1869.] 



of Cornwall and Devonshire. 



211 



| 15-092 



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

 lustre, and occurs in rhombs of C0° and 120° angles. Its oxygen ratios 

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



Oxygen Ratios. 



Silica 20-72/ \ 9 



Fluosilicon 0-9 18 J 



Alumina 10'692 



Iron peroxide 4*400 



Iron protoxide 0*514^ 



Manganese protoxide ... . 0-.310 



Lime 0*192 



Magnesia 0-427 > 4*292 



Potash 1-655 



Soda 0-254 



Lithia 0*940^ 



The oxygen ratio of this iron-potash Hica (which is undoubtedly a lepi- 

 domelane) for silica and bases is 



216 : 194, or 1 : 1. 



The granites of Cornwall and Devon, which have been frequently ex- 

 amined by me during the last sixteen years, appear all to contain 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, as 1 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 vary- 

 ing 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 ortho- 

 clase and albite, and by the absence of all the Lime Felspars, 



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

 known geological age, but probably subsequent to the Laurentian 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 or- 

 thoclase and oligoclase, or Labradorite, or some other of the Lime Felspars, 

 and by the absence of albite. 



