﻿[ 306 ] 

 XLII. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



[Continued from p. 235.] 

 Jan. 7, 1869. — Lieut.-General Sabine, President, in the Chair. 



npHE following communication was read : — 

 - 1 - " Notes of a Comparison of the Granites of Cornwall and De- 

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

 Haughton, M.D., D.C.L., F.R.S., &c. 



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

 tain albite 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 Tre- 

 wavas Head, where it has the following composition : — 



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



Silica 6576 



Alumina 2172 



Lime 0'89 



Magnesia trace 



Soda 9*23 



Potash 176 



Water 0*40 



9976 

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

 with quartz and orthoclase, which has the following composition : — 



II. Orthoclase {Trewavas Head). 



No. 1*. No. 2t. 



Silica 63-60 63*20 



Alumina 21*04 21'00 



Iron and manganese oxides .... trace trace 



Lime 0*90 0*68 



Magnesia trace trace 



Soda 3-08 275 



Potash 9-91 10-30 



Water Q-40 Q-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 essen- 



* From veins at foot of cliff associated with Cleavelandite albite. 

 t From the granite at summit of cliff. 



