August 4, 1892 J 



NA TURE 



321 



crept as they do to-day. Ice scored and polished rocks exactly 

 as it still does among the Alps and in Norway. There was 

 nothing abnormal in the phenomena save the scale on which 

 they were manifested. And thus, taking a broad view of the 

 whole subject, we recognize the catastrophe, while at the same 

 time we see in its progress the operation of those same natural 

 processes which we know to be integral parts of the machinery 

 whereby the surface of the earth is continually transformed. 



Among the debts which science owes to the Huttonian school, 

 not the least memorable is the promulgation of the first well- 

 founded conceptions of the high antiquity of the globe. Some 

 six thousand years had previously been believed to comprise the 

 whole life of the planet, and indeed of the entire universe. 

 When the curtain was then first raised that had veiled the 

 history of the earth, and men, looking beyond the brief span 

 within which they had suppoed that history to have been 

 transacted, behold the records of a long vista of ages stretching 

 far away into a dim illimitable past, the prospect vividly im- 

 pressed their imagination. Astronomy had made known the 

 immeasurable fields of space ; the new science of geology 

 seemed now to reveal boundless distances of time. The more 

 the terrestrial chronicles were studied the farther could the eye 

 range into an antiquity so vast as to defy all attempts to measure 

 or define it. The progress of research continually furnished 

 additional evidence of the enormous duration of the ages that 

 preceded the coming of man, while, as knowledge increased, 

 periods that were thought to have followed each other con- 

 secutively were found to have been separated by prolonged 

 intervals of time. Thus the idea arose and gained universal 

 acceptance that, just as no boundary could be set to the 

 astronomer in his free range through space, so the whole of 

 bygone eternity lay open to the requirements of the geologist. 

 Playfair, re-echoing and expanding Hutton's language, had 

 declared that neither among the records of the earth nor in the 

 planetary motions can any trace be discovered of the beginning 

 or of the end of the present order of things ; that no symptom 

 of infancy or of old age has been allowed to appear on the face 

 of Nature, nor any sign by which either the past or the future 

 duration of the universe can be estimated ; and that although 

 the Creator may put an end, as He no doubt gave a beginning, 

 to the present system, such a catastrophe will not be brought 

 about by any of the laws now existing, and is not indicated by 

 anything which we perceive. This doctrine was naturally 

 espoused with warmth by the extreme uniformitarian school, 

 which required an unlimited duration of time for the accomplish- 

 ment of such slow and quiet cycles of change as they conceived 

 to be alone recognizable in the record of the earth's past 

 history. 



It was Lord Kelvin who, in the writings to which I have 

 already referred, first called attention to the fundamentally 

 erroneous nature of these conceptions. He pointed out that 

 from the high internal temperature of our globe, increasing in- 

 wards as it does, and from the rate of loss of its heat, a limit 

 may be fixed to the planet's antiquity. He showed that so far 

 from there being no sign of a beginning, and no prospect of an 

 end to the present economy, every lineament of the solar 

 system bears witness to a gradual dissipation of energy from 

 some definite starting-point. No very precise data were then, 

 or indeed are now, available for computing the interval which 

 has elapsed since that remote commencement, but he estimated 

 that the surface of the globe could not have consolidated less 

 than twenty millions of years ago, for the rate of increase of 

 temperature inwards would in that case have been higher than 

 it actually is ; nor more than 400 millions of years ago, for 

 then there would have been no sensible increase at all. He 

 was inclined, when first dealing with the subject, to believe that 

 from a review of all the evidence then available, some such 

 period as 100 millions of years would embrace the whole 

 geological history of the globe. 



It is not a pleasant experience to discover that a fortune 

 which one has unconcernedly believed to be ample has some- 

 how taken to itself wings and disappeared. When the geologist 

 was suddenly awakened by the energetic warning of the physicist, 

 who assured him that he had enormously overdrawn his account 

 with past time, it was but natural under the circumstances that 

 he should think the accountant to be mistaken, who thus re- 

 turned to him dishonoured the large drafts he had made on 

 eternity. He saw how wide were the limits of time deducible 

 from physical considerations, how vague the data from which 



NO. II 88, VOL. 46] 



they had been calculated. And though he could not help 

 admitting that a limit must be fixed beyond which his chronology 

 could not be extended, he consoled himself with the reflection 

 that after all a hundred millions of years was a tolerably ample 

 period of time, and might possibly have been quite sufficient for 

 the transaction of all the prolonged sequence of events recorded 

 in the crust of the earth. He was therefore disposed to ac- 

 quiesce in the limitation thus imposed upon geological history. 



But physical inquiry continued to be pushed forward with re- 

 gard to the early history and the antiquity of the earth. Further 

 consideration of the influence of tidal friction in retarding the 

 earth's rotation, and of the sun's rate of cooling, led to sweep- 

 ing reductions of the time allowable for the evolution of the 

 planet. The geologist found himself in the plight of Lear when 

 his bodyguard of one hundred knights was cut down. "What 

 need you five-and-twenty, ten or five ? " demands the inexorable 

 physicist, as he remorselessly strikes slice after slice from his 

 allowance of geological time. Lord Kelvin is willing, I believe, 

 to grant us some twenty millions of years, but Prof. Tait would 

 have us content with less than ten millions. 



In scientific as in other mundane questions there may often 

 be two sides, and the truth may ultimately be found not to lie 

 wholly with either. I frankly confess that the demands of the 

 early geologists for an unlimited series of ages were extravagant, 

 and, even for their own purposes, unnecessary, and that the 

 physicist did good service in reducing them. It may also be 

 freely admitted that the latest conclusions from physical con- 

 siderations of the extent of geological time require that the in- 

 terpretation given to the record of the rocks should be rigorously 

 revised, with the view of ascertaining how far that interpretation 

 may be capable of modification or amendment. But we must 

 also remember that the geological record constitutes a voluminous 

 body of evidence regarding the earth's history which cannot be 

 ignored, and must be explained in accordance with ascertained 

 natural laws. If the conclusions derived from the most careful 

 study of this record cannot be reconciled with those drawn from 

 physical 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 admirable 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 assump- 

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

 been left out of sight, which 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 computations 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 completion of results achieved 

 by the same processes in the past. There is fortunately 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 cha- 

 racteristic of the earth's suriace, has been regarded as an ex- 

 tremely slow process. Though it goes on without ceasing, yet 

 from century to century it seems to leave hardly any perceptible 

 trace on the landscapes of a country. iMountains and plains, 

 hills and valleys, appear lo wear the same familiar aspect which 

 is indicated in the oldest pages of history. This obvious slow- 

 ness 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 draina<je- 

 basin and carried in the form of mud, 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 must be equivalent to each other. 

 As much material must be laid down in sedimentary accumula- 

 tions as has been mechanically removed, so that in measuring 

 the annual bulk of sediment borne into the sea by a rj>er, 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. 



