Geoscience Reference
In-Depth Information
professional geologists focus on one or perhaps two parts of the geo-
logical column - to develop an encyclopaedic knowledge of the com-
plete span would be virtually impossible. Today the discipline of
stratigraphical study is split into three sub-branches: lithostratigra-
phy, biostratigraphy and chronostratigraphy. Lithostratigraphy is con-
cerned with the naming, defining and description of rock units in
terms of their physical characteristics. Distinctive units that can be
marked on a map are termed formations, and several in close proxi-
mity to each other may be bundled together into a group. In biostrati-
graphy fossils are used for correlation between districts, and biozones
are the formal units that are characterised by a particular fossil con-
tent. Chronostratigraphy concerns itself with the definition of inter-
nationally agreed boundaries between units known as systems, series
and stages, and these boundaries are marked at particular points in
appropriate geological horizons (stratotypes) and are each known as a
'golden spike' whose absolute geological age is known. A 'System'
comprises all the rocks between its lower and upper boundary and it
corresponds to the division of geological time known as a 'Period'.
Thus rocks of the Silurian System were deposited during the Silurian
Period.
Such formality in dealing with stratigraphy has not always been
the case, and here we are primarily concerned with the establishment
of the divisions of geological time at a time when absolute ages were
not known. As has been discussed earlier, many geologists in the
eighteenth century followed the three-fold division of the rocks into
Primary, Secondary, Tertiary, while some included Quaternary as a
fourth division, a classification that owed much to the effects of the
Great Flood associated with Noah and the Ark. However, by the early
1800s the biblical interpretation of the formation of the geological
succession was largely discarded, except by some die-hard believers
in the Mosaic story. Dominick McCausland (1806-1873), the Irish
barrister and author of topics such as The Times of the Gentiles,
published a geological column as a frontispiece in his popular
Sermons in Stones. The geological periods are placed not in Eras but
