442 G. A. Lebour — On Western Brittany. 



rV. — On the Denudation of Western Brittany. 

 By G. A. Lebottr, F.R.G.S., of tte Geological Survey of England and Wales. 



I pi QTATED roughly, the geology of the Department 



O of Finistere may be said to consist of two masses 

 of granite, one to the north and one to the south, enclosing between 

 them nearly the whole of the sedimentary rocks of the district. These 

 consist of Cambrian slates and gneiss, Lower, Middle, and Upper 

 Silurian slates and grits, and very small and unimportant patches of 

 Upper Carboniferous shales. The entire mass of these deposits has 

 an east and west direction, and occupies the central part of the 

 Department. 



All the country to the north of Brest, and extending from Nor- 

 mandy to the Isle of Ouessant, forms part of the northern granitic 

 mass. Between this and the tongue of land to the south of Douarne- 

 nez, including the very indented peninsula of Crozon, we have the 

 Cambrian and Silurian rocks powerfully undulating and forming the 

 two loftiest chains of hills in the district — the Montagnes d'Arrhee 

 and the Montagnes Noires — which run across the country close to 

 and parallel to each other in an east and west direction. Immedi- 

 ately to the south of this wide band of Palaeozoic formations, comes 

 the southern mass of granite. Like the first, this has scattered here 

 and there, over its whole extent, numerous isolated enclosed masses 

 of metamorphic schists, which have, by Dufrenoy and others, been 

 referred to the Cambrian age. 



The coal shales mentioned above, as occurring in this district, are 

 to be found in thin elongated patches at Quimper and at Cleden, on 

 the west coast ; their extent is utterly insignificant. 



About two miles south of Quimper, near the hamlet of Toulven, is 

 to be seen a small series of horizontal beds of Tertiary age (the only 

 instance of the occurrence of such deposits in the Department) of 

 which more will be said presently. 



2. Coastline. — The coastline of Western Brittany is very irregular. 

 The two great bounding masses of granite forming comparatively even 

 and sweeping lines, interrupted only by the long, narrow, and winding 

 creeks which are to be found at the mouths of all the rivers. These 

 creeks are, however, quite minor features on these coasts, and are 

 many of them hewn out of the micaceous schists which are inter- 

 spersed among the granites. Where an estuary occurs in the granite 

 itself, it is usually wider, and is bounded by flat sandy shores. 



The cliffs along these coasts are low, with rounded and well- 

 weathered tops. They are higher when composed of the metamor- 

 phic schists, which are much more compact and harder to dis- 

 integrate than the granites of these parts, and naturally form bolder 

 and more striking cliffs. 



The Cambrian and Silurian beds, on the other hand, which are 

 enclosed between the two above-mentioned bands of granite, are, as 

 a rule, characterized by high, steep cliffs, the faces of which are 

 frequently much eroded into caverns, oftentimes of very considerable 

 magnitude. The general line of this part of the coast is very in- 



