THE MINING DISTRICTS OF CORNWALL. 329 



I. II. 



Water 1 h yg rometric • • '21 '20 



Watei \ combined ............ -24 -22 



Silica 47-90 48-10 



Alumina 19-26 19-37 



Ferrous oxide 7'55 7'79 



Ferric „ 1-67 1-42 



Manganous oxide trace trace 



Lime 10-57 10-51 



Magnesia 7*97 7'5 / 3 



Potassa -49 *42 



Soda 4-33 4-07 



100-19 99-66 



An examination of thin sections under the microscope shows that 

 this rock is largely composed of hornblende, which occurs in the 

 form of contorted crystalline patches, and as thickly matted acicular 

 crystals. Although consisting, to a great extent, of hornblende, it 

 contains a little chlorite, with some magnetite, while the spaces 

 between the hornblendic patches and crystals are filled by an amor- 

 phous transparent colourless base. 



This rock appears to be an altered hornblendic slate. 



Greenstone from Hose Sill, near Castle Sorneck. — This rock is not 

 unlike that from the cliff at Botallack, except that it is somewhat 

 darker in colour, has a decidedly schistose structure, and, when 

 freshly broken, presents more numerous, although smaller, crystal- 

 line facets. 



An analysis, in duplicate, of an unweathered specimen of this rock 

 yielded the following results (sp. gr.=3*15) : — 



I. II. 



Water I *W ometric - ••• 1*37 1-37 



w ater 1 combined 1-59 1-53 



Silica * 35-66 35-50 



Alumina 21-13 21-16 



Ferrous oxide 10-37 10-27 



Ferric „ 14-56 14-92 



Manganous oxide trace trace 



Lime 9-37 9-13 



Magnesia 2-75 2-84 



Potassa -93 -97 



Soda 2-10 2-12 



99-83 99-81 



A microscopical examination of this rock shows that it largely 

 consists of crystalline hornblende, among which a little flocculent 

 chlorite is sometimes sparingly disseminated. It includes numerous 

 rounded patches of some colourless transparent material, which is tra- 

 versed in all directions by hornblendic crystals, and which docs not 



