BY HUGH MILLEK. 317 



Was this the case here ? It would be easy to prove that gaps 

 have here also been cut at points where no breaks, fissures, or 

 even closed faults occur, and where the stream had an open 

 course elsewhere. But, unfortunately, this would be vain in 

 argument if it is possible that a barrier or false lottom of glacial 

 drift may have helped the water across. Even in the spaces 

 most free from drifts the occurrence of patches here and there in 

 the furrows, or of preglacial notches brimful, suggests that their 

 quantity may have formerly been sufficient to raise stream-beds 

 up to the level of the lowest points of egress.^ One strongly 

 feels, however, after an examination of the ground, that Jukes' 

 explanation is the philosophical and true one. It is a master- 

 key to unlock all difficulties : without it we must have recourse 

 to a whole bunch. And the feeling may become assurance when 

 the behaviour of the ridges in the main valley, ITorth Tyne, is 

 considered. Grant, for argument's sake, that the ridges in the 

 higher ground did originally dam up the water; and, keeping 

 in mind that there is no question of an origin for the furrows 

 by subsidence of the earth's crust, turn to the valley. The river 

 there practically flows alternately in a wide basin, and a compara- 

 tively narrow gorge ; the valley being constricted by compound 

 ridges, strictly representing, — as the Whinsill ridge, which is 

 continuous from the uplands, clearly demonstrates, — the usually 



* It seems in place here to give some passing account of the several origins of partial 

 gaps among a series of Escarpments such as these. 



1. Some are due to accidents of structure. The Nicks so characteristic of the Whinsill 

 are mostly of this class. Where this intrusive basalt has broken across from one stratum 

 to another it is both less homogeneous itself and contains portions of the ruptured bed. 

 In gaps traversed by faults a local shattering has had the same effect, that, namely, of 

 rendering the ridge liable to excision by atmospheric disintegration. 



2. Some narrow steep-sided notches in these escarpments may be the work of springs 

 rising in them. The rock beside a spring, being especially liable to saturation, must be 

 rifted by frosts elsewhere inactive; and transport for particles is always at hand. 



If it were possible to find a sufficient number of preglacial gaps the difficulty referred 

 to in the text might be overcome. But this can bo proved only in a few special instances. 

 These, however, demonstrate, I believe, that the escarpments arc, like the valleys, pre- 

 glacial. The glaciers between the Tynes passed along the ridges and could not cut narrow 

 notches across them. They simply dropped stones and clay into a notch previously there. 

 The best example of a drift-filled notch will be found by following the stream on tlu' 

 north side of Coldknuckles, a mile do\\m to the first gap. 



