274 The Rev. S. Haughton on the Optical Properties of 



Xo. 2. Mica from Glendalough Valley, county Wicklow; gray, 

 silvery, transparent, with specific gravity =2- 793. 



99-61 

 If we take the mean of these analyses, we find, — 



Average Mica of Dublin, Wicklow, and Carlow Granite. 



Per cent. Atomic quotients. 



Silica . . . 44-27 0-962 3 



Alumina . . 30-91 0-601 \ n . fifi7 9 



Peroxide of iron 5-27 0-066 J u DD/ 



Lime . . . 0-82 0-0291 



Magnesia . . 0-92 0-046 I n .ooo n 



Potash . . . 11-01 0-234 f 0338 1 



Soda ... 0-90 0029 J 



Loss by ignition 5-66 0-629 2 



There can be little doubt, from the foregoing analyses, that 

 the transparent, gray mica of the district under consideration is 

 a margarodite, having the rational formula 



RO, Si0 3 + 2(R 2 3 , Si0 3 )+2HO. 



The mica which deviates most from this formula is the mica 



