February 14, 1895] 



NA TURE 



369 



larately determined there from other evidence. Neverthe- 

 less stratigraphical arrangement gains much in precision, 

 ■\i well as in scientific interest, when changes in litho- 

 ogical characters are found to be accompanied by 

 ;hanges in organic forms ; or, on the other hand, when 

 the succession of animal or vegetable types is found to 

 be repeated in distant localities irrespective of local 

 variations in lithology. But where the rocks have been 

 so folded and broken that from mere mineral characters 

 their true order cannot be made out, the presence in 

 them of determinable life zones, elsewhere well es- 

 tablished, may enable their complicated structure to be 

 unravelled. How this task can be successfully accom- 

 plished, has been well shown by Messrs. Lapworth, 

 Peach, and Home, in regard to the excessively convoluted 

 ■structure of the Silurian uplands of the South of Scotland. 



There is, however, some risk of error in the application 

 of this valuable aid in tectonic investigation, i Jbviously 

 :he existence of a life-zone, which will be of general 

 utility, must be determined upon a basis of evidence 

 .^ul^l'lciently wide to eliminate mere local peculiarities. It 

 should rest not on the presence of a single species, but 

 in a group of species or genera, for the narrower its 

 .lalivontological range the greater will be the risk of 

 elevating accidental into general characters. We cannot 

 suppose that a given species began and ended everywhere 

 at the same time or on the same platform. In some 

 areas the conditions would be favourable for its earlier 

 ippearance or longer continuance, so that we may expect 

 :he /ones not to be very sharply defined, but to blend 

 nto each other, and in such a way that if we were to 

 lefine them by single species we should find them to pre- 

 sent exceedingly variable limits. In a restricted region, 

 where the sequence of life-zones has been accurately 

 Ascertained, these platforms are of great value in work- 

 ing out questions of geological structure. But as we 

 recede from that region the necessity of caution increases. 

 The broad features of biological sequence will no doubt 

 remain, and we shall be able to say where the upper or 

 the lower members of a sedimentary series lie, but we 

 may be led into mistakes by trying too rigidly to make 

 the pateontological zones of one country agree with 

 those of another. 



In the department of geotectonics, one of the most 

 interesting features has been the increased attention 

 bestowed upon the nature and results of the great move- 

 ments that have affected the crust of the earth. The 

 early experiments of Hall, showing that the stratified 

 rocks have undergone enormous lateral compression, 

 have been repeated and extended, and many of the 

 remarkable structures of mountain-ranges have been 

 successfully imitated. More detailed investigation has 

 been bestowed upon plicated and disrupted rocks, espe- 

 cially in Switzerland, Saxony and Scotland. The effects 

 of mechanical deformation in producing foliated struc- 

 tures, even in what were originally massive rocks, have 

 been copiously illustrated. The study of these questions 

 has led to a belter appreciation of the enormous plica- 

 tions, inversions, and dislocations which mountain-chains, 

 modern as well as ancient, have undergone. In the 

 .A.lps and in the Scottish Highlands, the subject has 

 been pursued with great ardour, and these regions will 

 henceforth be classical examples of some of the great 

 features of geotectonic geology. 



Another distinguishing characteristic of the last 

 quarter of a century of geological progress has been the 

 increased interest taken in the history of the earth's 

 surface. It is strange that while, generation after 

 generation, men laboured zealously to investigate the 

 history of the planet as recorded in the rocks of the 

 terrestrial crust, they neglected to take account of the 

 superficial topography. They did not realise that every 

 land-surface is a kind of palimpsest, on which the chron- 

 icles of a long series of ages may be more or less dis- 



NO. 1320, VOL. 51] 



tinctly traced, and thus that every landscape has, as it 

 were, two histories : first, that of the rocks which form 

 its framework, and, secondly, that of the configuration 

 into which these rocks have been carved. It was in 

 Britain that this fascinating branch of geological inquiry 

 first took definite form in the early days of Hutton and 

 Playfair, and it is here that, after long neglect, it has 

 within the last twenty or thirty years been renewed and 

 pursued with most success. The varied geological struc- 

 ture of these islands, their changeable climate, their 

 mountainous groups, and long lines of sea-beaten coast, 

 make them exceptionally suitable for the prosecution of 

 this inquiry. But this branch of geology is now receiv- 

 ing even more attention in the United States than among 

 ourselves, and in many respects the geological structure 

 of North America offers peculiar advantages for its 

 cultivation. 



It is impossible within the limits of this article to do 

 more than present in brief outline a retrospect of a few 

 of the departments of so wide a science as geology. Let 

 me, in conclusion, make reference to but one more subject 

 which has greatly exercised the minds of geologists 

 during the last quarter of a century. It is more than 

 thirty years since Lord Kelvin pointed out that there 

 must be an ascertainable limit to the antiquity of the 

 earth, and that from the data at that time available the 

 limit could not be fixed at less than twenty, or more 

 than 400, millions of years ago. He based this calcula- 

 tion on the thermal conductivity of the globe. After- 

 wards returning to the subject, he placed the limit within 

 100 millions of years ; and still more recently, reviewing 

 the question in the light of the arguments from tidal 

 retardation and the age of the sun's heat, he has brought 

 down the period of the earth's antiquity to about twenty 

 millions of years. 



Geologists have not been slow to admit that they were 

 in error in assuming that they had an eternity of past 

 time for the evolution of the earth's history. They have 

 frankly acknowledged the validity of the physical argu- 

 ments which go to place more or less definite limits to 

 the antiquity of the earth. They were, on the whole, 

 disposed to acquiesce in the allowance of loa millions 

 of years granted to them by Lord Kelvin, for the trans- 

 action of the whole of the long cycles of geological 

 history. But the physicists have been insatiable and 

 inexorable. .As remorseless as Lear s daughters, they 

 have cut down their grant of years by successive slices, 

 until some of them have brought the number to some- 

 thing less than ten millions. 



In vain have the geologists protested that there must 

 somewhere be a flaw in a line of argument which tends 

 to results so entirely at variance with the strong evidence 

 for a higher antiquity, furnished not only by the geological 

 record, but by the existing races of plants and animals. 

 They have insisted that this evidence is not mere theory 

 or imagination, but is drawn from a multitude of facts 

 which become hopelessly unintelligible unless sufficient 

 time is admitted for the evolution of geological history. 

 They have not been able to disprove the arguments of 

 the physicists, but they have contended that the physi- 

 cists have simply ignored the geological arguments as of 

 no account in the discussion. 



So here the matter has rested for some years, neither 

 side giving way, and with no prospect of agreement. 

 Within the last few weeks, however, .is readers of 

 Nature will have observed, the question has been taken 

 up anew from the physical side.' Prof Perry, feeling that, 

 after all, the united testimony of geologists and biologists 

 was so decidedly against the latest reductions of time, 

 that it was desirable to reconsider the physical argu- 

 ments, has gone over them once more. He now finds that 

 on the assumption that the earth is not homogeneous, 

 as postulated by Lord Kelvin, but possesses a much 

 1 Nature, January 3 and February 7, iSgs; pp. 224-341. 



