IxXit" PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



The next movement is that of which the North of England affords 

 such distinct e^ddence. It took place either immediately after the 

 conclusion of the Silurian period, or, as M. de Beaumont has con- 

 cluded, immediately after the deposition of the Tdestone, and pre- 

 vious to that of the Old Red Sandstone. Its effect on the position 

 of the Silurian beds is shown by the great want of conformity between 

 them and those of the Carboniferous Limestone deposited on their 

 upturned edges. It extended towards the South-west as proved by 

 a similar unconformity in the coal districts of Shropshire*. In the 

 whole of this area, from the North of England to the Severn, the Old 

 Red Sandstone is wanting, except in a few localities atiout the Lakes 

 district. It commences on the south and west of the Severn and 

 passes mider the coal-field of S. Wales, resting conformably, as 

 already stated, on the subjacent Silurians, thus showing that the 

 movement of which we are now speaking did not extend into this 

 region. It would seem probable that it so elevated the bottom of 

 the sea throughout the area over which it extended as to prevent in 

 a great measure that deposition of the Old Red Sandstone which 

 succeeded this epoch, in that part of the sea to which the movement 

 did not extend. The elevation of the sea-bottom might probably 

 extend into N. Wales. 



The masses which formed this elevated sea-bottom (or possibly, in 

 some parts, dry land) must have been subjected to denudation for 

 an enormous period, during which the deposition of the Old Red 

 Sandstone might be taking place further to the south. The smaller 

 depth of the sea in the Shropshire region and in that of N. Wales 

 in the same latitude, would also seem to be indicated by the thin 

 deposits of Carboniferous Limestone, or its entire absence. 



This movement between the Upper Silurians and Old Red Sand- 

 stone is also recognized in the S.E. of Ireland. In the counties of 

 Wicklow, Wexford, and Waterford, it gave a general strike of about 

 N.N.E. and S.S.W. to the beds, instead of the N.E. and S.W. direc- 

 tion of the corresponding beds in Englandf . In Scotland the discon- 

 tinuity between the Old Red Sandstone and inferior beds is distinctly 

 recognized, but in the Grampians the strike of the former is about 

 N.E. by E., differing from that in Ireland by about 30°. I have 

 already stated that the movement produced no sensible effect in 

 S. Wales. The slight discontinuity sometimes observable in Pem- 

 brokeshire and elsewhere J, in an apparent overlapping of the Old 

 Red Sandstone and Carboniferous Limestone over the lower forma- 

 tions, must be attributed to that gradual depression which, it must 

 not be forgotten, must have been almost incessantly going on during 

 the periods to which we are referring. 



It appears probable, as Sir Roderick Murchison has pointed out, 

 that frequent outbursts of volcanic matter took place, and often along 

 pre-existing lines of vent, during the interval between the Silurian 

 period and that of the Coal-measures ; but the elevatory effects of 



* Vide supra, page Ixs. 



t Sir H. De la Beche, Memoirs of the Geological Survey, vol. i. p. 222. 



X Memoirs of the Geological Survey, vol. ii. part i. p. 197. 



