120 -^>''?/- Bonney — 8ome Notes on Gneiss. 



of rocks in which the bands of mica or of other minerals are thin, we 

 must deny it in all cases where they are even moderately thick. 



But was the old notion entirely wrong ? Cannot a gneiss be in 

 an}' case an altered sedimentary rock ? Beyond all doubt not a few 

 schists have thus originated : all calc-schists and quartz-schists ; man,y 

 mica-schists ; perhaps also some hornblendic and chloritic schists. 

 I am aware that it has been suggested that certain basic rocks might 

 be changed into calc-schists. It is true a hornblendite or a pyrox- 

 enite might produce an ophicalcite, but both these are rare rocks; 

 also certain calc-mica schists might come from rocks of the doleritio 

 group; but I doubt whether we have evidence of such transmuting 

 power as would produce on the one hand anything like a marble or 

 one of the purer calc-schists, on the other a rock so siliceous as certain 

 quartz-schists. They are interbedded and associated ; they alternate 

 with and graduate into one another, now rapidly, now slowly ; they 

 present macroscopic resemblances to stratification, even more per- 

 fectly than the examples which we have considered above, while, 

 in the case of not a few, the possibility of an igneous origin is 

 excluded by their mineral composition. Thei-e seems, however, no 

 reason why a sediment of the proper chemical composition should 

 not, as a result of metamorphic processes, be changed into a gneiss. 

 But, as a rule, clays (for to some variety of this rock we must look) 

 are rather deficient in alkalies, and seem to produce micas and 

 minerals such as andalusite more readily than felspar. 



In cases of contact alteration, though felspar occurs more frequently 

 than has been supposed, it seems not to be very common ; while 

 pressure appears to bring about the conversion of felspar, where it 

 already exists, into quartz and white mica. Into these minerals, 

 indeed, it sometimes appears to break up without any such dis- 

 turbing cause. ^ No doubt felspar of secondary origin is a more 

 common constituent of crystalline schists than has hitherto been 

 supposed, and the mineral has been often overlooked in consequence 

 of the close resemblance to quartz which this variety presents, but 

 this fact, so far as our present knowledge goes, is more applicable 

 to the products of basic than of acid igneous rocks, and does not 

 seem likely to remove our difficulties in the case of gneisses. 



It is not, however, possible for a rock like the Torridon sandstone 

 to be reconsolidated by secondary enlargment of its constituent 

 gi'ains, and to pass back, as do some quartzites, into a really 

 crystalline condition ; thus producing a rock similar to one of the 

 Laurentian gneisses ? Such a change would be theoretically possible, 

 but it is not easy to find satisfactory evidence that it has really 

 occun-ed. The Torridon sandstone of Scotland, the gres felspathiqne 

 of Normandy, where they have been subjected to pressure, approach 

 nearer to the mica-schist group, the felspar disappearing in the 

 manner already mentioned. I have not myself seen any marked 

 case of contact-metamorphisra in either of these rocks, so that I 

 cannot be certain what would be the effects of raising them to a 

 fairly high tempei'ature under moderate pressure and in the presence 

 1 Quart. Journ. Geol. Soc. 1888, p. 36. 



