GEOLOGIC DIVISIONS MAPPED. 7 



and those divisions in which the system under description is known are taken up in 

 numerical order. The intervening map divisions are skipped. This arbitrary 

 order traverses all natural and political boundaries. The scope of some publica- 

 tions covers more than one zone or one column, in such a way as to necessitate the 

 discussion of two or more divisions under one heading. 



It is assumed that the text and map wUl be used together. To refer from the 

 map to the text, note the letter and number of the map index for the area under 

 examination and look them up in the chapter Avhich describes the geologic system 

 studied. If the reader has simply a certain spot of color on the map in view, the 

 corresponding color in the legend of the map will give the name of the system and 

 chapter. 



To refer from the text to the map, note the chapter heading and map index 

 under which a citation occurs and by aid of the legend and index on the margin of 

 the map identify the corresponding spot of color. 



GEOLOGIC DIVISIONS MAPPED. 



The geologic divisions represented on the map are adjusted to the requirements 

 of cartography. They involve a compromise between the accepted standards of 

 geologic science and the facts of history. The latter point can not be adequately 

 discussed here, but it is true that the European standard (or that of New York in 

 so far as it conforms to that of Europe) is a close approximation to the natural 

 classification in the Atlantic and Gulf provinces only, and that it does not correspond 

 satisfactorily with the natural order of events in the Pacific and interior provinces. 



The limitations of cartographic representation are inherent in the scale of the 

 map and the processes of printing. The scale of the map was selected with reference 

 to its use as a wall map and the major features of the continent are intended to be 

 apparent at a distance. They must therefore have at least a certain minimum 

 size and color distinction. The employment of the map for local reference is, how- 

 ever, desirable, and to this end more refined drafting with slighter distinctions of 

 color is desirable. These minor distinctions have been pushed to the limit of litho- 

 graphic methods. 



Whatever geologic systems, series, groups, or formations may be adopted, it will 

 be found that they can not be carried uniformly and with constant value over so 

 broad a territory as North America, for several reasons. 



The major problems arise from one or another of several conditions — the 

 scale of the map; differences of knowledge and ignorance; differences among classi- 

 fications peculiar to the surveys of Canada, Mexico, and the United States; unlike 

 development of the geologic series in diverse provinces. 



' The scale of the map sets a limit to refinement of possible classification. On a 

 large-scale map may be drawn or printed legibly narrow strips of color, which may 

 represent formations or even subdivisions (members) of formations. On a map 

 of smaller scale a color strip of the same minimum width covers the space represent- 

 ing a thick formation or a group or a series. Thus the desirability of printing a 

 map that shall be legible controls the width of a color band, and the latter in tttrn 

 determines the least geologic division that can be recognized. The real limitation 



