ON THE OLDER ROCKS OF BRITTANY. 



301 



23. Notes on the Structures and Relations of some of the Older 

 Rocks of Brittany. By T. G. Bonney, D.Sc., LL.D., F.R.S., 

 Professor of Geology in University College, London, and Fellow 

 of St. John's College, Cambridge. (Read March 23, 1887.) 



[Plate XVII.] 



Some unavoidable engagements prevented me from availing myself 

 of the cordial invitation of the Societe Geologique de France to our 

 Members, and joining their excursion to Brittany last summer. My 

 friend the Rev. E. Hill was, however, more fortunate, and his account 

 was so interesting that I determined to spend part of my autumn 

 holiday in learning something of the geology of a country to which 

 many years ago I had paid a flying visit for the sake of its antiquities. 

 I am greatly indebted to Mr. Hill for the loan of specimens, and for a 

 number of valuable hints, both verbal and manuscript. Even greater 

 are my obligations to Dr. Charles Barrois, who, being detained by 

 official duties in the neighbourhood of Redon, most kindly wrote to 

 me several times, pointing out the best localities for study, and in 

 subsequent correspondence has imparted to me his views in the most 

 frank and cordial manner. Some excuse may perhaps be needed for 

 writing at all upon a district which is in the hands of geologists so 

 competent as the members of the French Survey. The impressions 

 formed during a passing visit may not unreasonably be deemed of no 

 value in the presence of their more elaborate work. But in one 

 respect (as Dr. Barrois urged in writing to me) I may hope my 

 remarks may be not without use. I did not go to Brittany to 

 criticize, but to compare. I looked at its rocks in the light of other 

 districts with which I am well acquainted, and sought to ascertain 

 their bearing on general questions of metamorphism and the genesis 

 of crystalline schists. 



Three questions, then, were always prominently before me as I 

 worked : — 



(1) Among the crystalline schists what structures, if any, are 

 presumably original ? and what are due to subsequent disturbance 

 primarily mechanical? 



(2) What is the probable age of these schists? How are they 

 related to, or in what respect do they differ from, beds indubitably 

 Palaeozoic ? 



(3) How far are both the crystalline and the sedimentary rocks 

 affected by the more important intrusive igneous masses ? and do the 

 results in case of the latter resemble the schists and gneisses generally 

 regarded as of Archaean age ? 



In regard to each of these questions Brittany supplies us with 

 evidence which appears to me valuable. 



T travelled round the peninsula by the railway, halting at Yannes, 

 Plouharnel, Lorient (whence I visited the He de Groix), Quimperle, 

 Quimper, Brest, Morlaix ; but my remarks will be confined mainly 



Q. J. G. S. No. 171. y 



