292 PROF. W. MOEKIS DAVIS ON [Aug. I909, 



formed on the weakest Mesozoic beds ; mature or oldish valleys 

 were opened in the less resistant rocks of the older mass, sub- 

 mature valleys in the more resistant rocks of the uplands (dotted), 

 and young valleys in the most resistant rocks of the monadnocks. 



Fig. 1 . — Diagram of two cycles of erosion in Devon. 



These two provinces, Wales and Devon-Cornwall, differ in area 

 and in relief, and in the proportion and distribution of the 

 several parts here specified ; they probably differ also in a multitude 

 of minor details ; yet the general resemblance appears to hold, and 

 upon this resemblance it seems fair to conclude that the dissection 

 of the peneplain and its monadnocks in Wales must have been 

 somewhat less developed in pre-Glacial times than the dissection 

 of the peneplain and monadnocks in Devon-Cornwall is to-day. 

 But for glaciation, Wales might be pictured as follows : — The areas 

 of less resistant rocks would have been reduced to rolling hills in 

 the stage of late maturity or of early old age, in which all traces 

 of the uplifted peneplain would have been lost ; the areas of more 

 resistant rocks might still retaiu traces of the peneplain in the even 

 surface of their uplands between open, early mature valleys ; while 

 the most resistant rocks would still surmount the uplands as monad- 

 nocks, and the valleys within them would have hardly reached 

 even a submature stage in the new cycle of erosion ; indeed, in 

 their upper parts, they might not show any distinct signs of revived 

 activity in the streams. The main valley-floors in the peripheral 

 areas of less resistant rocks would be worn down low and open ; 

 but the headwater valleys in the most resistant rocks of the 

 monadnocks would be little changed from the form which they had 

 assumed in the previous cycle before uplift. Such is the stage of 

 Devon-Cornwall to-day, and such would seem to be, but for 

 glaciation, the present stage of development of Wales. Pre- 

 Glacial Wales would have been in a somewhat less advanced stage 

 of development. 



X. Pub-Glacial Form or Snowdok. 



It is upon grounds such as the foregoing that a tentative restora- 

 tion of the pre-Glacial form of the Snowdon district is based. At 

 the time just before the Tertiary cycle of erosion was interrupted by 

 uplift, the then low-standing group of AVelsh monadnocks presum- 

 ably had well-subdued crests and ridges, only here and there 



