Reviews—Professor Joly’s Radioactivity and Geology. 128 
Many readers will turn expectant to the chapter on ‘‘ Uranium and 
the Age of the Earth’’; but here we must confess some degree of 
disappointment. From comparative determinations of radium and 
helium, especially in phosphatic nodules,! Strutt has arrived at high 
estimates of the age of the stratified sequence. Since the most hkely 
source of error is loss of part of the helium, these should be under- 
rather than over-estimates. Our author cites these and some other 
results, but only to emphasize the elements of doubt attaching to 
them. He then throws over radioactivity, and offers us, instead, two 
methods of calculating geological time which lead to very much lower 
figures. One is based on the rate of sedimentation, and the other on 
the amount of sodium in the sea. Both have already been before the 
geological public, and it is not necessary to enter into them. To us 
they appear far more hazardous than the helium method. 
The manipulation of figures possesses an undoubted fascination. 
There is a feeling of security in striking a mean of discordant results, 
and a certain sense of generosity in allowing a margin on the side of 
safety; and it is easy for the enthusiastic speculator to forget how 
enormously errors may accumulate when several rough estimates are 
taken as links in a chain of argument. Even the taking of a mean 
demands more judgment than it sometimes receives. For instance, on 
p. 284 of this book, we are given the ratio of sediment carried in the 
waters of nine different rivers. ‘his ratio ranges from 1 in 291 to 
1in 10,000. We do not believe that an average of nine figures so 
discordant can afford any useful information concerning the average 
amount of sediment carried by the rivers of the world. But, further, 
the author has taken a harmonic instead of an arithmetic mean. The 
true mean is not 1 in 2781, but 1 in 1182, which at one blow reduces 
the calculated eighty millions of years to about thirty-five millions ! 
Personally we do not attach any weight to one or other of these 
figures. 
We are far from wishing to disparage the introduction of quanti- 
tative considerations into dynamical and historical geology; but we 
feel at the same time the need for sounding a note of warning at the 
present juncture. Thanks to the discovery of radium, geology -is 
finally delivered from the tyranny of a certain school of physicists, 
and Lord Kelvin’s arguments based upon the consistentior status are 
swept away. Let us take heed lest, freed from one bondage, we fall 
straightway into another not less galling. 
Probably Professor Joly does not look for unhesitating acceptance 
of all his conclusions. However much opinions may differ on some 
of the points raised, he has here introduced us to a large number of 
important and curious questions, and his book should be in the hands 
of every geologist who is interested in the modern developments ot 
science. 
A. H. 
_ | Strongly confirmed, since this was written, by examination of zircons from 
igneous rocks of various ages. 
