﻿Royal Society. 307 



tially from the white mica of Leinster and Donegal, and proves to be 

 a variety of lepidolite. 



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



Silica (Si0 3 ) 47*60 



Fluosilicon (SiF 3 ) 5-68 



Alumina 27*20 



Iron peroxide 520 



Manganese protoxide .... 1'20 



Lime 0*45 



Magnesia trace 



Potash 10-48 



Soda . 0-72 



Lithia 1-14 



99-67 

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

 and 120°. Its oxygen-ratios are, reckoning for the fluorine its equi- 

 valent of oxygen : — 



Oxygen-Ratios. 



IP 3 ;. •'••• 2 f 7141 26-461 8 . 9 



Fluosilicon 1*74/ \ 



^ lumina ., 1 ?£2 1 14-270 4-8 



Iron peroxide l*5o/ J 



Manganese protoxide . . 0*268^ 



Lime 0*127 I 



Ma g nesia I 9-Q82 1*00 



Potash 1-776 f Z ^ Z iUU 



Soda 0-184 



Lithia 0*627J 



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 quan- 

 tity 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 

 X2 



