18 



TUTIRA 



have tried to gauge wear and tear have themselves moved. There is, 

 however, growing on the stream's edge immediately above the fall, a 

 certain aged kowhai tree whose bole is, I believe, five or six feet farther 

 from the chasm's rim than thirty-seven years ago ; the rim has receded 

 that distance. At all events there can be no doubt that the fall is 

 slowly retreating lakewards. Attrition is at present almost imper- 

 ceptible, yet there are reasons to suppose that under certain circum- 

 stances it might become rapid, and that then the alluvial deposits of 

 the lake-basin accumulated during centuries might be washed away in 

 weeks. Because there has been almost no movement for years, it does 

 not follow that such conditions will continue. 



Instances of sudden erosion have occurred not infrequently even 



Waterfall as at present. 



in my time. After years of quiescence the ditch three feet deep 

 and two feet across draining Kahikanui flat, amply wide enough 

 for the normal flow of water, became in a single flood and in a few 

 hours' time a chasm 140 feet wide, 15 feet deep, and 300 feet long. 

 In the three days' deluge of 1917, 600 yards of Tylee's Valley were 

 gutted to the width of a chain and a depth of 20 feet. Flood-water 

 had got into softer strata and gouged out in a few hours these great 

 weights of soil. Some such catastrophe might likewise happen in the 

 far future to the big waterfall. Already there is a cavern extending 

 far beneath the ledge over which its water flows, proving thereby the 

 existence of a softer rock beneath. Should, therefore, the hard upper 

 crust give way or wear out, as must eventually happen, and should the 



