66 
The N.Z. Journal of Science and Technology. 
[Mar. 
formation of the gas helium and of lead as the final products, radium itself 
being a comparatively short-lived intermediate product in the change. This 
transformation appeared to be the one chemico-physical process that is 
absolutely unaffected by conditions of pressure, temperature, or state of 
chemical combination. If, then, we know the rate of change, and have a 
uranium-bearing mineral that was originally completely free from lead or 
helium, the ratios between the present amounts of either of these elements 
and the residual uranium (supposing all products to have been retained 
within the mineral) will indicate the length of time the transformations 
have been in progress—that is, the age of the mineral. On this assump¬ 
tion several physicists, notably Boltwood, and geologists, notably Holmes, 
have made collections and analyses of uranium-bearing minerals of which 
the geological age is known, and there is a remarkable concordance between 
their relative ages and the ratios of lead to uranium. Owing to the escape 
of the helium, the ratios of the helium to uranium cannot be expected to 
vary so consistently ; and, as a matter of fact, Strutt has shown that such 
inconsistencies occur. 
Arguing from the most recent determination of the rate of derivation 
of lead from uranium, the following are the figures that have been assigned 
to the main divisions of geological time, a more elaborate table being given 
by Barrell in the paper cited below :— 
From the present to the commencement of Tertiary times, 55 to 65 
millions of years ; 
From the present to the commencement of Mesozoic times, 190 to 
240 millions of years ; 
From the present to the commencement of Palaeozoic times, 550 to 
700 millions of years ; 
From the present to the period of intrusion of the oldest granites, 
about 1,400 millions of years. 
The remarkable concordance of the relative ages determined by this 
method with the results of geological investigation predisposes one in 
favour of this method of age-determination, and, indeed, it has been used 
by Holmes to determine the relative ages of the pre-Cambrian formations 
of Mozambique with apparently encouraging results. The mineral which 
is most frequently studied to find the ratio of lead to uranium is zircon, 
in which the uranium is present in small quantity only. It would be 
of very great interest to collect and analyse as many samples of 
zircon as could be obtained from known sources in New Zealand, for the 
ratio so determined might aid in fixing the geological age of the schists 
and granites which have been assigned by geologists to very widely 
different periods. 
But how can these long periods be reconciled with the results of the 
various chemico-geological methods of estimating the age of the earth ? 
This is the problem that Professor Barrell has considered in his recent 
paper entitled “ Rhythms and the Measurement of Geological Time ” [Bull. 
Geol. Soc. America, vol. 28, pp. 745-904, 1917). He believes that there is 
in these earlier geological estimates a fundamental error, which lies in the 
assumption that the present conditions represent the average intensity of 
geological process during the past. Geological activities, he holds (as do 
the majority of geologists also), have not an approximately constant value, 
