Mesozoic _ 

 Paleozoic. 



28 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



contravene the principle of utility, which requires that a color scheme for a given 



province shall be as nearly uniform as practicable. 



To illustrate the application of the first and second principles that have been 



proposed, we may consider certain examples. A general geologic map of North 



America requires few distinctions in the Mesozoic but many in the Proterozoic, 



Paleozoic, and Cenozoic. We may therefore allot the colors as follows: 



„ . [Yellow. 



Cenozoic r[^^^^^_ 



[Blue. 



Violet. 

 Purple. 



Proterozoic I Red. 



A general geologic map of Europe makes different requirements— very few dis- 

 tinctions in the Proterozoic, few in the Paleozoic, but very many in the Mesozoic 

 and Cenozoic. Hence for such a map we may allot the colors as follows: 



(Yellow. 

 Cenozoic [JGreen. 



Mesozoic ] Blue. 



T, 1 • [Violet. 



Paleozoic ^ _ , 



Purple. 



Proterozoic Red. 



The braces are overlapped to avoid too precise an assignment — for it is the 

 purpose of the second principle to preserve that elasticity which alone can make 

 the scheme available everywhere. Yet through the first principle we secure an 

 orderly sequence, which will always convey the idea of relative age. 



The preceding considerations apply to general maps on a small scale, for 

 instance, the accompanying geologic map of North America, or maps on the scale of 

 1 : 1,000,000, the standard international base. A cartographer having such a map 

 to construct should consider the geologic development of the province which the 

 map is to represent in whole or in part and should devise the application of his 

 color scheme in such a way that it accords with the two principles suggested and 

 yet gives the greatest range of colors where there "is the greatest development of 

 the geologic column. 



Detailed maps, on larger scales than 1 : 1,000,000, present special conditions; 

 they may be simple or comprehensive. 



If they are simple, it is desirable that the colors used should correspond with 

 those that represent the same general time divisions on the general map. Where 

 one or two effects suffice for the latter, there may be two or three times as many 

 on the simple detailed map, but the range of colors may not need to be greatly 

 increased. 



If the detailed map shows terranes that are greatly subdivided, the range of 

 colors must be increased; but in that case the dominant effects may be and should 

 be selected to correspond with colors indicating the general time divisions on the 

 general map and only so many of the colors above and below as may be needed 

 should be used. The reader will then know that (a) if warm ruddy tones predomi- 

 nate, the earlier periods are represented; (b) if cold blue tones are most evident. 



