Geology Reference
In-Depth Information
the Astronomer Royal, after whom the famous comet is named,
proposed determining the age of the world by a rather novel
method, utilising this information. He assumed that originally
the oceans did not contain any salt but were pure water which
had become salty over time due to 'saline particles brought in
by rivers'. He therefore considered that if one measured how
much salt was in the oceans now and then measured the levels
again in a few hundred years time, one could estimate from the
rate of increase how long it had been since the oceans had not
contained any salt at all, and thus obtain the age of the Earth -
or at least the age of the oceans. He lamented the fact that no
one had thought of doing this in Greek or Roman times so that
we would now know the answer!
But Halley's proposal that measurements should start imme-
diately seems not to have been heeded for a further hundred and
eighty years until John Joly, a Professor of Geology at Trinity
College Dublin, became the leading advocate of this approach
in 1897. Joly considered that when Kelvin's 'molten globe' had
cooled su~ciently to allow water to condense, a primeval ocean
formed that did not contain much, if any, sodium. Therefore, if
he measured the amount of sodium in the sea today and com-
pared it with the amount extracted from the land and deposit-
ed in the sea each year by rivers, then he should be able to deduce
how many years it had taken to arrive at the sea's present level
of sodium. This method was fraught with assumptions and
di~culties, not least of which was the measuring of sodium in
rivers. Nevertheless, Joly arrived at an age of 89 million years
since the formation of the first ocean; a value then comfortably
within Kelvin's age of the Earth.
The second method for evaluating the age of the Earth was
epitomised by Samuel Haughton, an Irish geologist, who intro-
duced the principle that 'the maximum thicknesses of the strata
are proportional to the times of their formation'. In other words,
the thicker the strata, the longer it had taken to form. Having
calculated that sediments were deposited on the ocean floor at
