118 Prof. Bonney — Some Notes on Gneiss. 



• ferent explanation. This occurs on the south side of the St. Gothard 

 Pass, about 1900 feet above Airolo, where the road between two 

 bridges is nearly level, and about a furlong from the foot of the 

 second set of zigzags. 



The dominant rock is a dark biotite schist, but it is locally inter- 

 banded with a light-grey granulitic rock. The latter varies from 

 mere lines to bands one or two inches thick, which, however, are 

 rather impersistent, thickening or thinning or splitting, generally in 

 the course of two or three feet, and stopping sometimes with a 

 moderately pointed end. Of this last case two explanations seem 

 possible: the structure may be original and " fluxional " in origin, 

 produced by a partial melting of the darker rock in consequence of 

 the intrusion of the lighter one, the two then moving on together; or 

 it may be due to the crushing of a solid mass, in which the one rock 

 formed veins in the other. I have examined, however, a good many 

 cases in which a " veined " mass has been crushed and somewhat 

 sheared, and, while I cannot deny that this explanation is a possible 

 one here, I feel bound to remark that, if so, the case is a rather 

 exceptional one. But it seems to me impossible thus to explain 

 either the first two cases or the very numerous instances of banded 

 gneisses which I have seen in many lands and extending over large 

 areas. In regard to these, as it appears to me, we must choose 

 between two hypotheses : that of fluxional movements anterior to 

 complete consolidation in a mass not perfectly homogeneous, and 

 that of the metamorphism of a stratified rock. For reasons which I 

 will state presently, I think the latter to be only rarely applicable, and 

 the former to be generally the correct one. This, as has been shown 

 in papers already published,^ must be the origin of some banded 

 gneisses in Sark,^ and (we may perhaps add) at the Lizard. Fluxional 

 structure undoubtedly, as I believe, sometimes occurs in the gabbro 

 of the Lizard Peninsula,^ and of the Saas valley,^ in the diorites of 

 Guernsey, and in the dolerite of Mount Koyal (Canada). Most of 

 these instances of mineral banding are afforded by districts on which 

 pressure has not produced any marked effects. Some rocks at the 

 Lizard certainly show pressure-metamorphism, but not in the vicinity 

 of the banded gabbi-os, where the serpentine in which they are 

 intruded is practically undisturbed, and even in the Alps the 

 tough gabbros have not seldom succeeded in resisting it. Still, as 

 some persons are sceptical on this matter, it may be interesting to 

 add another case where veins of intrusive granite exhibit a well- 

 banded structure which cannot be explained by anything but a 

 movement of the material while still in a plastic condition. This 

 may be found in a mass of ice-worn rock near the base of the 

 Allalin glacier. Granite veins, generally narrow, break in all 

 directions through a mass of greenish schist (the Griiner schiefer of 

 the Swiss geologists), apparently not having any necessary con- 

 nection with the foliation, which is conspici;ous in the latter and is 

 probably the result of pressure. This, however, cannot have pro- 



1 Quart. Journ. Geol. Soc. 1892, p. 122. 2 jjj^ iggi, p. 464. 



^ loc. cit. p. 483. * Phil. Mag. 1892, p. 237. 



