ORDEB V. EHOMBOHEDBAL TALC-MICA. 



the prismatic system, though they have not yet been ob- 

 served. It occurs in granular compositions of a peachblos- 

 som-red colour, sometimes passing into several pale shades 

 of green. The specific gravity of Lepidolite is constantly 

 lower than that of the rest of the varieties of mica, having 

 been found in a very pure variety = 2*832. Its chemical 

 constituents are, according to WENZ, 



Alumina 33-61. 



Silica 49-06. 



Oxide of Manganese 1'40. 



Magnesia 0-41. 



Lithia 3-60. 



Potash 4-18. 



Fluoric Acid 3-45. 



Water 4-18. 



and a trace of oxide of iron. Before the blowpipe upon 

 charcoal, it intumesces, and fuses very easily into a trans- 

 parent globule. It has been found near Rozena in Mora- 

 via, and at Uto in Sweden, in primitive rocks. It is cut 

 into snuff-boxes and various ornaments. 



7. Clay-slate is in a very close connexion with the differ- 

 ent substances comprised within the name of Mica, and 

 uninterrupted transitions may be found from those posses- 

 sing low degrees of perfection in their cleavage into the 

 varieties of clay-slate. This substance, besides, cannot be 

 considered as a particular species, being generally not only 

 compound, but even mixed. Its varieties are in immediate 

 connection with mica-slate, which is again united to granite 

 by means of gneiss. The proportion of mica, which forms 

 one of the constituents of the latter, increases ; and while 

 the others diminish in quantity and size, so as to be not 

 observable at all in clay-slate, it imparts to this rock most 

 of the properties by which it is characterized. This is the 

 result of immediate observation, and hence may also be 

 explained the varieties met with in clay-slate, and which 

 have given occasion for designating some of them by parti- 

 cular names. Most of the other slaty rocks are more or 

 less allied to clay-slate. 



