GEOLOGICAL SECTIONS 1117 



Continuous and Discontinuous Mapping. 



Since rock outcrops are, as a rule, scattered over a consider- 

 able area witii intervening portions in which the rock is covered 

 by glacial or other loose soil deposits, two modes of mapping on 

 the same scale have come into usage. The first is the mapping of 

 outcrops only, forming what may be called outcrop maps. The 

 intervening covered spaces may be left blank or may be colored 

 for the superficial deposits. The result will be a map very difficult 

 to read and to follow, while the structure of the region is not 

 readily ascertainable from such a map. The practice of printing 

 the pattern, representing the superficial unconsolidated deposit, 

 over the color pattern of the formation is adopted in some quarters, 

 as in the case of the International Geological Map of Europe. 

 American maps, as a rule, represent rock formations only, with- 

 out the overtint for superficial unconsolidated deposits. These de- 

 posits are either entirely omitted or represented on a separate map. 

 It is, of course, understood that in such cases the map does not 

 represent an accurate picture of the surface features of the litho- 

 sphere, but is hypothetical so far as the covered portions are con- 

 cerned. In a region of simple structure no appreciable errors are 

 likely to arise from such a mapping, but in a complicated region this 

 may readily be the case. 



Sections. 



Types of Sections. Geological sections are of three kinds : ( i ) 

 the natural cross-section, (2) the columnar section, and (3) the 

 ideal section. The natural cross-section represents structure (in 

 so far as it is ascertainable) and surface features, and is the one most 

 generally employed in connection with geological representation. 

 It gives the third dimension of the land form, the other two being 

 furnished by the map. Cross-sections should, whenever the scale 

 permits it, be drawn to the natural scale, i. e., vertical and horizontal 

 scales should be alike. In some instances this is not possible, owing 

 to the smallness of the scale and the large number of structural 

 features to be represented. In such cases an exaggeration of the 

 vertical over the horizontal scale is necessary, but this should not 

 be over five times, or. in rare cases, ten times, the horizontal. It 

 must be borne in mind that vertical exaggeration of the scale always 

 involves an increase in the steepness of dip of the strata and a cor- 

 responding distortion of other characters. 



