.176 PROCEEDINGS OF THE GEOLOGICAL. SOCIETY. [Apr. 20, 



Geological Survey, sheets 5, 6, and 36), the Cambrian and Llan- 

 deilo rocks had suffered immense denudation before the deposition 

 of the Wenlock shale, which rests often nearly at right angles on the 

 upturned edges of the older rocks. 



Respecting other and, in great part, later denudations, that of the 

 Merionethshire district is at first sight the most remarkable, and is 

 therefore here mentioned b}'^ way of a special example. From the 

 lowest exposed beds of the Cambrian strata to the top of the Snowdon 

 rocks on one side, and to the Bala limestone on the other, there is at 

 least a thickness of 23,000 feet. All this has been removed by 

 denudation, for the sequence of rocks round the Cambrian central 

 axis is perfect and quite conformable ; and, for the reason, that, if 

 we could sink deep enough through the Snowdon and Bala rocks, we 

 should certainly arrive at a given depth at the Harlech grits ; there 

 can be no doubt that originally these grits were covered by the Lower 

 Silurian strata, that surround them in Merionethshire, on the south, 

 east, and north. Indeed, so far from the rocks that form Snowdon 

 (the Bala beds) being the highest rocks that originally overlaid the 

 Merionethshire Cambrians, there can be little doubt that the addi- 

 tional 8000 feet of strata that lie above the Bala limestone must also 

 have been once continuous across this space ; and the amount of 

 matter removed, and, perhaps, the curves of the strata during the 

 process of denudation, may be deduced from the arrangement of the 

 beds as shown in the published geological sections of this part of 

 North Wales, which have been drawn with much accuracy of detail. 



Respecting the amount of matter removed ; I believe that the 

 subject should be treated in the same manner that the author has 

 already dealt with the denudation of South Wales and Somersetshire *. 

 This can only be done by the employment of sections on a corre- 

 sponding vertical and longitudinal scale. Numerous other questions 

 in geological physics will by-and-by be opened out and settled by 

 this style of work, — questions that it is rash to touch upon with data 

 less perfect than such sections afford. 



* Memoirs of the Geological Survey of Great Britain, vol. i. p. 297. 



