1& 



TAB. CCCXCL 



S I L E X Mica. 



Brown and Black Mica. 



1 his substance may be found of almost all colours, except 

 blue; at least there have not yet been observed any speci- 

 mens of that colour. The darkish Irown variety, figured at 

 the upper part of the plate, is not uncommon in small specks 

 or specimens in Granite, but rather more rare of a larger 

 size : the group is not as usual convenient for dividing into 

 plates, but is seemingly a congeries of scaly crystals ar- 

 ranging in a peculiar angular manner, sometimes according 

 with the angles of the primitive rhomb, meeting and min- 

 gling so as to become quite zigzag, while the accumulated 

 planes are diverging from different centres in confused order. 

 Its colour gives it rather the appearance of thin horn, but 

 it has a more glassy lustre. 



The lower specimen is rather rare, being of a true shining 

 black, i. e. black without any mixture of brownish, blueish, 

 or any other colour : it has a more glassy lustre than the 

 upper specimen. The congeries of plates on one side of the 

 specimen are small and on the other larger, more continu- 

 ous, partly undulated, and altogether less orderly, and not re- 

 gularly enough grouped to need particularizing. 



Common brown Mica of the shops is often two feet or 

 more square, and when used very thin for microscopes ap- 

 pears nearly colourless. 



