210 



Rev. S. Haughton on the Granites 



[Jan . 7 



III. White Mica, Lepidolite (Tremearne, near Treivavas Head). 



Silica, Si0 3 47-60 



Fluosilicon, SiF 3 5-68 



Alumina 27*20 



Iron peroxide' 5*20 



Manganese protoxide 1*20 



Lime 0'45 



Magnesia trace 



Potash 10-48 



Soda 0-72 



Lithia 1-14 



9967 



This lepidolite is white, pearly, and occurs in rhombic tables of 60° and 

 120°. Its oxygen ratios are, reckoning for the fluorine its equivalent of 

 oxygen, — 



Oxygen Ratios. 



Silica 24714 



Fluosilicon 1*747 



Alumina 12 /13 \ 



1*557 J 



26-461 8-9 

 4-8 



Iron peroxide 



Manganese protoxide . . 0*268^ 



Lime 0-127 | 



Magnesia [ 2 . g 2 VQQ 



Potash 1*776 j 



Soda 0-184 ' 



Lithia 0-62/ 



This corresponds with a theoretical formula, in which the oxygen of the 

 silica is to that of the bases as 3 : 2. 



The Black Mica of the Cornish granites seems to be more abundant than 

 the White Mica already described. I found a sufficient quantity of it at 

 Coron Bosavern, near St. Just, to enable me to make the following analysis : — 

 IV. Black Mica, Lepidomelane {Coron Bosavern, near St. Just). 



Silica (Si0 3 ) 39*92 



Fluosilicon (SiF 3 ) 3*04 



Alumina 22 '88 



Iron peroxide 15*02 



Iron protoxide 2*32 



Manganese protoxide .... 1 -40 



Lime 0*68 



Magnesia 1°07 



Potash 9*76 



Soda 0-99 



Lithia 1*71 



98-79 



