﻿308 Royal Society. 



The Black Mica of St. Just is of a blackish-bronze colour and 

 metallic lustre, and occurs in rhombs of 60° and 120° angles. Its 

 oxygen-ratios are, reckoning for the fluorine its equivalent of oxygen, — 



Oxygen-Ratios. 



Silica 207271 21 . 645 



Fluosilicon 0-918 J l 



1 15-092 



Alumina 10*692 



Jron peroxide 4*400 



Iron protoxide 0*514 "l 



Manganese protoxide. ... 0*310 I 



Lime 0*192 | 



Magnesia 0*427 > 4*292 



Potash 1*655 



Soda 0*254 



Lithia 0*940 



The oxygen-ratio of this iron-potash Mica (which is undoubtedly 

 a lepidomelane) for silica and bases is 



216: 194, or 1 : 1. 



The granites of Cornwall and Devon, which have been frequently 

 examined by me during the last sixteen years, appear all to con- 

 tain the two felspars and the two micas above analyzed. In a future 

 communication I hope to describe their composition in detail, and to 

 give a comparison of this composition with that of the granites of 

 Ireland. 



The following generalizations will be found,. I believe, capable of 

 proof. 



(1) The granites of Ireland may be divided into two distinct 

 classes, marked by characters both geological and mineralogical. 



(2) The First Class of granites consists of Eruptive rocks, of ages 

 varying from the Silurian to the Carboniferous periods. To this 

 class may be referred the granites of Leinster and Mourne, and the 

 granites of Cornwall and Devon. 



(3) The first class of granites is characterized by the presence of 

 orthoclase and albite, and by the absence of all the Lime Felspars. 



(4) The Second Class of granites consists of Metamorphic rocks, 

 of unknown geological age, but probably subsequent to the Lauren- 

 tian period. To this class may be referred the granites of Donegal 

 and Galway, and the granites of Scotland, Norway, and Sweden. 



(5) The second class of granites is characterized by the presence 

 of orthoclase and oligoclase, or Labradorite, or some other of the 

 Lime Felspars, and by the absence of albite. 



