Prof. Bonney — Some Nutes orirQneiss. 119 



duced the structure of the granite, which is roughly parallel with the 

 sides of the veins, and thus inconstant in direction. The granite is 

 moderately fine-grained, consisting of quartz and felspar, with only 

 a little white mica. Taking one vein, about six inches wide, as an 

 example (Fig. 4), we find the outer band (rather more than one inch 

 thick) somewhat more coarse-grained than the rest, the felspar 

 varying in size from large hemp-seeds to very small peas. The next 

 band is richer in quartz ; the interior differs somewhat from both and 







4^. 



Fig 4. — Vein of streaky and banded granite intrusion in green -scliist. Rock near 

 the foot of the AUalin glacier. Diagrammatic, the spots indicating the more con- 

 spicuous felspar-grains. 



seems to exhibit a slight fluxional structure. Parts of some of the 

 veins were rather more micaceous, that mineral also occurring in 

 streaks, sometimes apparently connected with small inclusions of the 

 green schist. The banding may be very clear for thi-ee or four feet, 

 then may become less distinct; one vein may show it, another not. 

 A movement of the constituents where the mass was not yet perfectly 

 consolidated is rendered probable by the rounded form of the felspars, 

 and this hypothesis alone seems possible as an explanation of the 

 banding. 



In regions stich as the Alps, which have been more than once 

 affected by great earth movements, there is always a possibility 

 that after a structure had been produced by one set of disturbances, 

 the mass in the lapse of time might become completely consolidated — 

 perfectly cemented as it were, by secondary mineral deposits — after 

 which it might be modified by a new set of earth movements. 

 This, very possibly, may be the history of certain banded gneisses, 

 generally rather fine grained, in which the layers of mica, though 

 very regular, are quite thin — hardly thicker than a stout piece of 

 cardboard ; such, for instance, as the rather fine-grained gneiss 

 which occurs on the east coast of Scotland about and to the north 

 of Muchals. But I have never been able to find any proof of the 

 notion that a broad banding can be produced by any amount of crush- 

 ing in an ordinary holocrystalline rock, while I have demonstrated, 

 as I venture to think, that it can come from movements anterior 

 to the consolidation of the mass ; so that while we may admit the 

 possibility of the former explanation being true for a certain group 



