205 



margin of the more southerly of the two High Green dykes. The 

 decomposed rock shows a cellular structure, evidently connected with 

 its mode of jointing. The walls of the cells are formed of a deep brown, 

 comparatively hard substance, evidently rich in ferric oxide ; while the 

 cells themselves are filled with a white or cream coloured friable substance 

 resembling kaolin. The following analyses by Mr. STEAD, of Middles- 

 borough, show the composition (I.) of the comparatively unaltered rock 

 (II.) of the cell walls, and (III.) of the cell contents : 



I. II. III. 



Si0 2 ... 53-70 ... 43-50 ... 66 20 



A1 2 3 ... 18-21 ... 14-61 ... 23-92 



Fe 2 3 ... 10-64 ... 31-14 ... 0-78 



CaO ... 6-66 ... 0-44 ... 0-95 



MgO ... 5-24 0-11 ... 0-33 



K 2 ... 0-99 ... 0-09 ... 0-04 



Na 2 ... 2-65 ... 026 ... 0'57 



H 2 ... 1-55 ... 9-50 ... 7-00 



C0 2 ... ... ... tr. 



99-64 99-65 99-79 



On comparing these analyses with each other we see that the effect 

 of the alteration has been to remove the alkalies and alkaline earths, 

 to introduce water and to concentrate the iron along the joint planes. 

 It is not difficult to trace the probable steps by which these results have 

 been brought about. In the first place, the silicates would be decomposed 

 by the action of water charged with carbonic acid and the carbonates 

 of lime, magnesia, and iron thus formed would be in part held in solu- 

 tion by an excess of carbonic acid. This solution would come in con- 

 tact with oxygen along the ioint planes and hydrated ferric oxide 

 would be precipitated. Diffusion would tend to cause the carbonates 

 held in solution in the interior of the cells to move outwards, and thus 

 fresh portions of the ferrous carbonate would be brought under the 

 influence of oxygen, obtaining access by means of the joint planes, 

 and a further precipitation of ferric oxide would take place. This 

 process would go on until the whole of the iron had been concen- 

 trated in the neighbourhood of the joint planes. The carbonates of 

 lime, magnesia, and the alkalies would of course also be removed 

 from the interior portions of the cells ; but, as there would be no 

 chemical action tending to precipitate the bases of these salts at 

 the joint planes, they would be wholly removed from the rock. 

 Here then we witness the result of an interesting chemical operation 

 iron is separated from calcium, magnesium and the alkalies by the action 

 of oxygen on a solution of the carbonates ; iron is precipitated as ferric 

 oxide, while the other substances remain in solution. 



In concluding this account of the dykes oi the North of England 



