ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixxxi 



small amount of unconformity in the stratification throughout that 

 region between the lowest beds and the Coal-measures inclusive^, as 

 already stated. 



The most obvious way of judging of the importance of this move- 

 ment, is to replace in imagination the whole sedimentary mass in the 

 position which it occupied immediately anterior to the movement. 

 For this purpose we have only to conceive the inclination of the beds 

 of every formation to be changed at each point throughout the whole 

 area, to an amount which should render the Coal-beds horizontal. 

 At those points where Coal-strata do not now exist, we must judge 

 of the corresponding amount of change to be given to the inclination, 

 by means of the continuity and other circumstances of the stratifica- 

 tion, exhibited in passing from those points where the Coal-beds do 

 exist. Now it follows from the facts already insisted upon, that the 

 beds of the whole series of formations in S. Wales would thus be 

 brought into a position in which their inclination to the horizon 

 would be so small that the eye would probably be unable to detect 

 it. I am not acquainted with the amount of discontinuity in Den- 

 bighshire, but I venture to assert that it is small compared with the 

 actual inclination of the beds of Coal and Movmtain Limestone ; and 

 if so, the Silurian beds of that region would be reduced to approximate 

 horizontality if the exact horizontality of the Coal-beds were restored. 

 Such would be the case also in the neighbouring counties of England 

 where Coal is found ; and such also we may conclude, by the most 

 legitimate induction, would be the case approximately at those points 

 where Coal does not now exist, always excepting those limited 

 localities in which the beds may have been disturbed by local volcanic 

 action. It is only in this manner that we can form an accurate 

 conception of the enormous influence of the movements subsequent 

 to the Coal period, in producing the observed phsenomena in the region 

 of which we are now speaking. 



Another strong reason for supposing the great movement of N. 

 Wales to have been subsequent to the Coal-measures, is to be found 

 in the fact already stated (p. Ixii), that those measures are dislocated 

 by the great Dolgelly and Bala fault, at its north-eastern extremity. 



But if the disturbances throughout N. Wales and the adjoining- 

 region are to be referred principally to the same epoch as those of 

 S. Wales, how shall we account for the various directions of the 

 numerous lines of elevation which this part of the country presents 

 to us? This is a grave difficulty in M. de Beaumont's theory. To 

 account for such facts, as well as for the existence of curvilinear lines 

 of elevation, our author has recourse to the notion of mixed systems. 

 They are supposed to arise, if I understand him correctly, either, in 

 the first place, from the resulting effects of successive movements on 

 the superficial strata in which we observe the phsenomena ; or 

 secondly, from a single movement acting on such superficial strata, 

 the mechanical cause producing the movement being modified in its 

 operation by the previously dislocated state of the subjacent portion 

 of the whole mass, caused by anterior movements in directions 

 different from that of the movement which alone has aifected the 

 superficial beds. 



VOL. IX. / 



