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Microscopic Constitution of certain Cornish Rocks. 91 



I. IT. 



Water* 366 3-60 



Silica 63-00 63*20 



Titanic acid . . . trace trace 



Phosphoric acid . . trace trace 



Alumina .... 20-50 19*80 



Ferric oxide . . . 3*56 3*46 



Ferrous oxide . . . 3*10 2*83 



Oxide of manganese . trace trace 



Lime 1-35 1*20 



Magnesia .... trace trace 



Potassa -95 -95 



Soda 3-07 322 



99-19 98-26 



When microscopically examined, this rock does not differ ma- 

 terially from that obtained nearer the surface, excepting that the 

 grains of oxide of iron are larger but less numerous, and that it 

 contains fewer of what appear to be broken crystals of a horn- 

 blendic mineral. The granular fragments of which it is composed 

 are also larger and more distinct than in the former case ; and on 

 examination of transverse sections it becomes evident that, in the 

 majority of instances, their larger surfaces are nearly parallel 

 with the planes of cleavage of the slate f. The chloritic mineral, 

 mentioned as occurring in killas from the adit-level, is also very 

 abundant ; and about the margin of many of the patches of this 

 substance peculiar markings are observed. These, which may 

 perhaps be the result of forces producing cleavage, will be more 

 fully noticed when describing the appearance of sections of clay- 

 slate from Polmear Mine. 



Killas from Polgooth Mine, one-hundred-fathom level. Sp. gr. 

 = 2*73. — The killas at this depth is of a greenish-grey colour, 

 slightly streaked and spotted with yellow, and is rather harder 

 than that found at shallower levels. Its chemical composition 

 was found to be as follows : — 



I. II. 



Water t 333 3*20 



Silica 51-00 50-83 



Titanic acid .... trace trace 



Alumina .... 20-67 20-90 



Ferric oxide . . . 13-44 13-39 



Ferrous oxide ... 4*70 5-14 



Oxide of manganese . trace trace 



Lime 1-68 1-56 



Potassa -95 -91 



Soda 3-96 4-20 



99-73 100-13 



* Of which '66 was lost in the water-bath. 



t This was observed to be the case in all the specimens of clay-slate 

 which were examined. X Of which 2 - 00 was lost in the water-bath. 



