432 G. W. Eawes—Lithia-hearing variety of Biotite. 



case of such combined crystals of mica. Some of the specimens 

 examined by Rose came from granitic veins similar to those of 

 Portland and Middletown. The cleavage lines begun in one 

 mica often run some distance into the other without a change 

 of direction, proving the complete continuity of the two. 



The biotite of Portland is black, and possesses a high luster. 



In thin plates it is transparent, clear, and brown by 

 mitted light. It is optically uniaxial. Specific grav" 

 When heated before the blowpipe it imparts to the f 



characteristic carmine-red color of lithia, and it appears thus to 

 differ from all the described varieties of the species. It afforded 

 me, on analysis, the following results : 



L II. Mean. 



Silica 35-59 3562 35-61 



Alumina .1 9*99 20-08 20-03 



Ferric oxide -15 -11 -13 



Ferrous oxide 21-83 21-86 21-85 



Manganous oxide. . . 1-19 1*18 1'19 



Magnesia 5-26 5-20 5-23 



Potash 9-61 9-76 9'69 



Soda. -49 -55 -52 



Lithia -96 -94 '93 



Titanic acid 1-45 1-47 1 '46 



Fluorine -76 -76 '76 



Chlorine tr. tr. tr. 



Water 1-84 1*90 1'87 



99-12 99-43 99*27 



The ratio of the R : fi : Si is 1 : 1 : 2 ; and the analysis hence 

 shows that the mica is an iron biotite, which has lithia replacing 

 part of the potash, and which contains much more ferrous 

 oxide and less magnesia than is common. It fuses before the 

 blowpipe to a magnetic globule. Some specimens fuse with 

 greater difficulty and give little color to the flame, mdicatmg 

 a transition toward the more ordinary variety. I would call 

 attention to the fact that the iron of this mica oxidizes with 

 great readiness both when heated and when exposed to the 

 weather, and this shows how easy it would be to denve 

 lepidomelane from it by alteration. This lithia-biotite seems 

 to be widely distributed in granitic veins. Specimens from 

 similar granitic veins in New Hampshire, Massachusetts and 

 North Carolina were examined which imparted more or less ot 

 the lithia coloration to the flame, fused to magnetic globul^, 

 and which possessed the same physical appearance and toe 

 L as the Portland biotita 



