H-20 T rdiixacfions . 



accepted opinion that in their -present form, the Southern Alps are a mountain - 

 range of the alpine type. They were undoubtedly at one time such a 

 range, though one in which the folding was not acute, being some- 

 what of the natiire of a series of isoclinals ; but they have been base- 

 levelled subsequently, and then raised and partially dissected. Dissection 

 has not reached a moderately advanced stage, and a residual divide is 

 still in existence. This is crossed by numerous passes, the lowest of 

 which is Haast's Pass (1,716 ft. high). Such low passes are extremely 

 unlikely to occur in a range of the alpine type unless it has suffered 

 denudation for a long period of time In this connection compare the 

 Southern Alps with the European Alps, the Himalayas, or the Andes. 

 The Canterbury peneplain formed from the original range of the alpine 

 type was no doubt continuous with that of Otago, which, according, to 

 Professor Park* and Dr. Marshall,t has been traced with certainty as far 

 north as the Waitaki River, with a general ascending slope from south to 

 north. It is extremely unlikely that it broke off suddenly at the northerly 

 boundary of the province, and it must have continued further north towards 

 Mount Cook and the head of the Godley River. From an area of high land 

 in that neighbourhood, or from a ridge continuing north and south from 

 it, the present principal lines of drainage proceeded outward, and this may 

 explain the remarkable orientation of the valleys of the Canterbury rivers 

 noted by E. Dobson, who pointed out that the main valleys all appear to 

 radiate from a point in the Tasman Sea about twenty miles west of Hoki- 

 tika. McKay has suggested that the arrangement is due to a series of 

 radiating faults, the lines of which are usually followed by the valleys. 

 Of this there is not the slightest direct evidence available at present, 

 the suggested explanation not being founded on observation, but is pro- 

 bably in sympathy with a somewhat mistaken tendency at the present to 

 attribute a large proportion of landscape forms to crustal movements with- 

 out positive evidence of these movements being brought forward. In this- 

 case it seems more reasonable to attribute the undoubted arrangement of 

 the valleys to the shape of the original land-surface on which the drain- 

 age was established. 



The peneplain explanation of the uniformity of the moimtain-tops 

 apparently fits the case best, although there is one consideration which 

 must not be lost sight of — viz., the tendency of all mountain-summits in an 

 area subject to similar conditions to approximate to a general even height. 

 The dominant erosive agent on the mountain-slopes of this region is frost. 

 Under the influence of its powerful action they are covered with immense 

 quantities of moving debris which has been riven from the solid rocks. 

 The pointed masses which form the highest peaks are just those which 

 respond most readily to it. Owing to the more rapid weathering of the 

 highest elevations they are gradually reduced to the level of the lower 

 ones, and Avhen the rocky eminences which crown them are destroyed 

 the summits take on a more or less dome-shaped form owing to the accumu- 

 lation of vast amounts of debris, which is formed faster than it can be 

 removed by transporting agents. Although these are very active on the 

 flanks of the mountains, they are somewhat sluggish on the tops, and 

 become more and more so as weathering proceeds. This coating of debris 

 acts as a protection, retards degradation of the mountains which have had 



* N.Z. Geol. Surv. Bull. No. 5{n.s.) — Cromwell Subdivision. 

 t " Geography of New Zealand." 



