20 EEPORT 1892. 



considerations, it is surely not too much to ask that the latter should be 

 also revised. It has been well said that the mathematical mill is an ad- 

 mirable piece of machinery, but that the value of what it yields depends 

 upon the quality of what is put into it. That there must be some flaw 

 in the physical argument I can, for my own part, hardly doubt, though 

 I do not pretend to be able to say where it is to be found. Some as- 

 sumption, it seems to me, has been made, or some consideration has been 

 left out of sight, whicb will eventually be seen to vitiate the conclusions, 

 and which when duly taken into account will allow time enough for any 

 reasonable interpretation of the geological record. 



In problems of this nature, where geological data capable of numerical 

 statement are so needful, it is hardly possible to obtain trustworthy com- 

 putations of time. We can only measure the rate of changes in progress 

 now, and infer from these changes the length of time required for the com- 

 pletion of results achieved by the same processes in the past. There is for- 

 tunately one great cycle of movement which admits of careful investigation, 

 and which has been made to furnish valuable materials for estimates of 

 this kind. The universal degradation of the land, so notable a character- 

 istic of the earth's surface, has been regarded as an extremely slow 

 process. Though it goes on without ceasing, yet from century to cen- 

 tury it seems to leave hardly any perceptible trace on the landscapes of 

 a country. Mountains and plains, hills and valleys, appear to wear the 

 same familiar aspect which is indicated in the oldest pages of history. 

 This obvious slowness in one of the most important departments of 

 geological activity, doubtless contributed in large measure to form and 

 foster a vague belief in the vastness of the antiquity required for the 

 evolution of the earth. 



But, as geologists eventually came to perceive, the rate of degradation 

 of the land is capable of actual measurement. The amount of material 

 worn away from the surface of any drainage-basin and carried in the form 

 of mud, sand, or gravel, by the main river into the sea, repi'esents the 

 extent to which that surface has been lowered by waste in any given 

 period of time. But denudation and deposition must be equivalent to 

 each other. As much material must be laid down in sedimentary accu- 

 mulations as has been mechanically removed, so that in measuring the 

 annual bulk of sediment borne into the sea by a river, we obtain 

 a clue not only to the rate of denudation of the land, but also to the 

 rate at which the deposition of new sedimentary formations takes 

 place. 



As might be expected, the activities involved in the lowering of the 



