THE NEW ZEALAND 
JOURNAL OF SCIENCE 
AND 
TECHNOLOGY. 
]g » O n f""*- -- v ' • ~ 
Vol. II. WellingtoJiU March, 1919. No. 2. 
( ^JUN5-fgf9 Y 
SOME ESTIMATES ,’ OFC^E AGE OF THE EARTH. 
By W. N. Benson, B.A., D.Sc., F.G.S., Professor of Geology, University 
of Otago. 
During the past few decades there have been several attempts to express 
numerically the probable age of the earth. Lyell and Darwin guessed that 
it was perhaps more than 200 millions of years, and other geologists were 
inclined to “ draw unlimited drafts on the bank of time,” but were checked 
by Kelvin’s declaration that the consideration of the present rate of loss 
of heat from the earth showed, upon certain assumptions, that the pro¬ 
bable age of the earth was not more than 20 millions of years, or, according 
to Tait, only half that figure. These estimates few geologists could accept, 
feeling sure that some error had been involved in the assumptions upon 
which they were based. 
The geological estimates rested on the measurement of the rate of 
deposition of sedimentary rocks at the present time, and the thickness of 
all sedimentary rocks ; or the annual addition to the ocean by river-waters 
of dissolved salts derived from the weathering of rocks, and the estimation 
of the total amount of such derived salts in the ocean. These arguments 
at first assumed that the present intensity of geological processes might be 
considered as equal to the average intensity of such activities in past 
times. Estimates on these assumptions showed a probable age for the 
earth of rather less than 100 millions of years. 
Recently, however, the physicists have again attacked the estimates of 
geologists, this time to declare them far too low, and to suggest as more 
probable a figure fifteen times as great. The error in Kelvin’s assumption, 
it is explained, was shown when the presence of radium was discovered in 
the crust in sufficient amount to account, by its heat of transformation, for 
the heat that is steadily lost by radiation from the earth. In effect, they 
said, “ The earth is not gradually cooling, it is being kept on the hob.” But 
the significance of the discovery of radio-activity in this connection has now 
to be extended. The element uranium is steadily breaking down, with the 
5—Science. 
