XIV 



Austell, for Mr, David Forbes, F.E.S., F.G.S., who found* its 



" Orthoclase, from the same neighbourhood, is composed of: — 



Veins at the foot • Mass at the top 



of the cliff. of the cliff. 



Silica 63-60 63-20 



Ahimina 21-04 21-00 



Iron and manganese oxides trace trace 



Lime 0-90 0-68 



Magnesia trace trace 



Soda 3-08 2-75 



Potash 9-91 10-30 



Water 0-40 0-40 



98-98 98-33 



" Through the kindness of Mr. Argall [of Wheal Vor] I obtained from 

 Tremearne, in Breage, a sufficient quantity of white mica (a variety of 

 Lepidolite) to determine its composition, which proves to be : — 



Silica 47-60 



Fluosilicon 5-68 



Alumina 27-20 



Iron peroxide 5-20 



Manganese protoxide 1-20 



Lime 0-45 



Magnesia trace 



Potash 10-48 



Soda 0-72 



Lithia 1-14 



99-67 



•'Black miea {Lepidomelane) from Cam Bosavern, Saint Just in Pen- 

 with, consists of : — 



Silica 39-92 



Fluosilicon 3-04 



Alumina 22-88 



Iron peroxide 15*02 



,, protoxide 2-32 



Manganese peroxide 1-40 



Lime 0-68 



Magnesia 1-07 



Potash 9-76 



Soda 0-99 



Lithia 1-71 



98-79 



" The granites of Cornwall and Devon appear all to contain the two 

 felspars and two micas above mentioned." 



Haughton, Proceedings of the Royal Society (7th January, 1869), xvii, 

 pp. 209 — 11. London, Edinhurqh, and Dublin Phil. Mag. (April, 

 1869), xxxvii, pp. 306—8. (Abridged), 



* London, Edinburgh, and Dublin Phil. Mag., xxxvii (May, 1869), p. 323. 



