676 Transactions. — Geology. 



and then observe the rapidity with which denudation would 

 proceed. The average annual rainfall for the district aggre- 

 gates 5-6 cubic miles of water for distribution. This quantity 

 would be increased to 7 '5 cubic miles, as it was in 1893-94^ 

 when disastrous floods and breakaways occurred. If we 

 assume that one-third of the total rainfall is carried back to 

 the ocean by streams and rivers, then the carrying capacity 

 of the rivers is represented by 1-9 cubic miles. This quantity 

 was increased to 2-5 cubic miles in 1893. The amount of sedi- 

 mentary matter held in suspension by the rivers varies very 

 much according to the time of the year. In winter it is greatest, 

 in summer least, in what may be termed normal conditions of 

 flood ; but in times of heavy flood the quantity of material 

 suspended in the water is much increased. In some cases I 

 have observed along the coast quite one-twentieth of the whole 

 stream made up of earthy matter, but the instances are local 

 and rare ; and from numerous experiments carried on by me in 

 the case of the river-waters of the Tutaekuri and Ngaururoro 

 I conclude that, taking the three periods, winter, summer, and 

 flood, into account, not less than 1 in 450 parts by volume of 

 all the river-waters in this district consists of organic and in- 

 organic matter held either in suspension or solution. On this 

 estimate there is annually carried to the sea by our rivers 

 0-004 of a cubic mile of denuded earth ; or, in other words, one 

 cubic mile of the surface is carried away by the rivers and de- 

 posited in the sea in 250 years. The average height of the 

 land throughout the district is certainly not more than 900ft., 

 so that the 8,970 square miles which it embraces contain 1,530 

 cubic miles of land above sea-level, and which is available for 

 denudation. Assuming the same rate of river - denudation 

 as is now in progress, it will take on this estimate 382,500 

 years to plane down the land to sea-level and bring about con- 

 ditions when the circulation of water in the land would be im- 

 possible by means of rivers. I have purposely worked out this 

 case to show how useless as a geological index of time it 

 would be to apply the known rate of denudation by means of 

 our rivers to the age of the earth, in the face of the changes 

 — the surface-changes — which are shown to have taken place 

 over this district, and which are altogether independent of 

 river-denudation. The 7,693 acres of breakaways, in a total 

 area of 1,158,237 acres, represents 0-66 of an acre per cent., 

 and if the average movement of each slip is only 50ft. the 

 lowering of the entire district amounts to over 4in. As the 

 entire area of the district under notice contains 5,740,800 

 acres, I do not think the estimate is overdrawn by naming 

 20,000 acres as having slipped away during the period of 

 1893-94, or an area which represents the lowering of the land 

 by not less than 12in. To this must be added the enormous 

 denudation which is constantly proceeding along the coast, 



