OP CENTRAL AND EASTERN CORNWALL. 487 



I. II. 



Water I h yg rometric * 21 * 24 



w 1 combined 1-69 1-53 



Silica 47-32 47-21 



Carbonic anhydride trace trace 



Phosphoric „ trace trace 



Alumina 18-10 18-19 



Ferric oxide 6-21 6-27 



Ferrous oxide 7*72 7'66 



Manganous oxide trace trace 



Lime 8-24 8-13 



Magnesia 7*05 7*15 



Potassa -62 -50 



Soda 3-18 3-27 



100-34 100-15 



An analysis of a specimen of the fine-grained green slaty rock 

 from this locality, also made by Mr. Brown for the author, shows 

 that it contains more silica and alumina, as well as a smaller pro- 

 portion of magnesia, than the crystalline rock. 



It therefore follows that, if both had originally a similar com- 

 position, one of them at least must have subsequently undergone 

 considerable alteration. 



Analysis, in duplicate, of this rock afforded the following per- 

 centage results (sp. gr.=2-89) : — 



I. II. 



Water i h y gr0metric * 26 ' 19 



Water \ combined -37 -40 



Silica 52-54 52-57 



Phosphoric anhydride trace trace 



Alumina 23-83 23-69 



Ferric oxide 2-44 2-50 



Ferrous oxide 4-87 4-83 



Manganous oxide trace trace 



Lime 7-89 7'91 



Magnesia 2-67 2-74 



Potassa 1-95 1-78 



Soda 3-11 3-29 



99-93 99-90 



The coarser -grained greenstone of St. Cleer-Down does not 

 materially differ from various much-altered rocks of the same class 

 found in other localities. Hornblende is plentiful, both in crystal- 

 line patches and in bacillar aggregations, disseminated in a ground- 

 mass probably resulting for the most part from the decomposition of 

 felspar, which is usually coloured by viridite. 





