Hill. — Denudation as a Facto?- of Geological Time. 677 



there being a glacier-like movement of the rocks seaward for 

 not less than a hundred miles between the East Cape and 

 Turnagain. 



These modifying results of denudation are to be met 

 with everywhere, and operating more or less under all con- 

 ditions and climes. Eiver-basins become lowered, the rate of 

 How and the power of carriage are diminished, new basins 

 are formed, and these in combination with the varying 

 character of climate make it impossible to accept results 

 depending upon so many complex and modifying causes in 

 estimating the past age of the earth and as a standard 

 measure of denudation in j^ast time. "We are certain that long 

 periods of time must have gone by to bring about changes of 

 the earth's surface and the differentiations in the flora and 

 fauna such as are recorded in the rocks themselves ; but 

 whether the years are to be reckoned by tens of thousands, 

 hundreds of thousands, or by millions there is no means of 

 knowing. No matter how we strive to arrive at a correct 

 index of geological time the difficulties are the same. The 

 animal equally with the vegetable kingdom has reached a 

 complex. — indeed, a highly complex — period of differentiation ; 

 but this differentiation has not been brought about by corre- 

 sponding differentiations of the earth's surface, acted on as it 

 has been from the beginning by forces all of which primarily 

 depend on the sun for their activity and effectiveness. The 

 world has grown from the simple to the complex by ever- 

 changing and ever-modifying conditions. Every to-day differs 

 essentially from its yesterday ; and, although we know that 

 animals and plants have come down through the ages by a 

 constant advance in adaptation and specialisation, it cannot 

 be asserted what conditions prevailed in times past to produce 

 all those earth-changes without which changes in the flora 

 and fauna were impossible. One thing, however, is certain : 

 that time cannot be measured by river-denudation, and the 

 estimates given by Houghton and others as to the time neces- 

 sary for the deposition of the sedimentary rocks are just 

 as uncertain and unsatisfactory as the estimates which the 

 physicists and mathematicians have given us. The facts 

 relating to surface-denudation which appear below supply 

 evidence of rapid changes of surface irrespective of river- 

 denudation, and the time will no doubt come when similar 

 facts will be collated for other areas and countries. Such 

 facts, however, whilst they supply valuable information to the 

 geologists as to surface-changes now in progress, and to the 

 possibility of great changes when physical conditions deviate 

 but slightly from what may be termed the normal standard, 

 cannot supply a reliable factor in the determination of geolo- 

 gical time, any more than the inferences to be drawn from 

 denudation by rivers, whether European, Asiatic, or other. 



