26 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



at a little distance. A geologic map of eastern North America printed in these 

 dark colors with so little difference of hue or shade would fail to present adequately 

 the great Appalachian zone as distinguished from the broad plateaus of the coal 

 measures and the domelike uplifts of the Cincinnati axis. In thepre-Cambrian also 

 the number of formations recognized in North America is greatly in excess of those 

 distinguished in Europe, and the simplicity of the European scheme renders it 

 insufficient to dehneate the geology of the Lake Superior region and the Canadian 

 shield. 



For these reasons, after very careful consideration, the compilers of the geologic 

 map of North America found it necessary to abandon the European international 

 scheme not only for the map issued by the International Geological Congress in 

 1906 but also for the map accompanying the present volume. 



The color scheme adopted by the United States Geological Survey for the 

 Geologic Atlas of the United States is based on three essential principles — (1) that 

 the order of the colors shaU be that of their natural association, (2) that there 

 shall be a standard color for each of the great geologic periods, and (3) that sub- 

 ordinate distinctions under each color shall be obtained by the use of obvious 

 patterns. To these may be added the practice of using dull, subdued tones for 

 the sedimentary and brilliant colors for the igneous rocks, as weU as the mnemonic 

 association of patterns which assigns parallel lines to the sedimentary rocks, angular 

 patterns to igneous rocks, hachure patterns to metamorphic rocks, and dotted 

 patterns to the surficial deposits. This color scheme has been developed and 

 applied in the office and engraving division of the United States Geological Survey, 

 where special facilities and skill in technical processes have been available. In 

 attempting to apply the scheme to geologic maps printed under contract with 

 private firms, difficulties have been encountered both in technical execution and 

 in cost. Experience shows that the applicability of any color scheme which requires 

 a precise use of patterns is affected by the necessity of developing a different use of 

 patterns to secure economy in printing. For this reason primarily the color scheme 

 of the United States Geological Survey is unavailable in general practice. 



As color schemes that employ definite associations of colors and time divisions 

 are limited in application to provinces that have had a certain historic development, 

 and as patterns that may occasion excessive cost are unpractical, it follows that 

 neither of these usages can be adopted for a universal color scheme except to a 

 qualified degree. Nevertheless, a color scheme which shall serve as the universal 

 language of geologic cartography is extremely desirable. The writer has sought, 

 therefore, to devise certain principles which would occasion as little departure from 

 current usage as might be practicable and which should yet be of general application 

 in all continents. The adoption of such general principles would not find favor in 

 England, France, or Germany, as may readily be foreseen if we consider how long 

 established the several usages of those countries are; nor can it hope to supersede 

 the international color scheme already applied to a large part of the geologic map 

 of Europe and to be extended to the incomplete portions of the map. But Europe 

 represents only a small part of the earth's surface and the Americas, Africa, and 

 Asia need not be governed by its practice. It may be hoped that in making local 



