ns as to time based solely on this hypothesis must in the mean
time be viewed askance.

But the time controversy having taken root, new methods were naturally
found for testing it. The geologists sought to estimate the period of
time that must have been required for the deposit of the sedimentary
rocks now observed to make up the outer crust of the earth. The amount
of sediment carried through the mouth of a great river furnishes a clew
to the rate of denudation of the area drained by that river. Thus the
studies of Messrs. Humphreys and Abbot, made for a different purpose,
show that the average level of the territory drained by the Mississippi
is being reduced by about one foot in six thousand years. The sediment
is, of course, being piled up out in the Gulf at a proportionate rate.
If, then, this be assumed to be an average rate of denudation and
deposit in the past, and if the total thickness of sedimentary deposits
of past ages were known, a simple calculation would show the age of the
earth's crust since the first continents were formed. But unfortunately
these "ifs" stand mountain-high here, all the essential factors being
indeterminate. Nevertheless, the geologists contended that they could
easily make out a case proving that the constructive and destructive
work still in evidence, to say nothing of anterior revolutions, could
not have been accomplished in less than from twenty-five to fifty
millions of years.

This computation would have carried little weight with the physicists
had it not chanced that another computation of their own was soon made
which had even more startling results. This computation, made by Lord
Kelvin, was based on the rate of loss of heat by the earth. It thus
resembled the previous solar estimate in method. But the result was very
different, for the new estimate seemed to prove that a period of from
one hundred to two hundred millions of years has elapsed since the final
crust of the earth formed.

With this all controversy ceased, for the most grasping geologist or
biologist would content himself with a fraction of that time. But the
case for the geologist was to receive yet another prop from the studies
of radio-activity, which seem to prove that the atom of matter has in
store a tremendous, supply of potential energy which may be drawn on
in a way to vitiate utterly all the computations to which I have just
referred. Thus a particle of radium is giving out heat incessantly
in suffi