Great Geologists | 89
C O N T I N E N TA L B L O C K
D
OL
OC
BASA
N
EA
f
n o n
gio sio
Re pres
m
co
SUBS
Region of
tension
LT I C
UM
T R AT
NE W
n
ea
Oc eep
D
L AY E
Ascending Currents
(Early Stage 1)
Island or
Swell
SUBS
OC EA N
T R AT
R
OL
D
OC
Re
EA
co
N
mp gion
res of
sio
n
UM
P la te au
B as al
ts
Mo
u
Ra ntain
nge
ain
unt
Mo ange
R
SUB
E
g
clo
TUM
STRA
Basaltic Magma
rises with
Ascending Currents
(Stage 2)
ite
SUBS
Oc
De ean
ep
T R AT
UM
Ec
log
ite
A representation of the mechanism by which Holmes envisaged continental drift might operate,
first presented in Holmes’ Principles of Physical Geology in 1944.
isotopes as pioneered by the work of Frederick Soddy who
had shown that not all lead present in rocks was derived from
radioactive decay. Uranium decayed to 206 Pb, thorium decayed to
208
Pb, and 207 Pb was at the time believed to be of non-radiogenic
origin. It thus became necessary to determine the proportions
of each isotope, which in a time before mass spectrometers,
required measurement of their atomic weights. Unfortunately,
following some initial research, the war prevented access to the
laboratories at the Radium Institute in Vienna where such work
was carried out.
Following the war, Holmes, despite his growing reputation in
geological circles and investigation of radiometric dating using
isotopes of lead, was unable to get a permanent post at a British
university. This led him in 1920 joining the oil industry as the
Chief Geologist of the Yomah Oil Company, active in Burma
(modern-day Myanmar). This was both an unsuccessful and
tragic adventure – his three year old son died after contracting
dysentery. No major oil field was located in the company’s
acreage and the continuously precarious finances of the
company led to a generally unsatisfactory life. Not surprisingly,
Holmes returned to Britain in 1922 and for two years took on
various jobs including running a shop selling ‘oriental crafts’ in
Newcastle.
In 1924 his luck changed and he was appointed Head of the
newly-formed Geology Department at Durham University
(initially he was the only staff member) and this allowed him to
resume his geological research. In particular he was engaged
in the committee set up by the National Research Council in
America for the “Measurement of Geologic Time by Atomic
Disintegration”. This required reviewing the growing numbers
of radiometric analysis now being published. Few met his
high standards, with the results ultimately published in 1931
with the lead author being the American mineralogist Albert
Knopf. His search for more reliable radiometric methods led
him to contemplate how the original isotopic ratios of igneous
rocks could be determined, which in itself opened up a new
avenue of research of igneous petrogenesis. Collaboration with
William Urry of the Massachusetts Institute for Technology
suggested that a return to helium measurements might prove
fruitful, but the results were shown to be spurious. It was not
until Alfred Nier demonstrated the true nature of the isotopes
of uranium and lead that reliable results began to emerge and
these results were incorporated in the 1947 version of Holmes’
geological timescale which interpolated ages for the geological
periods between the radiometric control points using sediment
thickness as a guide to duration. Holmes himself knew that
there were flaws in this method, but it did provide the geological