Geological Society of London. ■ 237 



structures which, iu his opinion, lend themselves more readily to a 

 theory of some kind of original stratification of the constituents than to 

 any other. The amphibolites in this region are undoubtedly of igneous 

 origin (intrusive), but subsequently modified. In one part of the 

 district are granitoid gneisses, but little modified by subsequent 

 mechanical action, which in structure diifer greatly from the granites, 

 and much resemble the older Archaean gneisses of other regions. 

 A " halleflinta " to the north of Quimperle proves to be in part 

 a rhyolitic rock, modified by subsequent pressure ; part, however, may 

 be an indurated tuff of similar composition. 



(3). In this part of the p;iper were noticed the crystalline rocks of 

 Eoscoff, and (more briefly) the Palaeozoic strata about Morlaix, with 

 the mineral and structural modifications due to pressure and to the 

 action of intrusive igneous rocks. The author pointed out that, in the 

 latter case, the results either of pressure-metamorphism or of contact- 

 metamorphism differ much from the crystalline schists, which, both in 

 Brittany and elsewhere, are regarded as of Archaean age ; and that 

 here in the north at Roscoff. we have a series of banded gneisses, less 

 modified by subsequent pressure than in the south, the structures of 

 which are very difficult to explain on any theory of a "rolling out" 

 of a complicated association of igneous rocks, but which are such as 

 would naturally result from some kind of stratification of the original 

 constituents. 



The result of the author's work is to strengthen the opinion which 

 he has already expressed, that while the structures of some foliated 

 rocks may be regarded as primarily due to pi-essure operating on 

 suitable materials, the structure of others seems opposed to this explana- 

 tion. At any rate the latter rocks appear to have assumed a crystal- 

 line condition with a semblance of stratification in Pre-Cambrian 

 times; so that, whatever may be their genesis, they are rightly called 

 Archaean gneisses and schists. 



2. ''The Rocks of Sark, Herm, and Jethou." By Eev. E.Hill, 

 M.A., F.G.S. 



The author described the island of Sark, about three miles long by 

 two broad, with the smaller areas of Little Sark and Brecqhou. Little 

 Sark is attached to Great Sark by a narrow ridge (the Coupee), which 

 the weather is rapidly degrading, while Brecqhou is completely 

 separated by a narrow strait. The greater pai't of these islands consists 

 of dark hornblendic banded rocks, which closely resemble those of the 

 Lizard, and show by their alternation of materials and their phenomena 

 of current-bedding that they have originated by some kind of deposition. 

 These were shown to lie unconformably on a gneiss, seen only at the 

 eastern extremity of the island, in and around the Creux Harbour. 

 Over this the beds lie in a dome, and as they slope away on the N., 

 W., and S., they pass under a highly-crystalline rock, which has been 

 called a metamorphic gneiss. This rock was described, and evidence 

 given to show that it is really a granite, — an igneous rock which has 

 overflowed the hornblendic beds. 



Yeins and dykes were briefly noticed ; they include a dyke of mica- 

 trap. The islands of Herm and Jethou, lying between Guernsey and 

 Sark, were also described. Jethou contains a fine raised beach. They 



