HOENBLENDE-SCfllSTS, GNEISSES, ETC. OE SAEK. 145 



Note by Peof. T. G. BoiofEx. 



In papers published previous to 1890 ^ I have supposed banded 

 gneisses and schists — rocks in which a ' stratification-foliation,' as I 

 termed it, exists — to have originated, as a rule, from detrital 

 deposits. Misunderstanding may be prevented by indicating ia 

 general terms to which of these rocks I now conceive this hypo- 

 thesis of crystallization from a molten condition, but under excep- 

 tional circumstances (of which the intrusion of one rock into 

 another, so as to render it plastic, may or may not be one), can 

 be applied in order to explain their differences from normal igneous 

 rocks. 



They are : — The ordinary banded ^Laurentian' gneisses of Canada 

 and of Greenland (so far as I have seen them) ; similar gneisses in 

 Norway, in certain parts of the Alps, and in the Scotch Highlands, 

 especially the North-western district; the Grannlitic Group at 

 the Lizard, and a portion, at any rate, of -the Hornblendic Group; " 

 perhaps also a few rocks in Wales ; in short, those holocrystalline 

 rocks, fairly coarse, in the description of which I have laid stress 

 upon their banded structure and the peculiar forms of their mineral 

 constituents. 



But I still maintain that sundry other gneisses and certain schists 

 (among them often chloritic and actinolitic) are due to the crushing 

 of ordinary igneous rocks — granites, dolerites, &c. Eepresentatives 

 of this group occur, as I have described, in America, Norway, the 

 Alps, Scotland, the Malvern Hills, and locally in other parts of 

 England, in Wales, Britanny, Normandy, and Guernsey. In a word, 

 whenever I have already admitted pressure to be the dominant 

 agent in producing ' schistosity ' in a rock, I have nothing to alter. 



Also, I still maintain that certain schists are metamorphosed 

 sediments — e. g. the calc-schists which graduate into marbles, most 

 quartz-schists, and mica-schists, with their varieties, some chloritic 

 schists, and the like. Examples of these occur in South Devon 

 (Start district), Scotland (chiefly, so far as I know, in the Central 

 Highlands, but with these I am less familiar), in Anglesey, in 

 Norway (if I can trust my memory), and abundantly in many 

 districts of the Alps — the group, in short, which I have designated 

 the Upper Schists in my notices of that chain.^ Some gneisses 

 ultimately may be proved to have been originally sediments, but 

 the number of these is certainly much less than I formerly sup- 

 posed it to be, and in the present state of my knowledge I think 

 it better to leave the question open. 



In short, I think there are at least three modes in which a 



^ The list may be diminished by referring for papers of earlier date to my 

 Presidential address to this Society for the year l885 ; since then, to vol. xliii. 

 (1887) p. 301 (Britanny), toL xlv. (1889) p. 67 (Alps), vol. xlvi. (1890) p. 187 

 (Alps); also lecture to British Association at Bath (' Nature,' vol. xxxix. p. 89). 



^ After what I saw in Sark, I think it possible that the difficulties which' 

 prevented me in 1890 from adopting the hypothesis of fluxion might dis- 

 appear if I could again examine these hornblende-schists. 



^ Quart. Journ. Geol. Soc. vol. xlv. (1889) pp. 96, 98. 



