n. On such areas there is an accumulation of
soluble salts which the deficient rivers have not been able to
carry to the ocean. Thus the salt content of certain of

[1] See the essay on Denudation.

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the rivers draining to the ocean will be influenced not only by
present denudative effects, but also by the stored results of
past effects. Certain rivers appear to reveal this unduly
increased salt supply those which flow through comparatively arid
areas. However, the flowoff of such tributaries is relatively
small and the final effects on the great rivers apparently
unimportant--a result which might have been anticipated when the
extremely slow rate of the land movements is taken into account.

The difficulty of effecting any reconciliation of the methods
already described and that now to be given increases the interest
both of the former and the latter.

THE AGE BY RADIOACTIVE TRANSFORMATIONS

Rutherford suggested in 1905 that as helium was continually being
evolved at a uniform rate by radioactive substances (in the form
of the alpha rays) a determination of the age of minerals
containing the radioactive elements might be made by measurements
of the amount of the stored helium and of the radioactive
elements giving rise to it, The parent radioactive substances
are--according to present knowledge--uranium and thorium. An
estimate of the amounts of these elements present enables the
rate of production of the helium to be calculated. Rutherford
shortly afterwards found by this method an age of 240 millions of
years for a radioactive mineral of presumably remote age. Strutt,
who carried

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his measurements to a wonderful degree of refinement, found the
following ages for mineral substances originating in different
geological ages:

Oligocene - 8.4 millions of years.
Eocene - 31 millions of years.
Lower Carboniferous - 150 millions of years.
Archaean - 750 millions of years.

Periods of time much less than, and very inconsistent with, these
were also found. The lower results are, however, easily explained
if we assume that the helium--which is a gas under prevailing
conditions--escapes in many cases slowly from the mineral.

Another product of radioactive origin is lead. The suggestion
that this substance might be made available to determine the age
of the Earth also originated with Rutherford. We are at least
assured that this element cannot escape