360 Transactions. 



This is to be seen very clearly in the neighbourhood of the intake for 

 the tunnel in connection with the power-station, but clearest of all in the 

 sheltered bays on what is called " the Peninsula," about half-way along the 

 eastern side of the lake. These show clearly that the water within com- 

 paratively recent times was 60 ft. higher. The lake now drains out at its 

 northern end towards the Harper and Wilberforce Rivers, but there is a 

 clearly denned old river-channel, with terraces and everything complete, 

 leading from near Messrs. Murchison's station buildings towards the Acheron 

 at the other end of the lake, the road following the bed of this old stream 

 for about a mile. The highest point of this outflow channel above the lake 

 is about 60 ft., so that if the lake were filled up to the level of its old shore- 

 line it would discharge in the opposite direction to what it does at present. 

 The cause of this reversal is attributable to a lowering of the barrier at the 

 northern end. What this barrier was is not clear ; it may have been the 

 front of the retreating glacier of the Wilberforce, for if that acted as a 

 ponding agent it would be effective as long as its terminal was below the 

 upper end of the lake, but when the ice retreated farther it would allow the 

 outflow of water in the direction of the Wilberforce. This would no doubt 

 account for the phenomenon, especially as the change in direction of drain- 

 age appears to have taken place suddenly, and not by the slow removal of 

 a rock barrier by water erosion. 



The other explanation suggested is that the movements ■*on the line of 

 fault had not ceased when the lake discharged normally from its southern 

 end, but that a sudden lowering of the country north-west of the line of 

 the Harper River allowed of escape of water in a northern direction. The 

 slumped character of the sides of the Upper Harper Valley suggest recent 

 earth-movements. Further, if the line of the Upper Harper be continued 

 to the south-west it will pass close to two other Tertiary outliers, one near 

 Glenthorne, at the mouth of the Harper,* and the other near the Mount 

 Algidus Station, across the Wilberforce ; and it is possible that their 

 preservation may be associated in some way with the same set of circum- 

 stances that have accounted for the occurrence of a Tertiary remnant 

 in the Upper Harper Valley. By suggesting this continuation of the 

 fracture- line across the Wilberforce it is not intended to endorse McKay's 

 extended system of fault-lines as indicated in his map. Although this 

 writer did great service towards the proper interpretation of the struc- 

 tural features of the mountain area of Canterbury, he probably erred 

 in carrying his hypothesis too far. There is no evidence that I am 

 aware of pointing to the extension of this line of fracture farther to the 

 south-west. 



* J. von Haast, Geology <>f Canterbury and Westland, 187tt. geological map facing 

 p. 370. 



