HISTOEY OF THE DEE, WALES. 223 



measures ; and I have no doubt that these upper Carboniferous strata 

 at one time also spread far to the westward above the limestone 

 across those parts of the Silurian strata now under review ; but how 

 far, it may perhaps be impossible to determine. I feel certain, how- 

 ever, that what is now the high mountain region of Snowdonia, 

 at that time very different from its present form, was never, 

 except on its low western outskirts, covered by Carboniferous strata. 



The statement that so much of Wales was once buried beneath 

 Carboniferous rocks is, at first sight, somewhat startling, involving, 

 as it does, such large revolutions in the physical geography of the 

 district in early times. 



First. An immense amount of denudation undergone by the Silu- 

 rian rock before the deposition of the Upper Old Red Sandstone 

 (which sparingly exists in North Wales) and the Carboniferous 

 Limestone*. 



Secondly. It appears to me that this waste was accompanied by a 

 gradual depression of the old land, so that, over part of Wales, what, 

 many years ago, I termed " a plain of marine denudation " was pro- 

 duced, similar to that well-known high plain of Carboniferous Lime- 

 stone and Old Red Sandstone of the Mendip Hills, on which the 

 Liassic and Oolitic strata lie unconformably. In like manner the 

 Carboniferous Limestone overlain by Millstone Grit and Coal-mea- 

 sures, covered in flat-lying strata the much older Silurian plain 

 of marine denudation. 



Thirdly. By succeeding changes of physical geography after the 

 deposition of the Millstone Grit, the region was converted into part 

 of one of those vast deltas, or at all events broad swamps, under- 

 going slow intermittent depressions, during which the Coal-measures, 

 with all their successive terrestrial growths, were formed. 



Fourthly. The region was again disturbed and elevated before 

 and during the deposition of the Permian strata, which, taken on a 

 large scale, often lie unconformably on the Coal-measures, and were, 

 in my opinion, true continental deposits. Then by slow degrees, 

 under subaerial agencies, a vast amount of the Carboniferous rocks 

 were removed from above the Silurian formations, and the plain 

 of marine denudation was again to a great extent exposed, not 

 less high, but probably even more elevated than it is now, lying as 

 it then did in the midst of a broad continental area. 



This plain (or, rather, its relics) is of much wider extent than that 

 part of it with which I am now concerned, lying on either side of 

 the valley of the Dee ; for beyond that area it extends far and wide 

 into the northern part of Denbighshire, and also into Montgomery- 

 shire and South Wales. Standing on the summit of Cader Idris or 

 of Aran Mowddwy, 2960 feet high, and looking east and south, the 

 eye, as far as it can reach, ranges across a vast extent of old table- 



* This amount could be approximately estimated by any one who would take 

 the pains to restore the pre-Carboniferous anticlinal curves of the Silurian 

 strata; but the doing of this in detail is not essential to my present argument. 

 For the method see memoir "On the denudation of South Wales," etc., Memoirs 

 of the Geological Survey, vol. i. 1846. 



