Geology Reference
In-Depth Information
and lonely, but nevertheless excited at the prospect of starting
work again.
Although the physics laboratory was new at the time of
Professor Strutt's appointment, there had still been a serious
shortage of apparatus which by 1908 had become acute:
We are at present largely subsisting on loaned apparatus, some of
which belongs to other public bodies, such as the Royal Observatory, the
Royal Society, etc., while some has been borrowed of private friends. I
need hardly say that it seems rather below the dignity of an institution
like the Imperial College that its teachers should have to beg apparatus of
their personal friends for the purpose of teaching the students.
This appeal to the College authorities' dignity had the desired
e¬ect and elicited a grant of £700 for special equipment plus
£800 for annually recurring expenditure which, although still
not considered enough, did mean that by 1910 Holmes had
access to some of the best equipment available at the time. He
carefully unwrapped and wrapped it up, each time he used it.
The geologist in Holmes understood that rocks were made
up of individual 'grains' of information called minerals, and that
minerals held the key to understanding the origins of a rock
because each mineral has a diagnostic colour, shape and hard-
ness which result from the di¬erent elements it contains. To put
it simply: elements combine to make minerals, and minerals
combine to make rocks. So, when an igneous rock cools from
its molten state within the Earth, di¬erent types of minerals will
form depending on which elements are present in the melt.
Thus when a mineral contains radioactive elements such as
uranium and thorium, that mineral becomes a time-keeper for
the rock it is in, because, as soon as the melt is cool enough for
the minerals to crystallise, the radioactive daughter elements
become trapped inside the mineral and the clock starts ticking.
Using the new uranium-lead technique, Holmes had decided
to determine the age of a rock from Norway in which there were
no less than seventeen di¬erent radioactive minerals, each one
of which could, in theory, be used as an age-check on the others.
