484 



NA TURE 



[September 13, 1900 



present rate at which form flows to form seemed so slow as 

 almost to amount to immutability. How vast then must have 

 been the period during which by slow degrees and innumerable 

 stages the protozoon was transformed into the man ! And if we 

 turn to the stratified column, what do we find ? Man, it is true, 

 at the summit, the oldest fossiliferous rocks 34 miles lower 

 down, and the fossils they contain already representing most of 

 the great classes of the Invertebrata, including Crustacea and 

 Worms. Thus the evolution of the Vertebrata alone is known 

 to have occupied a period represented by a thickness of 34 miles 

 of sediment. How much greater, then, must have been the 

 interval required for the elaboration of the whole organic world ! 

 The human mind, dwelling on such considerations as these, 

 seems at times to have been affected by a sur-excitation of the 

 imagination, and a consequent paralysis of the understanding, 

 which led to a refusal to measure geological time by years at all, 

 or to reckon by anything less than "eternities." 



Geologic Periods of Time. 



After the admirable Address of your President last year it 

 might be thought needless for me to again enter into a con- 

 sideration of this subject ; it has been said, however, that the 

 question of geological time is like the Djin in Arabian tales, and 

 will irrepressibly come up again for discussion, however often it 

 is disposed of. For my part I do not regard the question so 

 despondingly, but rather hope that by persevering effort we may 

 succeed in discovering the talisman by which we may compel 

 the unwilling Djin into our service. How immeasurable would 

 be the advance of our science could we but bring the chief 

 events which it records into some relation with a standard of 

 time ! 



Before proceeding to the discussion of estimates of time drawn 

 from a study of stratified rocks let us first consider those which 

 have been already suggested by other data. These are as 

 follows: — (i) Time which has elapsed since the separation of 

 the earth and moon, fifty-six millions of years, minimum esti- 

 mate by Prof. G. H. Darwin. (2) Since the " consistentior 

 status," twenty to forty millions (Lord Kelvin). (3) Since the 

 condensation of the oceans, eighty to ninety millions, maximum 

 estimate by Prof J. Joly. 



It may be at once observed that these estimates, although 

 independent, are all of the same order of magnitude, and so far 

 confirmatory of each other. Nor are they opposed to conclu- 

 sions drawn from a study of stratified rocks ; thus Sir Archi- 

 bald Geikie, in his Address to this Section last year, affirmed 

 that, so far as these were concerned, one hundred millions of 

 years might suffice for their formation. There is then very 

 little to quarrel about, and our task is reduced to an attempt, 

 by a little stretching and a little paring, to bring these various 

 estimates into closer harmony. 



Prof. Darwin's estimate is admittedly a minimum; the 

 actual time, as he himself expressly states, "may have been 

 much longer." Lord Kelvin's estimate, which he would make 

 nearer twenty than forty millions, is founded on the assump- 

 tion that since the period of the "consistentior status" the 

 earth has cooled simply as a solid body, the transference of 

 heat from within outwards having been accomplished solely by 

 conduction. 1 



It may- be at once admitted that there is a large amount 

 of truth in this assumption ; there can be no possible doubt 

 that the earth reacts towards forces applied for a short time 

 as a solid body. Under the influence of the tides it behaves 

 as though it possessed a rigidity approaching that of steel, 

 and under sudden blows, such as those which give rise to 

 earthquakes, with twice this rigidity, as Prof. Milne informs 

 me. Astronomical considerations lead to the conclusion that 

 its effective rigidity has not varied greatly for a long period of 

 past time. 



Still, while fully recognising these facts, the geologist knows 

 — we all know — that the crust of the earth is not altogether 

 solid. The existence of volcanoes by itself suggests the con- 

 trary, and although the total amount of fluid material which is 

 brought from the interior to the exterior of the earth by 

 volcanic action may be, and certainly is, small — from data 

 given by Prof. Penck, I estimate it as equivalent to a layer 

 of rock uniformly distributed 2 mm. thick per century ; yet 

 we have every reason to believe that volcanoes are but the 

 superficial manifestation of far greater bodies of molten material 



1 The heat thus brought to the surface would amount to one-seventeenth 

 of that conveyed by conduction. 



NO. 161 I, VOL. 62] 



which lie concealed beneath the ground. Even the wide areas 

 of plutonic rock, which are sometimes exposed to view over 

 a country that has suffered long-continued denudation, are 

 merely the upper portion of more extensive masses which lie 

 remote from view. The existence of molten material within 

 the earth's crust naturally awakens a suspicion that the pro- 

 cess of cooling has not been wholly by conduction, but also 

 to some slight extent by convection, and to a still greater 

 extent by the bodily migration of liquid lava from the deeper 

 layers of the crust towards the surface. 



The existence of local reservoirs of molten rock within the 

 crust is even still more important in another connection, that 

 is, in relation with the supposed "average rate of increase of 

 temperature with descent below the ground." It is doubtful 

 whether we have yet discovered a rate that in any useful 

 sense can be spoken of as "average." The widely divergent 

 views of different authorities as to the presumed value of this 

 rate may well lead to reflection. The late Prof. Prestwich 

 thought a rise of 1° F. for every 45 feet of descent below the 



Fig. I. — Map of the British Isles, showing the distribution of rates of 

 increase of temperature with descent. The rates are taken from the 

 " British Association Report," except in the case of those in the south of 

 Ireland. 



zone of constant temperature best represented the average ; 

 Lord Kelvin in his earlier estimates has adopted a value of 

 1° F. for every 51 feet ; the Committee of this Association 

 appointed to investigate this question arrived at a rate of 

 1° F. for every 60 feet of descent ; Mr. Clarence King has 

 made calculations in which a rate of 1° F. for 72 feet is 

 adopted ; a re-investigation of recorded measurements would, 

 I believe, lead to a rate of 1° F. in 80 or 90 feet as more 

 closely approaching the mean. This would raise Lord Kelvin's 

 estimate to nearly fifty millions of years. 



When from these various averages we turn to the observations 

 on which they are based, we encounter a surprising divergence 

 of extremes from the mean ; thus in the British Isles alone the 

 rate varies from 1° F. in 34 feet to 1° F. in 92 feet, or in one 

 case to i°F. in 130 feet. It has been suggested, and to some 

 extent shown, that these irregularities may be connected with 



