Speight, Cockayne, Laixg. — MduhI ArrDir.^inifh Disfrirf. 819 



Those dift'erent conditions of rainfall have probably lasted for a very 

 long time, and thv'w effect is evidenced by the lower elevations to the west, 

 where the subaerial denudation has been excessive, owing to the heavy 

 rains causing frequent Hoods and the great amount of snow forming large 

 and powerful glaciers. There is evidence from other parts of the Alps that 

 the dominant western erosion has resulted in the capture by the western 

 streams of the upper tributaries of some of the rivers of the eastern 

 watershed. It is only in some such way that the formation of Arthur's 

 Pass can be explained.. This action is likely to continue when the marked 

 difference in the height of the floors of the eastern and western valleys is 

 considered^ — for example, the floor of the Bealey Valley near the tunnel- 

 entrance is at an altitude of 2,435 ft., while the western end, at Otira, five 

 miles and a quarter distant, is only 1,583 ft. — that is, it is 852 ft. lower. 

 The same is generally true for the other valleys of that part of the Southern 

 Alps, a feature well brought out by Mr. Edward Dobson's original surveys 

 for a road across the range. This remarkable physical peculiarity can be 

 most easily explained by the greater efficiency of eroding agents on the 

 western side of the range. The heads of the valleys have been sapped back 

 by glaciers, and the valleys have been deepened by ice and water action 

 so that they have been able to encroach on their eastern neighbours ; and 

 the marked overlapping of adjacent streams on either side of the crest of 

 the range intensifies this effect when capture of even one small tributary 

 has taken place. The more rapid erosion on the west will loAver the range 

 progressively from that direction, so that the eastern region will become 

 of relatively higher relief, and in future geological time Mount Hutt and 

 Mount Torlesse, on the eastern border of the mountainous country, will, 

 if similar meteorological conditions continue, become its highest elevations, 



(c.) Present Form of Mountain Region — a Dissected Peneplain. 



The rocks of which the area is composed consist of greywackes, slates, 

 and mudstones of Lower Mesozoic age, which have been folded subse- 

 quently by mountain-building movements into folds whose general axes 

 rmi in a N.N.E.-S.S.W. direction. Local variations appear to be frequent, 

 so that at times the direction is almost E.-W., and again become N.W.- 

 S.E. This variation is apparently due to change in the direction of the 

 thrusts by which the area was' folded. The date of the folding is probably 

 Upper Jurassic, but it may have been Upper Cretaceous. A feature of the 

 mountainous region which has thus been produced is the approximately 

 uniform height of the great majority of peaks. A very large number of 

 these are between 6,800 ft. and 8,200 ft. in height, with very few above 

 or below these limits except in the Mount Cook region. This suggests that 

 the whole area has been reduced to a level platform either by marine denu- 

 dation or, more probably, has been base-levelled by a former stream system, 

 and a few peaks, like Mount Arrowsmith, which dominate the rest are 

 the residual elevations on the peneplain. The higher mountains which no 

 doubt once existed further west, and have been removed by erosive agents, 

 would represent the higher mountains on this peneplain. It is possible, 

 however, that the present prominent elevations are the remains of an old 

 divide which existed on it, and from which streams once flowed east and 

 west. 



In advancing this theory for the present form of the mountain region 

 >of Canterbury, I am quite aware that this is contrary to the generally 



