March 14, 1925 
Canada 
Geological Survey 
Bulletin No. 39 
GEOLOGICAL SERIESINO. 44 
COLOUR PRINTING OF GEOLOGICAL MAPS 
The lithographic process of colour printing briefly described here 
has been in constant use since 1899 for the reproduction in colours of the 
maps of the Geological Survey, Canada. It is an application of the same 
principle applied in three-colour halftone printing, whereby all colours, 
tints, hues, and shades are produced by the superposition of impressions 
in yellow, blue, and red from specially prepared copper plates, called 
halftone blocks. 
The three above-mentioned colours have, from early times, been 
known as the 'primary , fundamental, or elementary colours in the textile 
arts and in painting. They yield practically all the colours of nature 
when their pigments are mixed together in various proportions; green is 
obtained from mixtures of yellow and blue, orange and buff from mixtures 
of yellow and red, violet and purple from mixtures of blue and red. Mix- 
tures of the three pigments, which theoretically should produce black, 
give, on account of impurities, brown colours. This physical property of 
the colours forms the basis of trichromatic printing, in which case the 
colour effect of the mixture of pigments is produced automatically during 
the progress of printing. 
It should be noted that mixtures of pigment colours, and not colours 
of the solar spectrum, are here dealt with. Spectrum colours are com- 
ponents of daylight or white light, whereas pigment colours (artists’ colours, 
pastels, printing inks, etc.,) are the unabsorbed components of white light 
which render those pigments visible. The three fundamental spectrum 
colours, or colour sensations, are red, green, and violet , which, if combined 
in proper porportions, will form white light. Red and green combined 
yield yellow; green and violet combined yield blue; and violet and red 
yield purplish red or carmine. A yellow pigment emits yellow colour 
because it absorbs one of the components of white light, namely, the 
violet rays. Likewise, a blue pigment absorbs red rays and a carmine 
pigment absorbs green rays. In other words, the primary pigment 
colours, yellow, blue, and carmine, are residues of primary colour sensations 
of red, green, and violet, unabsorbed by the pigments, whereas pigment 
brown or black is the result of a more or less complete absorption of white 
light. 
00591—11 
