Geoscience Reference
In-Depth Information
Proterozoic (first life) and the Phanerozoic. The Archean, which was
named by the American mineralogist James Dwight Dana (1813-1895)
in 1872, and the Proterozoic, named by another American geologist
Samuel Franklin Emmons (1841-1911) sixteen years later, comprise
the immense pile of largely unfossiliferous rocks that were produced
or deposited between the development of crustal rocks formed by the
differentiation of the Earth's material into a central core, middle mantle
and surface crust, and the point at which marine animals were able to
precipitate hard shells. The youngest Eon is the Phanerozoic, a term
coined from the Greek phanero and zo, meaning visible and life respec-
tively, where lithological variation is greater than before and where life
on Earth began to diversify, and at times wax and wane.
The Phanerozoic in turn is composed of three Eras - Palaeozoic
(ancient life), Mesozoic (middle life) and Cenozoic (recent life). The
first was named by Adam Sedgwick in 1838 in a paper published in the
Quarterly Journal of the Geological Society (of London) to encompass
the two lowermost geological Periods. However, just two years later
John Phillips redefined it to include the younger Devonian Period. It
now comprises six geological Periods. The Mesozoic and Kainozoic
(nowmore often written Cenozoic) were so named by Phillips in 1840
in a paper published in the Penny Cyclopedia, a widely distributed
popular magazine that did much to spread the geological word in
Britain. The Eras are themselves divided into Periods, which are
shorter time-spans often characterised by distinctive rock types and
recognised by their particular fossil constituents. It is with this level of
subdivision of the geological column that people are most familiar,
having heard on television of the 'Cambrian explosion', or seen the
blockbuster movie Jurassic Park. Nearly all Periods are themselves
subdivided into lesser Epochs, Stages and Series, and even into Zones,
which can comprise a very short span of time, and are recognised by
just one diagnostic fossil.
By and large the Stages and Series are named in a parochial way,
often reflecting an area where the rocks occur, and it can be difficult
when reading the geological literature to correlate packages of geology
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