1856.] 



HAUGHTON GRANITES OF IRELAND. 



175 



1. Three Rock Mica 53 8 



2. Glendalough Mica 70 4 



3. Mount Leinster Mica .... 72 18 



4. Lough Dan Mica 70 



5 . Glenmalure Mica 67 11 



Chemically considered, this mica is the Margarodite of mineralogists, 

 and is very constant in its composition, as appears from the following 

 analyses : — 



Table II. — Analyses of Grey Mica. 





1. 



2. 



3. 



Average. 



Silica 



44-71 

 31-13 

 4-69 

 1-09 

 0-90 

 9-91 

 1-27 

 6-22 



44-64 



30-18 



6-35 



b-72 

 12-40 



5-32 



43-47 

 31-42 

 4-79 

 1-38 

 113 

 10-71 

 1-44 

 5-43 



44-27 

 30-91 

 5-27 

 0-82 

 0-92 

 11-01 

 0-90 

 5-66 



Alumina 



Peroxide of iron . . 

 Lime 



Magnesia 



Potash 



Soda 



Loss by ignition . . . 





99-92 



99-61 



99-77 



99-76 



No. 1. Glendalough Valley, county Wicklow Specific gravity =2-793 



No. 2. Mount Leinster, county Carlow 



No. 3. Three Rock Mountain, county Dublin „ „ =2*771 



Dividing the last column of the foregoing Table by the respective 

 atomic weights of the constituents, we find the following : — 



Silica .... 



Alumina 0-601 1 



Peroxide of iron . . 0*066 j • * * • 



Lime 0-029^ 



Magnesia 0*046 I 



Potash 0*234 ["■ 



Soda 0-029J 



Water 0*629 



0*962 

 0*667 



0*338 

 0*629 



From the foregoing figures may be inferred the formula, 

 RO, SiOg + 2{ R2O3, SiOg } + 2H0, 



which coincides with the received mineralogical constitution of 

 Margarodite. 



4. Black Mica. — Besides the mica already described, black mica 

 in small grains presents itself in the granite of several parts of 

 Leinster ; it is never present in great quantity, but appears to be 

 distinct from the grey silvery mica, as it can be separated from it to 

 a considerable extent by levigation, proving that it has a difi'erent 

 specific gravity. I have never been able to procure it in sufficient 

 quantity for examination, either chemical or optical . It is sometimes 

 found, as in the granite of the Three Rock Mountain, county Dublin, 



