8 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



in this regard is the space occupied by a least geologic division and it is therefore 

 dependent on structure as well as on classification. Where strata lie flat thejr 

 present as a rule wide expanses, but where they dip steeply they exhibit narrow 

 edges. The latter is apt to be the case in mountainous regions, where the mosaic 

 is intricate and the difficulty is thus enhanced. Contrast for instance the geologic 

 map of the Great Plains and that of the Rocky Mountains. 



The scale of the geologic map of North America is limited by the great scope of 

 the map. Being continental, the scope demands a scale represented by 1 divided 

 by millions, and considerations of convenience led the compilers to fix on 1 : 5,000,000, 

 or approximately 1 inch to 80 miles. One-twentieth of an inch being about as 

 narrow a band of color as can be read distinctly, 4 miles is nearly the narrowest 

 zone of rocks that can be represented without exaggeration. Many zones narrower 

 than 4 miles are of such importance that they must be represented even though 

 exaggerated, but not many adjacent bands can be so treated. The scale therefore 

 imposes severe limitations on classification. 



As the limitations are more severe in mountains than in plains a refined set of 

 geologic divisions which can be delineated for plains will be impracticable in moun- 

 tains, whereas that series of terranes which is practicable for the former is likely 

 to be much too comprehensive for the latter. In maps of moderate scope these 

 differences are usually adjusted in such a manner that a uniform classification may 

 be employed over the entire map ; but where the scope is continental the differences 

 become so great that a uniform classification for the continent becomes unsatis- 

 factory. Even though the natural geologic divisions are the same, they must be 

 grouped or separated according to the requirement that the map shall show all 

 possible detail and yet be legible. Cambrian, Ordovician, Silurian, Devonian, and 

 Carboniferous must be distinctly mapped and to some extent individually sub- 

 divided in eastern North America; but on the scale adopted they must be grouped 

 where they are upturned on the flanks of the Rocky Mountains. Thus a uniform 

 classification is impracticable for the geologic map of North America, and an elastic 

 or varied classification is required by the conditions of printing and legibility 

 inherent in the scale. 



Knowledge contrasted with ignorance imposes a similar condition. In recon- 

 naissance surveys major divisions only are distinguished, and these often but 

 vaguely or along a narrow line of route. There are large areas of North America 

 regarding which present knowledge is no more precise. Within or adjoining these 

 regions are districts of greater or less extent which have been surveyed in detail. 

 The boundaries of these districts may be those of a State or Province, or those of 

 an arbitrary quadrangle. They are sharply defined. Beyond them lies compara- 

 tive ignorance; within them is displayed the key to an understanding of what the 

 unknown probably is. In depicting the well-known and the little-'known side by 

 side on the same map, we can not use one classification for both, without great loss 

 in the one or certainty of error in the other. The only alternative is to map each 

 according to our knowledge of it, using for the well-known district a detailed classi- 

 fication up to the limit set by the scale of the map and for the little-known region a 

 generalized classification adapted to our ignorance. This method has been adopted 



