100 
to our present standpoint than it is at present—the rocks that 
composed that old surface having been long since entirely removed 
by the action of subaerial forces. As the general level of the 
surface wasted away and sank somewhat nearer to its present level, 
part of the last-formed plain—the Subcretaceous Plain that I have 
mentioned as being traceable here and there over greater part of 
the British Isles—began to be re-exposed, and the rivers that had 
already established their courses in the Cretaceous rocks began to be 
guided downward into rocks of quite a different character. Still, 
they went on all the same. Just as the acid used by the engraver 
bites downward into the hard plate of metal only along the lines 
where the etching ground has been scratched away, so the erosive 
action of the streams was guided downward through the upper 
rocks they began to flow in until their courses were fairly established 
in the harder and more diversified rocks beneath. Let us confine 
our attention for the present to the Petteril, as we are actually on 
the river banks of that stream at the present moment. According 
to the view here advocated, the Petteril at first started to flow 
somewhere out on this side of Stybarrow Dod, probably along 
the course now taken by the upper part of Aira Beck. Thence it 
flowed away between what is now Great and Little Mell Fells, across 
what is now a wide east and west valley. Then past Penruddock, 
Greystoke, Plumpton, and thence taking the valley south of Lazonby 
Fell, and joining the Eden near Great Salkeld. There are many 
reasons for believing that the Petteril maintained this channel for 
a considerable period—quite long enough to give rise to the 
formation of such valleys as this before us, where the Petteril has 
excavated a few cubic miles out of what might otherwise have 
been a continuous escarpment of Carboniferous rocks extending in 
an unbroken ridge across from Summerground Crags to Fluska 
Pike. 
But the rocks about the base of the Carboniferous Limestone 
series waste away at a faster rate than the beds higher up; in other 
words, if we had a level surface to start with, composed partly of 
the softer beds below, and partly of the limestone itself of Sum- 
merground Crags, this limestone would dissolve fast, but the softer 
