Hill. — Denudation as a Factor of Geological Time. 671 



by leading geologists, exceeds the average height of the land- 

 surface by not less than sixty times. These sedimentary de- 

 posits have been removed, principally by the agency of water, 

 from some place where they were in situ, just as rocks are 

 now removed by means of denudation from a higher to a lower 

 level. Denudation is a iiecessary condition of inequality of 

 surface when acted on by physical agents. The greater the 

 instability of rock-masses the more certain is the prospect of 

 denudation, for every physical agent acts in breaking down 

 rocks and bringing them to one common level. But, recog- 

 nising all this, is it possible to formulate a standard for 

 the measurement of past time by an assumed standard of 

 denudation which, the world over, must vary according 

 to the character of the deposits composing every river-basin, 

 in conjunction with its specialised climatic conditions ? 



As a test of actual change now in progress a knowledge 

 of the amount of material borne by rivers to the sea is of 

 high scientific value, and the results obtained by actual ex- 

 periment give some idea as to the prospects to come. But 

 even here the knowledge is of relative value only, for rivers, 

 like everything else in nature, differentiate ; they have a 

 beginning, they undergo change, attain their maximum of 

 development, and in course of time they disappear. Every 

 river in Hawke's Bay affords evidence of the truth of this 

 statement, and every tributary is an illustration of the surface- 

 changes in progress, and of the differentiations which rivers 

 undergo during the course of their history. The index of 

 denudation of those rivers where experiments have been 

 made shows wide variations, as is seen in the case of the 

 Danube and the Po in Europe. The basin of the Danube is 

 said to be lowered at the rate of one foot in 6,846 years, and 

 that of the Po one foot in 729 years ; but these rates have 

 varied, and must have varied, ever since the rivers first began 

 to flow. The elevation of the Po and Danube in their upper 

 course, where denuda.tion is always most powerful, would 

 largely increase their effective power for transport of de- 

 nuded material ; and a similar remark applies to every river 

 on the earth's surface. The increase in the denuding-power 

 of moving water varies, according to Hopkins, as the sixth 

 power of the velocity of the current ; so that by simply 

 doubling the rate of flow in a river its effective carrying- 

 power is increased to no less than sixty-four times, and if 

 trebled its power is increased 729 times. And every geologist 

 must recognise the importance of this varying power in esti- 

 mating surface-changes as brought about by the action of 

 moving water. For years my attention has been directed to 

 this aspect of geological inquiry, and it has always appeared 

 to me that the plan of estimating the amount of material 



