SOME NORTH-OF-ENGLAND DYKES, 941 
of the alteration has been to remove the alkalies and alkaline earths, 
to introduce water, and to concentrate the iron in the form of 
hydrated peroxide along the margins of 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 de- 
composed 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 solution by an excess of carbonic acid. This solu- 
tion wouldcome into contact with oxygen along the joint-planes, and 
hydrated ferric oxide would be precipitated. Diffusion would tend 
to cause the carbonate of iron held in solution in the interior of the 
cells to move outwards, and thus fresh portions would be brought 
under the influence of the oxygen, obtaining access by means of the 
joint-planes, and a further precipitation of ferric oxide would take 
place. This process would be continued until the whole of the iron 
had been removed from the interior of the cell, and concentrated in 
the neighbourhood of the joint-planes. In short the action would 
be analogous to that described by Prof. Judd as having taken place 
in the case of the Northamptonshire iron-ore*. The carbonates of 
lime and magnesia would of course also be removed from the inte- 
rior of the cells; but as there would be no chemical action tending 
to separate the lime and magnesia at the joint-planes, these bases 
would be completely removed by the percolating waters. Here, 
then, we witness the result of an interesting chemical operation : 
iron is separated from lime and magnesia by the action of oxygen 
on a solution of the carbonates ; iron is precipitated as ferric oxide, 
while the carbonates of lime and magnesia remain in solution. 
We return now to the description of the unaltered rock. Under 
the microscope (Pl. XIII. fig. 2) one recognizes felspar, pyroxene, 
and.large plates of an opaque mineral, which appears to be ilme- 
nite 7, also colourless prisms of apatite. The rock is composed 
almost entirely of crystalline elements, and in structure is emi- 
nently diabasic (structure ophiteque of Lévy and Fouqué and dia- 
basisch-kornig of Rosenbusch). 
Felspar.—This mineral occurs in lath-shaped sections, usually 
three or four times as long as broad. In the Tynemouth dyke 
_ and its associates the corresponding felspars, as we have seen, are 
very long in proportion to their width. Another point of difference 
lies in the fact that the felspars in the High-Green dyke are more 
or less altered, while those in the altered portions of the Tyne- 
mouth dyke are comparatively fresh. This probably indicates an 
important difference in chemical composition. Under polarized 
light, and in cases where the alteration has not proceeded so far as 
to destroy all individual action, the crystals are seen to consist of 
binary, or more frequently of multiple twins. 
Pyrowxene occurs as isolated crystalline grains and irregular plates ; 
* «Geology of Rutland, p. 133. 
+ Titanic acid was not estimated in the analysis, and is therefore weighed in 
part with the silica and in part with the alumina and iron. 
Q.J.G.8. No. 158. = 
