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estimate the duration of geological time. I do not ask you necessarily 
to concur in this conclusion, but at least to suspend judgment in the 
matter ; for it will assuredly be a misfortune if geology, so lately freed 
from one bondage, should fall straightway into another no less galling. 
This at least is certain, that every one of the various geological pro- 
cesses which have been discussed in this connection, is controlled by 
conditions which cause its rate to be very variable. It is a clock 
which now hurries and now creeps, or stands still, and it can never be 
trusted as a time-keeper. Even for the most recent chapter of geo- 
logical history we can make no approach to certainty on these lines. 
Attempts have been made, for example, to estimate the time since the 
final retreat of the ice in North America from the rate of recession of 
the falls of Niagara ; but the evidence shows that this rate has varied 
widely even during the last half-century, and Gilbert, after a careful 
study of all the data, refrains from offering any opinion on this point. 
Must we then abandon all hope of any practicable measure of 
time in geology ? I do not draw this conclusion, but rather that we 
must search outside strictly geological phenomena for some physical 
process of which the rate is not affected by any disturbing conditions. 
There are, I think, only two classes of changes for which so much can 
be claimed — the transformations of the radio-active group of elements 
and the astronomical movements. It seems not improbable that in one 
or other of these two directions the solution of the problem may eventu- 
ally be found. 
The chemists have taught us that radium is derived from the 
spontaneous breaking up of uranium, the change taking place ap- 
parently in two stages and involving the liberation of three atoms 
of helium. But radium itself disintegrates spontaneously, giving the 
radium-emanation named niton and liberating another atom of helium. 
Niton in its turn undergoes disintegration, and so on through a suc- 
cession of changes. The final product is lead, and in the gradual 
conversion of uranium to lead eight atoms of helium in all are set free. 
Of these various spontaneous changes some proceed with extreme 
slowness, others with comparative rapidity ; but in each case there is 
a constant rate which, so far as experiment has tested it, is inde- 
pendent of temperature or pressure. 
Professor Strutt has shown that this gradual liberation of helium 
can be made the basis of a method of estimating the absolute ages of 
minerals and rocks. For example, phosphates and some iron-ores are 
rich in radium, derived from uranium. They also contain helium, 
