1869.] 



On the Granites of Cornwall and Devonshire. 



209 



III. Notes of a Comparison of the Granites of Cornwall and Devon- 

 shire with those of Leinster and Mourne." By the Rev. Samuel 

 Hatjghton, M.D., D.C.L., F.R.S., Fellow of Trinity College, 

 Dublin. Received December 18, 1868. 

 The granites of Mourne are eruptive, and can be proved to contain al- 

 bite as their second felspar. 



The granites of Leinster are also eruptive ; and although albite has never 

 yet been actually found to occur in them, its existence can be inferred with 

 considerable probability. 



During the past summer (1868) I have succeeded in proving that the 

 second felspar that occurs in the granites of Cornwall is albite. I found 

 this mineral as a constituent of the granite at Trewavas Head, where it has 

 the following composition : — 



I. Albite, var. Cleavelandite (Trewavas Head). 



Silica 65'76 



Alumina 21* 72 



Lime 0*89 



Magnesia trace 



Soda 9-23 



Potash T/6 



Water 0'40 



99-76 



This albite is opaque, cream-coloured, lamellar, and associated with 

 quartz and orthoclase, wbich has the following composition : — 



II. Orthoclase (Trewavas Read). 





No. 1*. 



No. 2t. 





63-60 



63-20 





21*04 



21-00 



Iron and manganese oxides . . 



trace 



trace 





090 



0-68 







trace 





3-08 



2-75 





9-91 



10-30 





0-40 



0-40 





98-93 



98-33 



The granites of Cornwall and Devon contain two micas, white and black. 

 I was fortunate enough to obtain, through my friend Mr. W. J. Henwood, 

 F.R.S., of Penzance, a sufficient quantity of white mica from Tremearne, 

 near Trewavas Head, to determine accurately its composition, which 

 proves to be highly interesting. It differs essentially from the white mica 

 of Leinster and Donegal, and proves to be a variety of lepidolite. 



* From veins at foot of cliff associated w ith Cleavelandite albite. 

 t From the granite at summit of cliff. 



