Geology Reference
In-Depth Information
During his trip to the United States in 1932 Holmes had dis-
cussed with his colleagues there the age dating results he had
obtained using Paneth's new helium method that appeared to
be so successful. On an earlier occasion he had put out a plea
to American universities to help him with his dream of building
a geological time scale, their wealthy research establishments
being the only ones that could really a¬ord to consider this
'Herculean task'
, so while in America, and with the positive
helium results from Paneth to support his case, he raised the
subject again. His reasoning was persuasive and it was agreed
that William Urry, then working with Fritz Paneth in Berlin,
would come to the United States and finally make Holmes'
dream come true.
The samples were carefully selected from sites all over the
United States, Canada and Europe and their geological ages
ranged from the Cambrian right up to the top of the Tertiary;
the crucial factor determining a sample's suitability being how
well its position in the geological column could be constrained.
Rocks would only be chosen if their geological age could be real-
ly well defined. The idea was to use basalt, the rock that flows
from volcanoes as molten lava, because it was easier to assign
a
geological
age to basalts than any other common igneous rock
type. In addition basalts were widely distributed in space and
time and so representatives of all ages were readily available.
When basalts are erupted from a volcano as a flow of lava,
they may travel great distances (hundreds of kilometres) and as
they do so they trap beneath them soil containing organic life
that will eventually become fossilised - as Lyell discovered when
he found the fossilised remains of shellfish beneath lavas while
examining the volcano Mount Etna. Once the volcanic action has
ceased and the lavas stop flowing, sediments will again be
deposited on top of the basalts. In time these sediments, also
containing living organisms, will eventually be turned into
