126 GE^^LOGICAL CHANGE, AND TIME. 



must he expliiiiie<l in accoidauce with ascertained nntnral laws. If th& 

 conclusions derived from the most careful study of this record can not 

 be reconciled with those drawn from physical considiM'ations, it is surely 

 not too much to ask that the latter should be also revised. It has been 

 well said that the matheinatical mill is an admirable piece of machinery, 

 but that. the value of what it yields depends upon the quality of what 

 is put into it. That there nuist be some flaw in the physical ar,i>'ument 

 I can, for my own ])art, hardly <loubl, though I do not i)retend to be 

 able to say where it is to be found. ISome assumption, it seems to me,, 

 has been made, or some consideration has been left out of sight, whicii 

 will eventually be seen to ^■itiate the conclusions, and which when duly 

 taken into account will allow time enough for any reasonable interpre- 

 tation of the geological record. 



In problems of this nature, where geological data capable of nu- 

 merical statement are so needful, it is hardly possible to obtain trust- 

 worthy computations of time. We can only measure the rate of changes 

 in progress now, and infer from these changes the length of time re- 

 (piired for the completion of results achieved by the same processes in 

 the past. There is fortunately one great cycle of movement which ad- 

 mits of careful investigation, and which has been made to furnish val- 

 uable materials for estimates of this kind. The universal degradation 

 of the land, so notable a chara(?teristic of the earth's surface, has been 

 regarded as an extrem(;ly slow process. Though it goes on witliout 

 ceasing, yet from centuiy to century it seems to leave hardly any ])er- 

 ceptible trace on the landscapes of a country. Mountains and plains, 

 hills and valleys api)ear to wear the same familiar aspect which is 

 indicated in the oldest pages of history. This obvious slowness in one 

 of the most important dei)artmeuts 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 degrada- 

 tion 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 nuid, sand, or gravel, by the main river into the sea 

 represents the extent to which that surface has been lowered by waste 

 in any given period of time. But denudation and deposition nuist be 

 equivalent to each other. As much material must be laid down in sed- 

 imentary accumulations 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 

 surface of the land are not everywhere equally energetic. They are 

 naturally more vigorous where the rainf\ill is heavy, where the daily 

 range of temperature is large, and where frosts are severe. Hence they 



