Geological Society. 539 



I am indebted to Professor Everett for permission to carry 

 ont the above work in the Laboratory of Queen's College, 

 Belfast, for suggestions regarding some of the methods 

 employed, and for the use of the necessary apparatus. 



LXIII. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from p. 387.] 



January 27, 1892.— Dr. W. T. Blandford, E.R.S., Vice-President, 

 in the Chair. 



'TVEE following communications were read : — 

 -*- 1. " On the Hornblende-schists, Gneisses, and other Crystal- 

 line Rocks of Sark." By the Rev. Edwin Hill, M.A., E.G.S., and 

 Prof. T. G. Bonney, D.Sc, F.R.S., V.P.G.S. 



The authors refer to Mr. Hill's paper, published in 1887, for a 

 general description of the Island. They were led to examine Sark 

 again in the hope that its rocks might afford some clue to the 

 genesis of the hornblende-schist of the Lizard. They describe the 

 structure, macroscopic and microscopic, of the various foliated 

 rocks. These are: — (a) The basement gneiss, a slightly foliated, 

 somewhat granitoid rock, probably of igneous origin, but with some 

 abnormal environment, and possibly intrusive into, instead of older 

 than the rock which succeeds it. (6) The hornblende-schists, almost 

 identical with those of the Lizard, but in one case yet more distinctly 

 banded, (c) Banded gneisses sometimes rather fine-grained, variably 

 banded: quartzofelspathic layers alternating with those rich in 

 biotite or occasionally hornblende. Some of these gneisses resemble 

 the " granulitic group " of the Lizard ; others recall certain of 

 the less coarse, well-banded gneisses of Scotland, e. g. south of 

 Aberdeen. Sometimes they are much "gnarled" by subsequent 

 earth-movements, by which, however, as a rule, the crystalline rocks 

 of the Island do not appear to have been very seriously affected. 

 (d) A very remarkable group of local occurrence which exhibits great 

 variety. In some places large masses of a dark green hornblende-rock 

 are broken up and traversed by a pale red vein-granite or aplite. 

 The former rock is drawn out into irregular lenticles, elongated 

 lumps, and finally streaks, and has been melted down locally into 

 the aplite. This then becomes a well-banded biotite gneiss, which 

 macroscopically and microscopically agrees with types which are 

 common among the Archaean rocks. Sark therefore presents an 

 example of the genesis of such a gneiss, and the authors are of 

 opinion that probably all the above-named rocks are of igneous 

 origin, but became solid ultimately under somewhat abnormal 

 conditions, to which the peculiar structures (which distinguish them 

 from ordinary igneous rocks) are due. They attribute the banding 

 to the effect of tiuxional movements, anterior to final consolidation, 



