EESULTS OF PRESSURE IN STRATIFIED PALJEOZOIC BOOKS. 11 



2. On some Results 0/ Pressure and of the Intrusioi^ o/Granite in 

 Stratified Paleozoic Rocks near Morliix, in Brittany. By 

 T. G. Bonnet, D.Sc, LL.D., P.ll.S., V.P.G.S., Professor of Geo- 

 logy in University College, London, and Fellow of St. John's 

 College, Cambridge. (Read November 9, 1887.) 



[Plate II.] 



In my paper on the " Structures and Relations of some of the Older 

 Rocks of Brittany "* I briefly referred to the slaty rocks in the neigh- 

 bourhood of Moriaix. As the main object of my journey was to 

 study the older and, presumably, Archsean rocks of Brittany, only a 

 moderate amount of time was devoted to those of later date, and 

 not many specimens were collected. These, however, have proved 

 of unusual interest ; so I venture to offer a few remarks upon 

 them to the Society, because, as I anticipated, they seem to throw 

 some light upon the history of the genesis of certain crystalline 

 schists. 



The rock beneath and in the immediate neighbourhood of Moriaix 

 consists of alternating bands, differing in texture and colour t- The 

 grey material, at the time of deposition, was evidently a fine-grained 

 silt or earthy sand, the dark a carbonaceous mud. The layers of the 

 former commonly do not exceed an inch in thickness, and are often 

 less ; the others are frequently more ; but extremely thin, more or less 

 lenticular streaks of the one material are occasionally interlaminated 

 with the other. Both are cleaved, the sandy band imperfectly, the 

 dark band very completely. Commonly the planes of cleavage and 

 stratification coincide, and then the rock has a compressed appearance; 

 but minor folds and " wriggles " are frequent, so that the " stripe " 

 is cut by the cleavage-planes at very different angles. In these 

 cases the change in amount and direction of the cleavage, the 

 thickening of the sandy bands before a thrust, their attenuation 

 along a line of strain, in short all the usual pressure-phenomena 

 in banded rocks are exceptionally well exhibited. Indeed, though 

 my opportunities of study have not been few, I never saw finer 

 examples. It was often difiicult to make progress by the side of 

 the rude terrace-walls which support the gardens towards the out- 

 skirts of the town, so interesting were the structures in the rough 

 blocks of which they are built. A cleavage-face in one of the 

 darker bands has a slightly micaceous lustre, exhibits, in short, a 

 " sheen surface." It is also wavy, being marked by a number of 

 flat " puckers," the crests being parallel, though not quite straight 

 lines. The specimen which I brought away (I obtained it in a road- 

 side cutting near the river, north of the town) shows four of these 



■* Quart, Journ. Greol. Soc. vol. xhii. p, 301. 



t It was considered to be Devonian by the French geologists, but Dr. Barrois 

 has recently informed me that the fossiliferous Devonian occupies a fold to the 

 south of Moriaix, the rock to the north of it, and perhaps that to the south (those 

 mentioned in this paper, which are lithologically identical), being Cambrian. 



