■170 McKay : Fault Revelations. 



thrown down impervious shale, against pervious limestone. 

 Along the planes and joints of the latter, underground water 

 has been and is making its unseen way. Coming against the 

 shale, it can no longer pursue its underground course, and is 

 forced to come to the surface. In doing so it clears out the 

 easily-moved shale until the face of the fault stands revealed. 

 The straight cleft thus formed is deepest at the centre, either 

 because the underground stream has its greatest volume there, 

 or because that is where the greatest ' throw ' of the fault lies. 

 The water which collects in this long, narrow excavation, in 

 finding its way by overflow into the major valley, eats out the 

 third valley at right angles to both. This automatically main- 

 tains the same depth as the cleft it drains, at the point where it 

 opens out of it, and thence thins down to the ordinary depth of 

 a hill-side stream. 



ADDITIONAL NOTE ON THE ' BUTTERTUBS.' 



Half an hour's walk over the divide from this section, along- 

 side the road, is a group of pot-holes which give their name to the 

 pass. They do not compare with Alum Pot and Gaping Ghyll 

 in depth (being at most only twenty-five feet deep) , but they are 

 in another way even more interesting. They hardly deserve 

 the name of ' Pot-Holes,' for the one in the photograph (which 

 is quite the best) is so full of pillars and walls of limestone, that 

 it is quite possible to walk across the ' pot,' from one side to the 

 other as on stepping stones. The sides of these pillars and 

 walls are uniformly vertical, and they present a beautifully 

 columnar or fluted surface, like many Cathedral pillars. (See 

 Plate IX:, fig. i). Into one of the pots a small stream plunges 

 and is lost. The bottom and sides of all of them are covered 

 with many kinds of ferns. 



The explanation of the peculiar featm-es of these pots could 

 only be arrived at through a detailed comprehension of 

 the geological structure of the district. The fluting and the 

 pillars are probably due, in the beginning, to certain unusual 

 stresses to which the limestone has been subjected ; which 

 stresses in some way not easily understood, have produced the 

 columnar jointing along the planes of which the pots have 

 been eaten out by the ordinary agents of subaerial denu- 

 dation. 



Naturalist, 



