46 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



not less than that of the Devon arch, and that in its extension (the 

 direction of the major axis of the ellipse) it was carried as far east- 

 wards as the meridian of London. 



Passing over to the district of South Wales, we may apply consi- 

 derations akin to those employed with reference to the South Devon 

 and Cornish area, and with like results. A mass of oldest sedimentary 

 strata makes its appearance near, and extends S. and E. of St. Davids ; 

 subordinate to which are bands of shingle. The lines of quartzose 

 conglomerate occurring in the grauwacke overlying the former — 

 so singularly like some of the beds of the Old Red Sandstone that 

 they were at one time mistaken for them, but which are now known 

 to be of Lower Silurian age — would imply that the source of such 

 a form of detritus was not far distant. 



This Silurian section is interesting, inasmuch as, in the imperfect 

 representation we get of the upper group, we see to what extent the 

 character of that fauna becomes modified by local conditions ; and 

 also that local conditions, and the variations they produce, are not 

 sufficient to account for that absence of the upper group which has 

 been remarked upon with respect to Cornwall and the West of 

 France. 



In like manner the cleavage theory shows that the Bristol Channel 

 area must at one time have been the line of an axis of elevation, of 

 which the direction coincided with the strike of the old group of 

 St. Davids, and that of North Devon. This was probably the eastern 

 extremity of an elliptical area, of which the maximum breadth was 

 placed at some distance to the west. 



Independently of these considerations, there is to be observed in 

 this same district the relation of unconformity between the Old Red 

 Sandstone and the Culm-series, as also that of both of the foregoing 

 with the Silurian and older rocks, — relations which must always exist 

 towards the original limits of formations where the destruction and 

 removal of mineral material alone take place. Here then as at the 

 western opening of the English Channel we have clear indications of 

 an old coast-line, and of land which has long since disappeared. 



The whole terrestrial surface thus restored would present an in- 

 dented outline produced by advancing ridges of old palaeozoic sedi- 

 mentary strata, and extending from the main Atlantic mass in east 

 and west directions. This growth of land from the Atlantic side, 

 towards the European area, by means of the successive elevation of 

 palaeozoic sea-bed, commenced from very early times. It was a phy- 

 sical change of this kind, which produced what may be designated 

 as " the Mid European line," and which at length constituted a parting 

 barrier between two hydrographical areas, in the northern of which 

 the true upper Silurian fauna had its development together with 

 northern relations, and of which the southern equivalent is that as- 

 semblage of forms which is usually known as the Devonian*. 



* This view will form the subject of a special memoir, but it was necessary to 

 state it here, inasmuch as it helps towards a determination of the form which the 

 palaeozoic terrestrial area of Western Europe ultimately assumed. 



