444 G. A. Lehour — On Western Brittany. 



features. Even ■when drawn to a large scale the surface line is a 

 most even one, sloping insensibly towards the sea. The numerous 

 valleys, which give the country its wild appearance, are seen on the 

 section hut as a number of little insignificant nicks, breaking for an 

 instant only the continuity of the surface line, and even rendering 

 the evenness of the latter more obvious by their very unimportance. 

 The very slight and gentle undulations to which this surface line 

 is subject are coincident with the changes in the geological con- 

 formation of the district across which it runs, and are sufficiently 

 accounted for by the relative hardness and facility of disintegra- 

 tion possessed by the rocks which characterize them. 



Looking, then, at the country from this new point of view, we 

 have a long central line of hills standing boldly between two plains, 

 each of which slopes gently to the sea. These plains I look upon 

 as the work of the last great marine submergence of Western 

 Brittany, — as, in fact, being plains of marine denudation. The for- 

 mation of the valleys by which these plains are as it were gouged 

 in every direction, I regard as being due to quite different, and, in 

 part at least, to much more recent causes. 



4. — Date of submergence. — It has been repeatedly stated, and it is 

 indeed held by many as an established fact, that the whole of 

 Brittany has been dry land since very early geological times. 



That this may not be true for the median range of hills of Western 

 Brittany there is no evidence whatever to show ; and indeed it is 

 most probable that the old rocks of which they are composed have 

 not been exposed to the action of the sea since very ancient times. 

 They no doubt stood out as a tongue of dry land during the 

 subsidence, and planing down of what I will now term the northern 

 and southern plateaux of Finistere. 



Whether this submergence took place several times or only once, 

 and when, I am not aware of any means of determining at present. 

 But I think that there is sufficient evidence to induce us to believe 

 that the last great submergence was a comparatively recent event. 



The northern plateau is remarkably deficient in any deposits 

 of later date than the Devonian. At one particular spot on the 

 southern plateau, however, as has been mentioned above, there is 

 an isolated patch of sands and clays which has been referred to the 

 Tertiary age. These beds are lying perfectly flat upon the upturned 

 edges of a mass of micaceous schists ; they stand above the general 

 leA'el of the country, and if produced at their present horizon they 

 would spread over a great portion of the southern plain. They are 

 unfortunately quite devoid of any organic remains by which their 

 exact age might be satisfactorily settled. In general appearance, 

 however, in lithological character, as well as in relative position, they 

 agree very closely with some deposits of undoubted Miocene age 

 in the neighbourhoods of Dinan and Eennes. These last-named 

 beds are all marine, and in the total absence of any palagontological 

 evidence to the contrary, it may, I think, fairly be admitted that the 

 patch in question at Toulven, near Quimper is also of marine 

 origin. Indeed the appearance of the beds is such as seems directly 

 opposed to the notion of their freshwater origin. 



