140 Geological Society : — 



teresting geological sections in Brittany, in the autumn of last year. 

 The author is greatly indebted for information to the Rev. E. Hill, 

 who took part in the summer excursion of the Societe Geologique 

 de France, and to Dr. Charles Barrois, who has for long been en- 

 gaged in investigating the geology of Brittany. 



(1) The Author briefly noticed the glaucophane-amphibolites and 

 the associated schists of the He de Groix, which have been already 

 admirably described by Dr. C. Barrois. He considered the evidence 

 to be on the whole in favour of the view that the former were 

 originally igneous rocks intrusive in the latter, but modified by 

 subsequent pressure (the marks of which are very conspicuous in 

 the schists), and by mineral change, which probably produced the 

 glaucophane, the garnets being anterior to the mechanical dis- 

 turbance. 



(2) The next part of the paper treated of sections in the district 

 about Quimperle. Cases were cited of granite, modified by pressure 

 so as to result in a gneissoid rock, and of banded gneisses also modified 

 by subsequent pressure, but, in the Author's opinion, indubitably 

 banded gneisses anterior to the mechanical disturbance, and exhi- 

 biting structures which, in 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 sub- 

 sequent mechanical action, which in structure differ greatly from 

 the granites, and much resemble the older Archaean gneisses of 

 other regions. A '.' halleflinta " to the north of Uuimperle 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 paper were noticed the crystalline rocks 

 of Roseoif, 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 primarilv due to pressure operating on 

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

 planation. At any rate the latter rocks appear to have assumed a 

 crystalline 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. 



