NORTH-WKST REGION OF CHARNVVOOD FOREST. 81 



lias an aspect whicli suggests a " fragniciital" origin, and its mode 

 of weathering certainly more resembles that of an indurated homo- 

 geneous volcanic ash than of a lava. A rough cleavage can 

 generally be discovered on closer examination. Its strike is 

 approximately AV.S.W.-E.N.E. (sometimes more nearly approaching 

 W.-E.), with a very high dip on the northern side. 



Though much attention has been paid to the microscopic structure 

 of the rock, we have little to add to our former remarks. The 

 larger felspars are sometimes to a great extent replaced by epidote, 

 which, by the mode of its occurrence, appears to have formed direct 

 from the other mineral, almost as a paramorph, instead of resulting 

 from a general exchange of constituents with the neighbouring 

 minerals. This change would most readily occur in the case of an 

 iindesine- or labrado rite-felspar, provided some of the soda and of the 

 silica were removed bj" solution. But an addition of some iron 

 would seem requisite. This, however, might have been obtained from 

 enclosures of iron oxide or ferriferous glass. Epidote and "V"iridite 

 are sometimes associated : the latter also occurs alone, and varies 

 from an isotropic mass to an aggregate of flaky minerals, which 

 have only a feeble action on polarized light ; probably these belong- 

 to the chlorite group, although in some cases they may be nearer to 

 serpentine. These may indicate the former presence of a member 

 of the pyroxenic group, but neither the external form nor the 

 structure of the grain helps us to a conclusion. We think it 

 probable that the grains composed of viridite and abundant opacite, 

 for the origin of which garnet or even olivine was vaguely suggested 

 in our former papers, are an iron oxide, in some cases at least 

 ilmenite, where the grain has been partly converted into a mineral 

 allied to chloropal. The quartzes are cracked, but, as a rule, do not 

 exhibit strain-shadows. The cracks sometimes are occupied only 

 by viridite or an allied secondary mineral, and appear connected 

 with a linear structure indicative of crushing of the adjacent rock, 

 in which case we refer them to subsequent pressure. But occasionally 

 they are partly occupied by material corresponding with the matrix, 

 iiud suggest that thej' arc due to strains set up before it solidified. 

 Eor instance, in one slide, small angular bits of quartz are excep- 

 tionally numerous, and occup}' positions in relation to an inlet in 

 a large quartz grain (like a wedge cut out of a round cake) ; and 

 this seems as if they came from the gap. Sometimes the quartzes 

 have a fairly-defined crystalline outline, but generally they are more 

 or less rounded, and occasionally seem to have been invaded by the 

 matrix. 



Some difficulties as to the strnclui'e of the last have been cleared 

 up by additional study. We called attention formerly to a peculiar 

 spotted character produced by thin lines of a green mineral which 

 seemed to traverse the whole in a kind of network. This, it is now 

 dear, is a secondary product, replacing a black iron oxide, and so 

 it is most likely a variety of chloropal. As the rock cooled, pro- 

 bably the iron, as usual, separated out of the glass, and then a 

 concretionary action was set up in the latter, expelling the opacite, 



