Geoscience Reference
In-Depth Information
rays required to produce a certain intensity of halo. Using specimens
from the Leinster Granite of Carlow in Ireland, Rutherford, working in
Manchester, produced artificial halos in mica and measured the num-
ber of alpha rays required to produce the halos. Meanwhile in Dublin,
Joly measured the mass of the nucleii and between them tabulated the
age of thirty halos. These age estimates ranged from 20 to 470 million
years. They concluded that the age of the Devonian was not less than
400 million years. However, a proviso read: 'that if the higher values
of geological time are so found to be reliable, the discrepancy with
estimates of the age of the ocean, based on the now well-ascertained
facts of solvent denudation, raises difficulties which at present seem
inexplicable'.
Three years later Joly measured halos in younger rocks from the
Vosges, and later still he measured the radii of rare halos in the
Tertiary granites of the Mourne Mountains. The radius of the latter
were 7%smaller than those of the Leinster Granite, which themselves
were 10% smaller than those he had recognised in Archean rocks, and
in 1917 he concluded in triumph: 'It would seem as if we might
determine a geological chronology on the dimensions of these halo-
rings!' No sooner had he come to this exciting conclusion than he
discovered small halos in Archean rocks from Norway which seemed
to explode their chronological promise. Subsequently Joly's halo data
were re-examined in 1927 by D. E. Kerr-Lawson of the University of
Toronto who was unable to detect the differences in size Joly had
noted in the different rock types. Shortly afterwards another student
of pleichroic halos, Franz Lotze (1903-1971) of the University of
G¨ ttingen, noted anomalies in the rings caused by uranium and said
that this invalidated Joly's ages derived from halos.
In 1922, Joly had reiterated his contention that radioactive decay
rates were not constant throughout geological history. This was par-
tially based on his observations of halos the characters of which were
not consistent. In particular, the innermost rings produced by ura-
nium in some halos were not consistent with the known ionisation
curves of uranium alpha-particles. He suggested that the rings were
