01^ THE NORTH-EAST OF FIFE. 



431 



decomposed and removed in solution, while the silicates of alumina, 

 magnesia, and iron, taking up water, recrystallize in new combinations. 

 The second stage in the alteration consists in the formation of the 

 black opaque products for which Yogelsang proposed the name of 

 " opacite." That this largely consists of secondary magnetite there 

 cannot be the smallest doubt, but it is often intermingled with ferric 

 oxide and its hydrated products. The way in which the opacite is 

 developed at the expense of the viridite is admirably illustrated in 

 many of the Forfarshire rocks. In the midst of the more or less 

 crystalline green mass, black opaque centres appear with a colourless 

 zone around them (see Plate XIII. fig. 5). As these black nuclei 

 increase in size, the colourless zone around them extends itself out- 

 wards, till at last we have the whole mass becoming colourless by 

 the separation of the iron from the viridite and its concentration into 

 the opacite nuclei. "What is the nature of the hydrous alumino- 

 magnesian silicate left behind, and which also crystalHzes in spheru- 

 litic aggregates, we can only conjecture, but it is probably not far 

 removed from pennine and clinochlore in composition. 



The third stage of change in the alteration of the rock consists in 

 the more complete oxidation and hydration of the opacite, by which 

 the brownish and reddish products, designated as " ferrite '' by Yogel- 

 sang, are produced. 



Concurrently with the later changes which we have been describing, 

 the glassy base of the rock often becomes completely devitrified, the 

 magnetite-grains are all converted into hydrated ferric oxide, and 

 infiltrations of these substances stain the whole substance of the rock 

 and even the felspars themselves. In this way the rocks acquire 

 the red, brown, and purplish tints which usually characterize the 

 porphyrites. 



Last of all, the felspars may become completely kaolinized, calcite, 

 chalcedony, and other secondary products may be formed in fissures, 

 and thus the whole rock passes into the condition of " claystone," 

 which takes the same place in the intermediate series of rocks as the 

 wackes do in the basic. 



Of course this series of changes is liable to some modifications 

 according as the composition of the original rock varies, or the con- 

 ditions to which it is subjected difier. The transformation appears 

 to take place more rapidly when the rock is full of gas-pores, each 

 of which becomes a laboratory of synthetic mineralogy. But though 

 the several stages of the change may overlap to a certain extent, 

 the order stated is that which would seem to be the one which is 

 almost invariably followed. 



In the case of the dacite-glass, we find an instance of a special 

 kind of alteration of a very peculiar kind. Secondary change is 

 seen to commence along the perlitic cracks, and to gradually extend 

 inwards till the whole mass becomes white and opaque. In this 

 white mass the felspar and biotite crystals remain nearly unaltered. 

 It appears to be isotropic, and may be a hydrated acid glass, just as 

 palagonite is a hydrated basic glass. That it contains the bases of 

 the rock and is not a mass of opal is proved by the fact that the mass 



