G. A. Lehour — On Western Brittany. 445 



Now, taking this isolated patch to be the remnant of a large 

 extent of similar beds deposited over the district during its subsi- 

 dence beneath the Upper (?) Miocene sea ; we have, I think, enough 

 to prove that marine action has been at work over a considerable 

 part of Western or Lower Bi'ittany, at least during the middle of 

 the Tertiary epoch — a date of submergence much more recent than 

 has usually been supposed probable for this country. 



That the loose sandy deposits formed at the bottom of the Miocene 

 sea should have all but entirely disappeared from the surface is only 

 what one would expect to be the case, considering the vast time 

 which has elapsed since their deposition, and probably also since 

 their emergence and consequent exposure to the denuding power of 

 the various subaerial agents. Indeed this one little patch, which is 

 now the only memento left us of that ancient condition of things, 

 was only saved by its position, and probably consists of merely some 

 of the higher members of the series. 



Taking all these things into consideration — the two gently-sloping 

 plains of marine denudation, separated by a high pre-existing water- 

 parting ridge, from whose sides no doubt streams ran into the 

 Miocene sea during its slow advance and recess, as thej'' do now into 

 the Atlantic and the Bay of Biscay, — there seems but a very small 

 effort of the imagination required to suppose that the rivers which 

 are now winding their way across the face of the country are, in 

 their upper part at least, identical with those which were running in 

 a similar manner during and previous to this epoch of submergence. 

 That they have themselves worn and excavated the narrow gorge- 

 like valleys at the bottom of which we now find them, I think 

 no one who has visited this part of Brittany, will doubt for a 

 moment. Many of these river-valleys I have examined minutely, 

 and in no case have I found that they coincided with any marked 

 lines of weakness such as faults, fissures, &c. 



One of the principal difficulties connected with these deep and 

 narrow river- valleys, is the extraordinary constancy of their width 

 and depth, which, as I observed before, continues good, quite 

 regardless of the various hardness and capacity for erosion of the 

 different rocks through which they pass. 



The assumption that these valleys are the same as those which 

 were scooped out during the retreat of the middle Tertiary sea, 

 seems however, to afford a sufficient explanation of this curious 

 fact; inasmuch as the hardest rocks (the Silurian grits), being those 

 in the centre of the country (that is — those, a part of Avhich were 

 always dry land, and the rest of which were the first left uncovered 

 by the retiring sea), and the hardness of the different rocks forming 

 the mineral characters of the country decreasing regularly fi'om the 

 central chain to the sea, both north and soutli, — it is plain that the 

 time which has elapsed since they respectively emerged from the sea 

 and became a prey to the action of running Avater, would have a 

 compensating influence over the erosive power of the latter, the 

 softest rocks having been last under water. Tliis explanation 

 requires the existence of certain conditions ; such as, that the scale 



