Prof. Bonney — Some Notes on Gneiss. 



115 



" sheared " in addition. I have more than once described the results 

 of such a process/ so far as the different minerals are concerned ; 

 hence it is needless to enter into details, and I shall confine myself to 

 a few general illustrations of the making of a gneiss by pressure. 

 To exhibit the first stage we may select a porphyritic granite 

 from near the English church at Saas Fee (Valais). The rock might 

 be termed an "augen-gneiss." If, however, a surface transverse to 

 the foliation be examined, the felspar crystals are fairly regular in 

 shape, being only a little distorted or rounded at the angles, and are 

 not conspicuously orientated. But on a surface in the direction of 

 the foliation they appear as rather irregular, wavy streaks, parted by 

 streaks and lines of dark mica. The annexed sketch (Fig. 1) shows 



Fig. 1. — Augen-gneiss, near Saas Fee, section along the "dip plane" of the 

 foliation (the rock near the church is a little less crushed than this, but my sketch is 

 rather too rough to be copied). 



a slightly more advanced stage of the same rock. The larger felspar 

 crystals vary from lenticular to streaky in form, are sometimes 

 cracked and slightly separated, as is indicated by lines of dark mica 

 about the thickness of a sheet of writing-paper. The smaller felspars 

 have been often crushed almost to powder. 



A stage somewhat more advanced may be found in gneiss (common 

 in boulders) on the eastern bank of the Davoser See. Here the rock 

 hardly can be called an augen-gneiss, for generally it is a coarse, 

 streaky, slightly wavy, foliated gneiss — constituent minerals : quartz, 

 felspar, biotite and some white mica. But on close examination 

 traces of the original structure can be detected, as is shown in the 

 annexed sketch (Fig. 2), and from this stage a gradual transition 



Fig. 2. — Eemains of crystals in a much crushed augen-gneiss, near Davos Dorfli. 



can be traced backwards into a true augen-gneiss, in which the 

 white rounded felspars are over 2" in diameter. 



1 See Presidential Address, Quart. Journ. Geol. Soc. 1886, Proc. 67, 68. 



