tint 

 and outline as they recede. In some cases the crystals are brown, 

 and in others beautifully dichroitic, being bright green and pink 

 in the different azimuths of polarized light. 



As considerable forces must have been in operation during the 

 production of these phenomena, we may expect to see the effects 

 of them upon the surrounding mica. We accordingly observe 

 the polarization produced by pressure round almost all of these 

 crystalline groups. Rents and other marks of violence are di- 

 stinctly seen in the mica, and cracks or luminous streaks often 

 occur in the tourmaline plates themselves. • I have observed, too, 

 in portions of the mica where I cannot find any cavities or cry- 

 stals, distinct luminous sectors of polarized light, which could 

 only be produced by a force emanating from their centre. This 

 force may have been that of gas discharged from some neigh- 

 bouring cavity, and driven by change of temperature to some 

 other part of the mica plate ; and in the following remarkable 

 phenomenon we may perhaps find some evidence in favour of 

 this opinion. 



Plates of mica contain many beautiful systems of Newton's 

 rings, occupying a circular space where the laminse have been 

 separated by some cause or other, and where of course there 

 must be either air or some gaseous body. The colours of the 

 first order are at the circumference of the circular space where 

 the laminae are in optical contact, and the higher orders of colour 

 extend towards and often to the centre of the space. Now it is 

 a curious fact, that wherever there is a cavity which has projected 

 its fluid and probably gaseous contents, it is situated in the cir- 

 cumference of one of these circular spaces. When two cavities 

 have been near each other, the circular spaces unite and lose 

 their form ; and when the cavities have been more numerous, 

 the circular spaces unite into very irregular shapes. That these 

 circular hollows or spaces between the laminse have been pro- 

 . duced by something which has issued from the cavity to which 

 they are so constantly related, cannot admit of a doubt. That 

 it has not been a fluid is evident, and therefore it must have 

 been a gas, which is either there still, or has escaped through 

 some minute openings between the lamina?, where optical con- 

 tact has been restored*. 



There are some specimens of mica in which the crystals of 

 tourmaline are large and opake, and exhibit phamomena which 

 I believe have not been recognised in any other mineral. The 



* A fluid even may have thus escaped, and the circular hollow remained 

 as before. In support of this opinion, see Edinburgh Transactions, vol. x. 

 p. 11, but especially vol. xvi. p. 13; or Phil. Mag. vol. xxxi. p. 101, 

 August 1847. 



