Prof. Bonney — Some Notes on Gneiss. 



117 



the layers may exceed a quarter of an inch in thickness, can be 

 produced by pressure acting on a rock in which no such structure 

 originally existed. 



But there are many gneisses which are distinctly and broac^ly 

 banded. Such, for instance, occur among the Laurentian rocks of 

 Canada and Greenland, the " Hebridean " rocks of Scotland, the 

 ancient crystalline rocks of Norway, and those of several parts of 

 the Alps. As an example I ma}'^ refer to a gneiss in the neigbour- 

 hood of Saas Fee. A huge boulder, which in all probability came 

 from the lower part of the Mittaghorn, afforded the particular 

 instance. The rock consists of quartz, felspar, and a greenish-grey 

 mica, and is rather fine-grained. The banded structure is very 

 distinct, the lighter zones consisting chiefly of quartz and felspar 

 with small flakes of mica, the darker being richer in mica, which 



Fig. 3.— Gneiss with "rucking" and traces of strain-slip cleavage in the broadest 

 micaceous band. Fragment about 15" long, from part of a fallen block, near Saas Fee. 



occurs also in larger flakes (Fig. 3). The former are usually the 

 thicker; the latter vary from between two and three inches in thick- 

 ness to little more than films. In these the mica flakes are bent into 

 zigzags, and in the bi'oader bands a strain-slip cleavage sometimes 

 may be seen crossing them obliquely. This structure, the result 

 of pressure, is clearly later than the formation of the bands. 



As another example a boulder may be cited which I found lying 

 on the flank of the Schafberg at Pontresina. Tn this also— a fairly 

 coarse gneiss — the mineral banding is distinct, the broadest of the 

 more micaceous zones being nearly three inches across. But in 

 addition to this the whole block shows an oblique " lamination " of 

 more or less micaceous stratulfe, even in the more quartzo-felspathic 

 bands. This structure, which is not very uncommon, curiously 

 resembles current-bedding in a stratified rock, to which, indeed, 

 it has been referred by more than one well-qualified observer. 

 Formerly I should have so regarded it myself, but I am now con- 

 vinced that the resemblance is illusory and that it is a result of 

 pressure.^ 



A third example may be mentioned, because it may require a dif- 



1 See Quart. Journ. Gaol. Soc. Proc. 1886, p. 97. 



