Geology Reference
In-Depth Information
the conclusion respecting solvent denudation to which radioactivity drives
us. If the sodium in the ocean has taken 1400 million years to accumu-
late, the rivers are now bearing to the sea about 14 times the average per-
centage of the past. It seems quite impossible to find any explanation of
such an increase.
It is di~cult for us now to appreciate just how traumatic the
discovery of radioactivity and all its implications must have been
for the traditional geologists of the time. In a matter of a few
years their whole world and conventional way of thinking about
the Earth had been turned upside down. They had been given
vast time scales to fill with sediments of which there was no evi-
dence, so it was only natural that they should first assume a flaw
in radioactivity, the young transgressor who had not yet shown
his credentials. Quite reasonably Joly questioned some of the
assumptions:
With these di~culties in view it is excusable to direct attention to the
foundations of the radioactive method and ask how far they are secure.
The fundamental assumption is that the parent radioactive substance,
uranium, has always in the past disintegrated at the present rate. Is this
assured? . . . I venture to suggest - I do so with di~dence - that our
assumption of a constant rate of change for the parent substances - ura-
nium and thorium - is really without any very strong basis.
Because, he argued,
we know nothing as to the origin of the primary radioactive elements
. . . the rate of change 150 million years ago may have been many times
what it is now.
Many geologists felt he had a point, although Holmes with
his understanding of the laws of physics, never considered it for
a moment.
Given the extremely crude methods that he had at his dis-
posal, compared with the sophisticated techniques available
today, Holmes' first attempt at a geological time scale has with-
stood the 'test of time' remarkably well. But in addition to his
mineral analyses, Holmes' work confirmed Boltwood's ideas
that lead was the ultimate decay product of uranium. Holmes
