18 Transactions. 



likely that they had reached an approximately similar con- 

 dition in Oligocene times, as pointed out hy Captain Hutton. 



Although the mountain tract of the province has heen tho- 

 roughly dissected, the plains are practically undissected, if we 

 omit consideration of that dissection which is due immediately 

 to the rivers themselves. They receive hardly any tributaries 

 after they leave the mountains ; the rain which falls on the 

 plains soaks rapidly through the porous beds, and finds its way 

 to the sea by percolation through the underlying shingle. The 

 rivers do receive some tributaries — e.g., the Kowhai runs into the 

 Waimakariri, and four rivers coalesce to form the Ashburton — 

 but they all rise in the foothills, and derive little of their water 

 from the rainfall on the plains. It is therefore evident that 

 there is a marked contrast between the physiographic conditions 

 of the upper portion of the rivers and that of their lower courses, 

 and hence the conditions which affect the terrace-development 

 are highly dissimilar. 



If we examine the valleys of the large rivers we find that 

 their courses may be divided into four parts, relative to their 

 terrace-development: (1.) The torrent path, where terraces are, 

 as a rule, absent. (2.) A wider valley path, where the rivers are 

 aggrading their beds, river terraces being absent, but glacial 

 terraces or shelves common. (3.) A gorge path, where rivers 

 burst through the outer range of Palaeozoic rocks on a line running- 

 through Mount Hutt and Mount Torlesse : in this case the 

 terraces have their highest development. (4.) A plain path — 

 i.e., the path from the foot of the mountains to the sea, where 

 terraces are again strongly developed, but are, as a" general 

 rule, of a simple and continuous character. 



The Torrent Path. 



The rivers begin as fair-sized streams from the terminals- 

 of glaciers, and this part of their course shows the general 

 characters of torrent and glacial erosion. The valleys are 

 typically U-shaped, with flat floors and sides so steep as to be- 

 at times unscalable for miles. They show signs of having been 

 recently swept clean, but are filling again with waste coming in 

 from the sides. There are no terraces except those due directly 

 to glacier action. Lateral moraines occasionally form terraces, 

 but only in those places where they have been protected from 

 the scouring action of the wild torrents which sweep this portion 

 of their valleys. A frequent position for these terraces is round 

 the end of a spur, and they slope down the valley at a steep 

 angle, indicating a rapid fall in the level of the surface of the 

 glacier, owing to its expanding as it accommodated itself to a 

 part of the valley where the cross section was greater. The 



