COLOR SCHEME. 27 



geologic maps of other continents than Europe, even European cartographers will 

 strive toward uniformity of expression, especially in color schemes, and that the 

 American nations which are not yet committed by established practice to diverse 

 usages will unite upon common principles. 



In order to promote this desirable end, the following principles, suggested by 

 the writer, have been proposed by the United States Geological Survey for discus- 

 sion with a view to the development of a common usage among the official surveys 

 of the Americas and for application to the geologic maps of the world published 

 on the standard 1:1,000,000 base. These principles have already been indorsed 

 in a general way by the Geological Survey of Canada and the Instituto Geologico 

 de Mexico. 



PRINCIPLES FOR COLORING GEOLOGIC MAPS. 



The first principle shall be that the relations of color and relative age shall be 

 invariable — that is, when applied to distinguish divisions of a sequence of strata, 

 colors shall always be used in a definite succession which shall express relative age. 



Let it be agreed that the sequence red, purple, violet, blue, green, and yellow 

 shall be adopted to represent succession of formations, groups, or series of sedi- 

 mentary rocks from older to younger and let the order of colors be invariable accord- 

 ing to the principle stated above, no matter what part or how much of the geologic 

 column is represented. Then red will always represent something older than that 

 which is shown in purple, or violet, or blue, etc. Blue will always stand for some- 

 thing older than that shown in green or yellow. In looking at any geologic map 

 thus colored the student would at once know which were the older and which the 

 younger sedimentary rocks. The essential features of sequence and structure 

 would be immediately obvious. 



The second principle is that color and geologic time divisions shall be measur- 

 ably independent ; in other words, no color shall be regarded as precisely indicative 

 of any particular time division. 



This principle is a radical departure from general usage in any one of the several 

 countries which have well-established color schemes, but it is simply a recognition 

 of general practice if we take account of the differences of these established schemes. 

 For instance, in Europe blue means Jurassic, but in the United States it stands for 

 Carboniferous. Thus blue in each country represents something different, and to 

 a student of one country examining the maps of the other it suggests something 

 other than it should. Let us agree that blue shall not necessarily mean Jurassic 

 or Carboniferous, but that it may represent either Jurassic or Carboniferous, or in 

 general some Mesozoic or Paleozoic time division. Such an agreement will give 

 that elasticity of usage which is essential to a general color scheme designed for 

 application to different provinces. If applied with appropriate recognition of the 

 first principle the elasticity thus introduced will not occasion confusion, because 

 we will always know the relative age of those formations represented by blue as com- 

 pared to those represented by green on the one side and violet on the other. Indeed, 

 a certain orderliness will take the place of the disorder that is forced by arbitrary- 

 usages upon any student of the maps of several countries. This principle is not 

 intended, however, to give license to use any color whate^'er for any period, nor to 



