G. K. Gilbert — Congress of Geologists. 445 



French, and the word I have translated shade {nuance) is one 

 which applies popularly to either hue or tone, while in the 

 scientific terminology of chromatics it applies to hue only. The 

 committee on the map has taken it in its popular sense, and has 

 represented some subdivisions by hues, and others by tones ; 

 for example, Pliocene and Miocene are assigned two tones of 

 the same hue, while Oligocene and Eocene have each a sepa- 

 rate hue. The upper Cretaceous and part of the lower Creta- 

 ceous are assigned a green hue in two tones, while the Gault and 

 the Wealden, classed as subdivisions of the lower Cretaceous, 

 have independent hues of green. Of the six reds assigned to 

 volcanic rocks, two agree in hue and differ in tone, while the 

 remainder have distinct hues. As the legend stands, both 

 major and minor distinctions, that is to say, the discrimination 

 of groups, the discrimination of systems, and the discrimination 

 of divisions smaller than systems, are all accomplished by 

 differences of hue, while the discrimination of minor divis- 

 ions is accomplished indifferently by variation of hue and by 

 variation of tone. The same means performs several functions, 

 and the same function is performed by several means. 



It is stating the same thing from another point of view to 

 say that the Congress and its committees have used the term 

 color in its popular rather than its scientific sense. Scientific- 

 ally, a color is a particular tone of a particular hue, and the 

 number of colors is infinite. Popularly, a color is an assem- 

 blage of contiguous hues and their tones, to which a name has 

 been given. Each hue and tone within the range covered by 

 the name is a shade of the color. It is in this popular sense 

 that the resolutions assign a color to each system, and assign 

 shades of the system-color to the subdivisions of the system. 



Now if in the variation of a system-color, by textures or 

 otherwise, a single hue is adhered to, the system-color remains 

 distinct from other system-colors throughout all its modifica- 

 tions and their modifications, but if hues as well as tones are 

 varied, the inevitable result is confusion, for some of the hues 

 of one system-color will approach too near to hues of other 

 system-colors. With a multiplicity of minor distinctions the 

 main distinction of system from system will be lost. 



Another difficulty lies in the fact that the Quaternary and 

 Devonian colors, while strongly contrasted in tone, are nearly 

 identical in hue. This does not affect their use in a general 

 map, but in a detail map the stronger tones of the Quaternary 

 gray will approach too closely the paler tones of the Devonian 

 brown. 



These criticisms apply to those features of the scheme which 

 affect its adoption for general and detail maps of^European 

 countries. There is one of equal or greater importance affect- 



