282 



NATURE 



[October 27, 1921 



ever, this allowance was reduced to forty, or pre- 

 ferably to twenty, millions, and by the uncom- 

 promising- Prof. Tait to ten millions. 



These estimates proved very embarrassing to the 

 geologist, who found it impossible to compress the 

 events of the earth's history into so restricted an 

 interval without unduly "hurrying up the pheno- 

 mena." Lord Kelvin, however, was inflexible, 

 and impressively asserted that he could conceive of 

 no escape from his conclusions. 



With the discovery of radioactive elements the 

 inconceivable happened, and Lord Rayleigh was 

 amongst the first to perceive that the rate of dis- 

 integration of uranium might be used to provide 

 the geologist with a trustworthy timekeeper. By 

 his experiments and reasoning he not only en- 

 larged our views on the duration of geological time, 

 but also opened the way to other methods of 

 investigation which in the hands of Prof. Joly and 

 Dr. Holmes have yielded concordant results. 



The age of the earth was thus increased from 

 a mere score of millions to a thousand 

 millions and more, and the geologist who had 

 before been bankrupt in time now found himself 

 suddenly transformed into a capitalist with more 

 millions in the bank than he knew how to dis- 

 pose of. 



The consequences have been far-reaching ; 

 already some geologists, thus newly enriched, 

 chief among them the brilliant Barrell, whose loss 

 we still deplore, have begun to rebuild their 

 science on a new and magnificent scale, while 

 more cautious people, like myself, too cautious, 

 perhaps, are anxious first of all to make sure that 

 the new clock is not as much too fast as Lord 

 Kelvin's was too slow. Lord Rayleigh does 

 not regard this as inconceivable, but as 

 unlikely. Prof. Joly, on the other hand, can not 

 only conceive a source of error, but has obtained 

 evidence which seems to show where it lies. This 

 is furnished by a study of the well-known pleo- 

 chroic haloes which surround minute uranium- or 

 thorium-bearing crystals included in the black 

 mica of granite. By a very elegant method of 

 investigation he shows that these furnish esti- 

 mates of geological time of the same order as 

 those established by Lord Rayleigh and Dr. 

 Holmes ; but he does not stop there ; he goes 

 further. The haloes consist of a number (seven) 

 of concentric rings due to the bombardment of the 

 mica by the a-rays which are emitted by the 

 uranium or the thorium, as the case may be, and 

 their products of disintegration. The outermost 

 of these rings is due to radium C, the innermost 

 to uranium or thorium. From data provided by 

 experiment it is possible to calculate the dimen- 

 sions of the rings, and in the haloes due to 

 thorium the length of the radii obtained by direct 

 measurement agrees very precisely with that 

 obtained by calculation, and this agreement holds, 

 not for some of the rings only, but for all. A 

 similar agreement is found for the rings of the 

 uranium haloes with the remarkable exception of 

 the innermost two, due to uranium and its imme- 

 diate product, ionium. These are larger than 

 NO. 2713, VOL. 108] 



they should be; in fact, the length of the radius 

 of the uranium ring as actually observed is one- 

 sixth longer than that predicted by calculation. 

 This shows that when the haloes began to be 

 formed — i.e. in Caledonian times — the range of 

 the o-rays emitted by the uranium-bearing crystal 

 was greater than it is now, and hence probably 

 that a metope of uranium then existed with pos- 

 sibly very different properties from the uranium 

 now known to us. 



If Prof. Joly's conclusions are sound, it is clear 

 that the uranium clock has not been keeping uni- 

 form time, and the change of rate in the dis- 

 integration of uranium is as much in question as 

 the age of the earth. The problem is a physical 

 one, and geologists must leave it in the hands of 

 the physicists while anxiously awaiting its solu- 

 tion. 



It would not be fair to end here without admit- 

 ting, what Prof.- J. W. Gregory's remarks will suf- 

 ficiently reveal, that geologists are not an undi- 

 vided family. There are some who welcome the 

 expansive vistas now opened to t^ieir view, and 

 Barrell has already attempted to readjust the geo- 

 logical perspective. He pointed out how the cal- 

 culations of the earth's age, based on the thick- 

 ness of deposits and the existing rate of deposi- 

 tion, as well as those based on the amount of 

 sodium in the ocean, may be vitiated by a too 

 servile interpretation of the doctrine of uniformity. 

 The rate of disintegration of uranium may have 

 changed, but so may the rate of denudation and 

 deposition ; so far from being constant, it may 

 have increased with the progress of time, so that 

 a foot of sediment which in the Pleistocene epoch 

 accumulated, according to Barrell, in the course 

 of 375 years would have required no less than 

 3700 years for its formation in the early days of 

 the Palaeozoic era. Thus at a period when the 

 earth was more highly charged with energy its 

 activities were diminished. We must no longer 

 picture a time when the earth was "young and 

 wantoned in her prime," but must suppose that 

 she has exchanged the passive indolence of youth 

 for the fiery activity of old age. 



In support of his views Barrell pointed out 

 that the continents of the present day are more 

 elevated as a whole than they were during a great 

 part of geological time, and that their interior is 

 not flooded to so great an extent by continental 

 seas. It is doubtful, however, whether this would 

 greatly affect those estimates which have been 

 based on the maximum thickness of sedimentary 

 deposits, for this is only to be found in the fore- 

 deeps which lay in front of mountainous lands and 

 lands now vanished from our sight. 



Barrell also laid great stress on the occurrence 

 of gaps in the stratified series, unconformities, 

 disconformities, and still smaller lacunae which 

 he termed diastemata. Of the important bearing 

 which unconformities must have upon this dis- 

 cussion there can be no doubt. They were not 

 overlooked in arriving at an estimate of lOO 

 million years. The disconformities are only now 

 beginning to receive the attention to which their 



