116 Prof. Bonney — Some Notes on Gneiss. 



Further crushing, associated with definite shearing, at last prac- 

 tically obliterates all distinct traces of " augen " structure. The 

 streaks of different mineral composition, the more quartzo-felspathic 

 and the more micaceous, especially the latter, become longer and 

 thinner, until, as the ultimate result, we come to a fine banded rock, 

 with laminge so uniform as to suggest that the materials once were 

 stratified. Excellent illustrations of this may be found in a number 

 of large boulders strewn about the lower end of the St. Moritz lake. 

 One block may be a rather coarse granitoid gneiss, the quartz, 

 felspar and mica (especially) forming little streaks. Another affoi'ds 

 a rock more markedly fissile, the surface of the slabs being slightly 

 undulate and of a sage-green colour (from the crushed and hydrated 

 biotite or from secondary chlorite). Slight streaks of white here and 

 there show through the "varnish," which is very thin: that is, through 

 the film of secondary mica forming the " sheen surface." But 

 on examining the transversely fractured ends of slabs, we see white 

 wavy streaks of quartz and felspar, perhaps a couple of inches long 

 and a fifth of an inch broad at the thickest part, from which condition 

 we are led on to a still more fissile rock, composed of alternating 

 laminas of white and green : the former about one-eighth of an inch 

 thick, the latter still thinner, each of which may be traced for a 

 length, usually, of from three to four inches; these bands, it is 

 important to notice, not being continuous through the block. In 

 certain extreme cases the rock is more like a mica-schist than a 

 gneiss, but even then very thin white streaks may be detected on 

 examining a transverse fracture. The Central Oberland and some 

 parts of the Pennines aiford many like instances. This " linear 

 foliation " seems to be the utmost that crushing and shearing can 

 produce, and the latter is an essential factor. To obtain this 

 structure the rock must be granitoid and fairly coarse ; where it 

 has been also porphyritic, that probably is recorded by a little 

 irregularity in the linear foliation, i.e. the occasional presence of 

 slightly thicker and more lenticular streaks. The more regularly 

 " lined " varieties have come, I believe, from rather coarse but non- 

 porphyritic rocks. The more close the resemblance to a mica-schist, 

 the more complete, in all probability, the crushing. In these cases 

 it may be also necessary that the rock originally was rather fine- 

 grained. 



Analogous structures may be produced in more basic rocks, such 

 as diorites and dolerites, but in them mineral changes seem to take 

 place more easily, such as the formation of acicular hornblende, 

 chlorite, epidote, and secondary felspar. These I have elsewhere 

 described, so that I may sum up the result of at least ten years 

 of study in the following statement : That pressure is a potent 

 factor in structural and mineral change in granitoid rocks, whether 

 acid or basic, though more so in the latter than in the former ; that 

 the resulting structures pass through a somewhat wavy or lenticular 

 streaking to a thin banding, which may ultimately lead to an almost 

 slate-like fissility ; but that I have never found any reason to 

 suppose that a well-marked mineral banding, such as one in which 



