C PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May IQ06, 



discussing these changes it will be convenient to confine our atten- 

 tion chiefly to the drainage of the main uplift, although it will be 

 necessary to make incidental reference to the subsidiary uplifts ; and, 

 in the first place, we may briefly consider the effects of the three 

 types of Lower Palaeozoic rocks in modifying the surface-features. 



IV. Effects of the Theee Types of Rock upon the Scenery. 



The differences between the hills of the Skiddaw-Slate, Volcanic, 

 and Upper Slate rocks are so well known that little need he said 

 concerning them. The smooth outlines of the Skiddaw-Slate hills, 

 often rising into peaks, form a marked contrast with the craggy 

 and irregular hills of the volcanic tract ; these again differ widely 

 from the lower and smoother elevations of the Upper Slates, which 

 are in some degree comparable with those of the Skiddaw Slates. 



But, although the general differences are widely recognized, there 

 are some points which require closer consideration. 



It is sometimes stated that the volcanic group of hills owe their 

 height to their superior hardness. This is not the case. The 

 highest hill composed of volcanic rocks is Scawfell Pike (3210 feet) ; 

 while Helvellyn rises to 3118 feet, and some others nearly touch 

 3000 feet. The highest hill in the Skiddaw-Slate tract is Skiddaw 

 itself (3054 feet) ; Blencathara or Saddleback (2847 feet) and 

 Grasmoor (2791 feet) come next. Within the circumference of the 

 Lake-District dome the highest point in the Upper Slates is 1819 

 feet, in the watershed between Long Sleddale and Borrowdale : 

 but, on the Howgill Pells, the Calf attains a height of 2220 feet— 

 1000 feet lower than Scawfell, it is true, but still a considerable 

 eminence. 



The slight difference (156 feet) between Scawfell Pike and 

 Skiddaw, hills composed of rocks so markedly different in character 

 and hardness, requires some consideration. Height of land depends 

 upon three factors — the amount of uplift, the antiquity of the uplift, 

 and the resistance of the rocks to the particular agents of denudation 

 which operate upon them. 



If the uplift be slight, no matter how hard the rocks, the country 

 will be low. Witness the tracts of Anglesey, for instance, composed 

 of hard rock, but never rising to any great height. 



Again, when the antiquity of a land-tract is considerable, however 

 great the original uplift, and whatever be the resistance of the rocks, 

 the district must be worn down, as, for example, has happened in 

 great tracts of Canada. 



