Tourmaline } fyc. within Mica and other Minerals. 269 



is opake ; but at less thicknesses it has a brownish transparency, 

 becoming almost perfectly transparent at thicknesses which do 

 not seem to exceed the 2000dth part of an inch. In fig. 3 I 

 have given a drawing of an opake group executed for me with 

 minute accuracy by my celebrated friend M. Haidinger of 

 Vienna, during his residence in Edinburgh. The transparent 

 groups are much more beautiful than the opake ones, the cry- 

 stalline ramifications having the most diversified forms, resem- 

 bling often regular organizations. 



When the mica is removed from above the titanium, so that 

 only an exceedingly thin film of it is left, the reflected light is 

 extremely brilliant, and consists of the most splendid colours. 

 These colours, which have always the form of the titanium, are 

 those which are produced by the thin film of mica which covers 

 the titanium, and are not produced, as has been supposed, by a 

 vacuity in the mica. 



In some specimens of mica from Bengal, the imbedded tita- 

 nium is spread out in a very irregular manner from a nucleus, 

 sometimes having the form of a thin film ; sometimes of oriental 

 characters; and sometimes it is disseminated in grains so ex- 

 tremely minute, that the flame of a candle seen through it is sur- 

 rounded with a halo of five or six perfectly-formed coloured rings. 



3. Distribution of Quartz in Mica. 

 In mica from various localities I have found large crystalliza- 

 tions of quartz, the quartz replacing the mica. I have never 

 even once met with a regular crystal of quartz ; and what is 

 curious, all the crystalline masses of it which I have examined 

 have their axis of double refraction in the plane of the lamina? of 

 mica. In some very large specimens of Bengal mica given to 

 me by Mr. Swinton, I have found layers of quartz, several inches 

 in area, and about the 200dth of an inch thick. The two sur- 

 faces of the plates are exceedingly unequal and corrugated, owing 

 to the circumstances under which they were formed, but they 

 possessed regular double refraction, and gave the colours of 

 polarized light. 



4. Distribution of Titanium in Amethyst. 

 While examining many years ago, along with the late Marquis 

 of Northampton, several bags of amethyst which had been im- 

 ported into Scotland from the Brazils, we were surprised to 

 observe a number of fine pyramidal crystals, which seemed to 

 have a powdery matter distributed through their mass. Upon 

 more narrowly examining these crystals, I found that this dust 

 formed an inner pyramid, all the faces of which were parallel to 

 the faces of the pyramid of amethyst. When two parallel faces 

 were ground upon the pyramid, and perpendicular to its axis, 



