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PROCEEDINGS OF SECTION C. 457 
in his anniversary address for the session 1888-89, to the effect that 
the question of colours to be adopted for geological maps was 
one of which very few geologists have any wide experience. 
The following remarks apply to detail maps to a large scale, say 
from two to six inches to a mile. 
I would first draw attention to a fact which appears to have 
been overlooked by the committee of the Bologna Congress, 
namely, the prevalence of colour-blindness. This peculiarity is 
far more common than is generally believed, as is shown by the 
results of the examinations in the Railway Department for 
guards and signal-men, where from two to three per cent. of those 
that present themselves are disqualified for this reason. Taking 
this, and the difficulty that many ordinary persons find in 
distinguishing between a series of tints of the same colour, the 
system that would appear to recommend itself as being most 
complete would be that adopted in the United States Geological 
Survey, of using light tints as bases and dark tints as overprints 
in various mechanical arrangements to represent the several 
formations in each system, or era, consisting of horizontal lines, 
vertical lines, right oblique lines, left oblique lines, broken lines, 
cross lines, dots, &c. This system admits of many variations, 
which can be used when desirable to represent a large number of 
members of an extended system. 
It is an easy matter to distinguish a pattern, and to carry it in 
the mind’s eye from the index to the map, or vice versa, whereas 
many who are not experts would frequently be at fault in this 
respect, if they had several grades or tints to deal with. By this 
scheme each epoch is represented by one colour only, although 
easily distinguishable in its several stages by the grading or 
over-print, while all attempts to establish a system by using shades 
of the same colour must prove inefficient. To my mind, it is of 
no great importance what colours are used, so long as they are 
distinct, yet I should in this again lean towards those used by 
the United States Survey, with two exceptions. The colours 
used are for 
Recent—Greys, for which I would prefer green. 
Tertiary— Yellows. 
Mesozoic—Green, for which I would substitute browns. 
Permian and Carboniferous—Blues. 
Devonian, Silurian and Cambrian—Purples, leaving the reds 
for the igneous rocks. 
The colours used would be in different tones and not tints of 
the same ; for instance, take ths Cainozoic or Tertiary system— 
for 
Pliocene sis ... chrome yellow 
Miocene i ... brown pink 
Eocene ee ... cadmium yellow. 
