C— GEOLOGY. 63 



farther west, near St. Davids, isolated rocky hills such as Pen Bery, Cam 

 Llidi and St. David's Head rise above the surface like islands out of the 

 sea. The general correspondence of the present surface with the Triassic 

 plain of erosion in various parts of South Wales suggests the possibility 

 that the well-defined boundaries of the isolated hills that rise above the 

 plateau level may, like the scarps overlooking the Vale of Glamorgan 

 farther east, be also of Triassic origin. 



The plateau of South Pembrokeshire is continued northward without 

 a. break into the remarkable even surface which truncates the Palaeozoic 

 rocks of Central and North Wales. This surface is relieved by occasional 

 hill masses which stand conspicuously above its level. These include the 

 summits of the mountains composed of Ordovician volcanic rocks which 

 range from Cader Idris northward through the Arans and Arenig. The 

 Plynlimon mass and a smaller mass near Drygarn belong to the same 

 category. Several writers have called attention to the way in which this 

 plateau abuts against the mountain masses which rise sharply to a height 

 of several hundred feet above its surface. Except for their greater altitude 

 above sea level, the relation of these hill masses to the surrounding plateau 

 is closely similar in North Wales and in South Wales, so similar, in fact, 

 ae to invite the idea that they are of similar origin. W. M. Davis' has 

 pointed out that, ' while the theory of marine planation was in vogue, it 

 was customary to interpret all evenly truncated uplands — that is, uplands 

 whose surface truncates their rock structure — as uplifted plains of marine 

 abrasion, more or less dissected since they were uplifted. When the 

 efficacy of sub-aerial erosion was recognised it became equally customary 

 to interpret truncated uplands as once base-levelled and afterward uplifted 

 i)eneplains. If Passarge's views be now accepted, it follows that no 

 truncated uplands should, without further inquiry, be treated as having 

 been eroded when their region had a lower stand with respect to base- 

 level ; the possibility of their having been formed during an earlier arid 

 climate as desert plains, without regard to the general base-level of the 

 ocean, must be 'considered and excluded before base-levelling and uplift 

 can be taken as proved.' 



Passarge, to whose views Davis refers, has given a description of great 

 plains which have been eroded under desert conditions. He states^ ' that 

 these desert plains are not undulating with low hills, but are true plains 

 of great extent, from which the isolated residual mountains rise like 

 islands from the sea. The residuals may be low mounds only a few meters 

 high, or lofty mountain masses rising several thousand meters above the 

 plains. The plain surrounds the steep slope of the mountains with a 

 table-like evenness ; there is no transitional belt of piedmont hills, and 

 no intermediate slope. . . . The bedding of the rocks is not flat, but 

 disturbed ; the plain therefore truncates the rock structures. . . . The 

 products of weathering are usually spread as a thin veneer on the plain ; 

 the waste does not lie in place on the rocks from which it was weathered, 

 but has been drifted about by wind and flood and has gathered in slight 

 depressions. The waste veneer increases the smoothness of the plain, 

 but the rock surface is also a plain. . . . Neighbouring areas contain 



' ' The Geographical Cycle in an Arid Climate.' Geographical Essays, p. 310. 

 •* Quoted from Davis, op. cit. sup. 



