Geoscience Reference
In-Depth Information
identification is given in a key printed alongside, and it will carry
numerous symbols, one of which will show the dip of the rocks, be
it at a steep or shallow angle. The standardisation of colours used on
geological maps was arrived at quite early, when they were applied as
watercolour washes; both Jean-
´
tienne Guettard's (1715-1786) Carte
mine´ralogique de France of 1784 and Walter Stephens (d. 1808) and
William Henry Fitton's (1780-1861) 1812 map of Dublin show
remarkably close resemblance to maps published only recently. This
standard use of colour is most helpful as it allows geologists from any
part of the world to interpret instantly the geology of any other part
from a map. In essence this colour standard is the Esperanto of geol-
ogy. All the Silurian strata are identically coloured, as are those of the
Jurassic.
Joseph Ellison Portlock, who was responsible for the triangulation
of Ireland and who mapped much of mid-Ulster, wrote in 1843 that it
was possible to adopt one of two colouring schemes. The first method
saw a particular geologically mixed district being given a distinct
colour and the mineralogical differences being picked out by the use
of symbols. The second method allowed for a different colour to be
applied to each particular and distinctive rock type, for example all
sandstone being given an identical colour regardless of its age, and the
age differences being indicated by symbols. De la Beche used shades of
blue to represent limestone of different ages. The second scheme was
favoured by most early geological cartographers including those in
the survey of Great Britain and in Ireland, although they recognised
that some difficulties with the colour pallet had to be overcome.
Greenough warned that the colours used should 'speak to the mind
as well as to the eye; and the relations of these to one another, must
convey clear and definite ideas of the natural relation of the objects
which they are employed to represent.' Ingeniously, William Smith
represented lithologies on his map in a colour that was as close as
possible to the actual colour of the rock itself.
A party of experienced geologists can look at the distribution of
the light blue coloration and know instinctively that it represents the
