Geology Reference
In-Depth Information
Houtermans knew of Holmes' paper on 'The Origin of Igneous
Rocks', published in the early 1930s, where Holmes first
proposed his ideas on initial ratios, or if Houtermans arrived
at his ideas independently from Holmes in the late 1940s,
but either way there is a strong case for renaming the
'Holmes-Houtermans' model for dating the age of the Earth as
simply the 'Holmes' model, since Holmes clearly got there first.
While waiting for the arrival of his calculating machine so that
he could complete the formidable calculations required to esti-
mate the age of the Earth from Nier's data, Holmes turned his
attention once again to his old dream of building a geological
time scale. By using Nier's data as control points in the geo-
logical column, he felt that at last the time had come when he
should be able to resolve the old conflicts between the hour-
glass and the radiometric methods
'in a reconciliation of con-
verging evidence'
from the base of the Cambrian up to Recent
times. The problem however, was twofold: the majority of Nier's
results were on Precambrian samples, in other words they were
too old, and of those that were younger than Precambrian, the
geological ages were not well constrained. This process of elim-
ination left Holmes with only five control points out of Nier's
twenty-five samples to use in the construction of his scale, so
how was he to estimate the duration of each geological period?
Samuel Haughton's celebrated principle of 1878 said, '
the
proper measure of geological periods is the maximum thickness of the
strata formed during these periods
' but at that time a rigid interpre-
tation of this principle assumed that the rate of accumulation
for any particular rock type was always the same, whatever the
conditions it formed in, and, as we have seen, this led to wildly
inaccurate estimates for the age of the Earth. If however, one
accepted the fundamental premise that the maximum thickness
of strata formed during any one geological period was indeed
