Geoscience Reference
In-Depth Information
wrote that he agreed with his predecessor's result. However, in 1914,
in a lecture to the Royal Dublin Society he argued that sediment mass,
not thickness, was a more accurate measure, and this yielded a mini-
mum of 47 million years and a maximum of 188 million years. He
reduced the mean of these limits to a figure of 87 million years on the
basis that he believed sedimentation rates were not uniform through
geological time.
THE HOUR-GLASS SHATTERS
It is clear that the nature of sediment production and its subsequent
deposition is dynamic and controlled by an infinite number of factors
that are difficult to quantify. Using sedimentation rates and accu-
mulation as a geological chronometer, geologists and biologists did
come up with dates for either the beginning of the Cambrian or the age
of the Earth, and reached a general consensus by 1900 that 100 million
years or less for this method was a reliable estimate. Although one
could scoff at the apparent foolishness of those who tried this calcula-
tion, such as its instigator John Phillips and his later disciples, one
cannot deny that the sediment accumulation concept which he first
considered in the mid 1830s was an interesting one, and no more
absurd to its advocators than other measures used earlier.
By 1910 these sedimentary chronologies were being supplanted
by the findings and age determinations generated by radioactive decay
methods. Although in its infancy, the study of radioactivity was
beginning to yield Earth ages that were considerably older than the
100 million years suggested by the sediment accumulation measure.
Even sedimentological chronologies were being revised upwards. At a
meeting of the Geological Society of America held in Albany, New
York, in 1916 the Yale professor Joseph Barrell (1869-1919) presented a
paper on 'Rhythms and the measurement of Geologic Time' that was
later published in full in the Society's Bulletin in 1917: it has since
achieved classic, almost cult, status amongst geologists. This may be
partly due to the fact that its author died young only three years later,
but also due to its revolutionary content in which he suggested that
