The Birth-Time of the World | Page 9

to Sect. D., Brit. Assoc. Rep., 1896.
[2] Lehybuch dev Kosmischen Physik, vol. i., p. 347.
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most reliable. Indeed these may be described as highly reliable, being
founded on some thousands of analyses, many of which have been
systematically pursued through every season of the year. These show
that Europe with a mean altitude of less than half that of North America
sheds to the ocean 25 per cent. more salts. A result which is to be
expected when the more important factors of solvent denudation are
given intelligent consideration and we discriminate between conditions
favouring solvent and detrital denudation respectively: conditions in
many cases antagonistic.[1] Hence if it is true, as has been stated, that
we now live in a period of exceptionally high continental elevation, we
must infer that the average supply of salts to the ocean by the rivers of
the world is less than over the long past, and that, therefore, our
estimate of the age of the Earth as already given is excessive.
There is, however, one condition which will operate to unduly diminish
our estimate of geologic time, and it is a condition which may possibly
obtain at the present time. If the land is, on the whole, now sinking
relatively to the ocean level, the denudation area tends, as we have seen,

to move inwards. It will thus encroach upon regions which have not for
long periods drained to the ocean. 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. Archæan - 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 by gaseous diffusion from the minerals.
Boltwood's results on the amount of lead contained in minerals of
various ages, taken in conjunction with the amount of uranium or
parent substance present, afforded ages rising to 1,640 millions of years
for archæan and 1,200 millions for Algonkian time. Becker, applying
the same method, obtained results rising to quite incredible periods:
from 1,671 to 11,470 millions of years. Becker maintained that original
lead rendered the determinations indefinite. The more recent results of
Mr. A. Holmes support the conclusion that "original" lead may be
present and may completely falsify results derived
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from minerals of low radioactivity in which the derived lead would be
small in amount. By rejecting such results as appeared to be of this
character, he arrives at 370 millions of years as the age of the
Devonian.
I must now describe a very recent method of estimating the age of the
Earth. There are, in certain rock-forming minerals, colour-changes set
up by radioactive causes. The minute and curious marks so produced
are known as haloes; for they surround, in