THE DEVELOPMENT OF BRITISH SCENERY. 277 



and Miocene uplifts, without attempting to indicate their 

 maximum periods with greater exactness. 



It is obvious that no drainage system can be older than 

 the rocks which cover the area, and in Great Britain there 

 are four large areas and several smaller ones which are un- 

 occupied by Mesozoic rocks ; the drainage of these areas 

 may, therefore, have been initiated in Devonian or Permian 

 times. The areas are : Scotland, the Pennine Chain, 

 Wales, and Devon and Cornwall, and it is of interest to 

 consider whether the river systems of these areas are of 

 very great antiquity. 



It may be remarked at the outset that if a tract has 

 existed above the sea for long ages and not undergone any 

 further uplift after its emergence from the sea to form high 

 land, the rivers must have reached their base-levels of 

 erosion and produced a plain of subaerial denudation (or 

 peneplain, to use Prof. W. M. Davis' term) unless, indeed, 

 the area has existed as a rainless region through these long- 

 periods. We cannot, therefore, suppose that the tracts 

 above mentioned have existed as land since Palaeozoic 

 times without subsequent uplifts (which would profoundly 

 affect, if they did not completely alter, the drainage- 

 systems) or their present eminences would have been 

 long ago worn away. 



In Scotland the main watersheds bear no direct relation 

 to the axes of uplift of the very ancient rocks which occupy 

 so large a portion of its surface and the great thickness 

 of old red sandstone and carboniferous rocks in the depres- 

 sion between the southern uplands and the Highlands 

 indicates the former extension of those rocks far beyond 

 their present limits, whilst evidence of the like extension of 

 still later rocks has been adduced by Prof. Judd, who 

 makes the following most suggestive remark : "In the face 

 of these facts, I believe that it is impossible to avoid the 

 conclusion that the whole of the north and north-western 

 portions of the British archipelago — now sculptured by 

 denudation into a rugged mountain-land— were, like the 

 south and south-eastern parts of the same islands, to a great 

 extent, if not completely, covered by sedimentary deposits, 



