BY E. C. ANDREWS. .513 



each of these, in turn, with the marvellous topography of disused 

 roads, the first-named is seen to represent merely a phase of 

 vertical cutting by ice-streams and thus to throw a flood of light 

 on the theory of peneplanation, for if we had no evidence of 

 similar strong stream-action excavating localty for thousands of 

 feet below base-level before commencing serious lateral cutting, 

 we might be inclined to imagine the transitional grade as a 

 negligible quantity only in the discussion. On the other hand, 

 unless we had compared these enormous ice-streams with modern 

 rivers and found them producing similar forms to those effected 

 by present day rivers along their flood channels only, it might 

 have been supposed that their containing fiord and canon contours 

 were due rather to diastrophism than to stream-erosion, and as 

 such a mention of them would have been irrelevant to the present 

 discussion. Or still again, unless we had ascertained that the 

 gravitative thrusts of ordinai'y flood streams, as also of storm sea 

 waves, acted to points considerably below base-level we might 

 have been prepared to deny the fact of glaciers having any 

 material influence in shaping the contours of their canons, and 

 this being so, we again could not hope, from glacial studies, to 

 get much light thrown on stream studies as illustrating peneplain 

 formation. 



In a word, then, streams of any material will, in homogeneous 

 plateaus, however high, cut their channel bases to points approx- 

 imating closely to base-level before lateral corrasion gains the 

 ascendancy. Even the weak and short New South Wales streams 

 are still actively corrading their channel bases in attempting the 

 •establishment of transitional grades, yet they have already incised 

 their ways thousands of feet into the old Upland Valleys, and 

 along their V-shaped torrent tracks even the channel bases are from 

 400 to 600 feet only above sea-level. The closer and quicker 

 approximation of certain streams to base-level than others, before 

 entering that transitional stage when vertical cutting tends to 

 approximate to zero, and lateral cutting gains the upper hand, is 

 simply an expression of their greater strength. Thus the New 

 Zealand ice-floods quickly lowered the preglacial stream grades 



