G. K. Gilbert — Congress of Geologists. 443 



The art of mapping geologic terranes by means of color is 

 well developed, and its methods, viewed from the geologist's 

 standpoint, admit of easy characterization. Color may be 

 varied in two distinct ways — in hue and in tone. Hues differ 

 in quality, as yellowish green and bluish green. Tones differ 

 in strength, as pale green and dark green. A color is printed 

 either solid or broken ; it is said to be broken when applied in 

 a pattern, as in lines or dots, or when it is interrupted by a 

 pattern. The difference between solid and broken colors is a 

 difference of texture. The primary discriminations in mapping 

 are through hue, tone and texture. 



The map engraver produces texture in three ways. In the 

 first way a single impression is made with a broken color. The 

 white of the paper, displayed where the color is interrupted, 

 combines with the color in the general effect, producing a paler 

 tone of the same hue. In the second way two impressions are 

 made, one with solid color the other with broken, and the two 

 impressions have the same hue ; they may or may not differ in 

 tone. This is monochromatic overprinting, and its general effect 

 agrees in hue with the single impression, but differs in tone, 

 being darker. In the third way two impressions are made, one 

 solid, one broken, nnd their colors differ in hue. This is bi- 

 chromatic overprinting, and its general effect differs in hue as 

 well as tone from each of the colors combined in it. The first 

 and second ways produce texture monochromatically and do 

 not yield a new hue; the third way produces texture bichro- 

 matically and yields a new hue. It is practically impossible to 

 obtain a texture effect without modifying the original tone. 



The natural gradation from hue to hue is absolutely contin- 

 uous and the number of hues is infinite; the number of tones 

 of each hue is likewise infinite. The number of hues and 

 tones the eye can discriminate is finite, but very great ; it is 

 stated that one thousand hues have been distinguished in the 

 solar spectrum. But the number of hues and tones that can 

 be combined in a map is small. As a matter of perception, 

 every color is modified by the colors adjacent to it. The same 

 hue affords different sensations when differently surrounded, 

 and different hues may afford the same sensation. The same is 

 true of tones; and there is a certain interdependence of hues 

 and tones in this respect. In a geologic map each color is liable 

 to fall into various combinations, and two colors little differ- 

 entiated occasion confusion. There is, therefore, a somewhat 

 narrow limit to the employment of hues and tones. The mat- 

 ter has not been fully worked out, but it is probable that 

 twenty is as large a number of hues as can be safely employed 

 in connection with tones. Texture admits of very great varia- 

 tion. The various color schemes submitted to the congress and 



