64 SECTIONAL ADDRESSES: 
the danger incurred by ‘the dictatorial hierarchy of exact scientists’ in 
raising a mathematical structure on an insecure foundation. 
The chief methods of determining the age of the earth, other than those 
based on radioactivity, are: the rate of decline of solar energy, the 
gradient of earth temperature, the quantity of salt in the seas, the rate 
of organic differentiation, and the rate of denudation of lands and of 
accumulation in the seas in relation to the known thickness of strata made 
throughout the geological ages. 
The determination of age by means of radioactivity depends on the 
fact that uranium and thorium break down into lead and helium, and 
that the rate of this disintegration is known. ‘The time required for half 
a given amount of these elements to break down is known as the half- 
value period. This period, according to Gleditsch, can be calculated to 
within 2 per cent.; for radium it is 1660 years and for uranium 
6 X<10° years. An atom of uranium breaks down into one atom of lead 
and eight atoms of helium; if the content in these elements can be 
measured and compared with the quantity of unaltered uranium in an 
equal volume of the mineral, it is evident that the age of the mineral can 
be deduced. 
The different methods of estimating the earth’s age have given results 
so divergent that it may be of interest to enumerate some of the outstanding 
computations as summarised by Barrell in 1917. 
To this list may be added the general statement, that on the basis of 
the gravitational infall of its own mass, Thomson calculated that the sun 
could not have illuminated the earth for more than 500,000,000 years, and 
probably for not more than 100,000,000 years. His latest estimate (1897), 
based on the assumption that the temperature gradient of the earth is the 
result of simple cooling, is 20,000,000 to 40,000,000 years. Clarence King 
in 1893, on the basis of the temperature gradient, calculated that the earth 
could not be more than 24,000,000 years old. Methods based on the supply 
of salt to the sea from the decay of primary rocks are very uncertain, 
and have led to widely divergent estimates as follows: Joly, 150,000,000 ; 
F. W. Clarke, 90,000,000 ; Holmes, 340,000,000 ; and Becker, 50,000,000 
to 70,000,000. 
Lyell long ago demanded 240,000,000 years for organic differentiation, 
and Darwin thought 200,000,000 too short for the purpose. On strati- 
graphic evidence, Barrell considered 250,000,000 a reasonable estimate for 
the duration of geological time since the Precambrian. 
The history of the subject shows that high figures were originally 
proposed by geologists and that, later, they tried to lower their estimates 
under the influence of the shorter time allowed by the physicists. More 
recently, the greater figures endorsed by the physicists permit the geologist 
ample time for his processes ; both lines of inquiry are now pointing to the 
it result—higher and higher estimates of the immense antiquity of the 
globe. 
Sedimentation and the Earth’s Age. 
Intimately connected with the estimation of time are the rates of erosion 
of old rocks and of deposition of new. Herein lies the most dependable 
geological means of determining the duration of the periods ; nevertheless, 
there are serious difficulties to overcome, among which may be mentioned : _ 
