Geoscience Reference
In-Depth Information
how great their influence was. Certainly the illustration of unconform-
ities by Steno was important, and these geological features were later
shown by James Hutton (1726-1797) to be of the utmost value in
many people across Europe, whether aristocrats, gentlemen of
science, or lowly mineral prospectors, shared a common interest in
geology, and many of these would have been familiar with at least one
of the various language editions of Steno's Prodromus. A number of
these readers realised that rocks were layered, or stratified, and it was
therefore natural for attempts to be made to classify these successions,
explain the differentiation of rock types and describe their composi-
tion and structure.
One of the earliest definitions of stratification was that of the
Reverend John Michell (1724-1793), according to the later Scottish
geologist Archibald Geikie who quoted it verbatim:
The earth is not comprised of heaps of matter casually thrown
together, but of regular and uniform strata. These strata, though
they frequently do not exceed a few feet, or perhaps a few inches in
thickness, yet often extend in length and breadth for many miles,
and this without varying their thickness considerably.
Michell, like many university geologists then and later, was appointed
to a chair in geology (in his case the Woodwardian Professor of Geology
at Cambridge) on the strength of little research in the field. An English
country rector, he wrote an important discourse on earthquakes,
which he based on observations of the catastrophic Lisbon earthquake
of 1755. This paper, which was read to the Royal Society of London in
1760, is important in that Michell suggested for the first time that
movement waves accompanied earthquakes. These radiating waves, if
mapped, he argued, could point to the centre of the earthquake. It
was nearly a century later that the Irish engineer Robert Mallet
(1810-1881) proposed the term 'epicentre' for the origin of the earth-
quake. At the time of Michell's paper it was thought that earthquakes
were caused by huge volumes of water vapour produced when water
