OLDER ROCKS OF BRITTAXY. 



319 



rocks is bounded on the north and south, and is probably underlain 

 continuously, by gneisses and schists. 



(2) Both the Palaeozoic and the older series have been pierced by 

 various igneous rocks, of more than one geological age, some being 

 even later than the last great earth-movements. 



(3) The intrusive igneous rocks have in some cases greatly altered 

 the Palaeozoic rocks, but appear to have produced little effect upon 

 the gneisses and schists. 



(4) The action of contact-metamorphism on the Palaeozoic sedi- 

 ments does not produce rocks which resemble the presumably Archaean 

 gneisses and schists. 



(5) The action of pressure-metamorphism on Palaeozoic sediments 

 has in no case produced a rock which is liable to be confounded with 

 the gneisses and schists of Archaean type *. 



(6) Although in certain cases igneous rocks have been, in conse- 

 quence of these mechanical actions, converted into gneisses or schists, 

 yet many of the gneisses and schists evidently were true foliated 

 rocks anterior to the above earth-movements, and the latter rocks 

 exhibit structures very analogous to those of stratification. At any 

 rate, even if they be only igneous rocks modified, this modification 

 was most probably anterior to the commencement of the Palaeozoic 

 age. 



Further, the close mimicry of stratification indicated by repeated 

 mineral banding in the gneissic series, not in rare and solitary spots, 

 but over considerable areas and in widely separated districts, makes 

 it difficult to understand how this can be explained by any rolling- 

 out of a complication of veins of igneous rocks rather diverse in 

 composition. The absence, from the best-preserved among these, of 

 structures which are known to be characteristic of igneous rocks, or 

 are indicative of crushing, and the presence of structures definite in 

 kind, whatever be their significance, suggests that either the meta- 

 morphism from which the present condition of the rock resulted 

 must be carried back to a very remote past, presumably before Palae- 

 ozoic times began, or there must be some unknown peculiarity in the 

 genetic history of the rocks themselves. 



(7) Hence, making every allowance for the various effects of the 

 above disturbances, there is to be found in Brittany, as maintained 

 by Dr. Barrois and other French geologists, a great fundamental 

 mass of true Archaean rock, that is, of rock which, whatever be its 

 genetic history, had become what we should call gneiss and schist 

 before the earliest Cambrian rocks were deposited f. 



* It is true I did not see the lowest group in the Brittany Cambrians, the 

 Schistes de St. Lo, but Mr. Hilt's description of the section of the Lower Cam* 

 brians at Crozon (on the sea-coast, south side of the inlet near Brest) was so 

 clear that I deemed it needless to spend a day in visiting it. The rock at Brest 

 is a rather micaceous gneiss, of very ancient aspect. 



t Note, June 6, 1887. — After this paper had been read, I received a copy of 

 a note on the French excursion to Brittany by Professor de Lapparent (Rev. 

 Scient. 1887, p. 38). He concludes nis description with this distinct expres- 

 sion of opinion :—" La Bretagne demontre-t-elle, d'une maniere indiscutable, 

 1' existence d'un terrain normal de gneiss et des schistes cristallins, anterieur a 

 la serie sedimentaire. Ce terrain est reinarquable par la persistance de sa com- 



z2 



