58 
THE UNITED STATES GEOLOGICAL SURVEY. 
[bull. 227. 
This combination of colors renders these topographic maps readily 
legible. On the reverse of each sheet is a description of the mode 
of reading- the map, and a legend, or series of conventional signs, 
indicating how the various facts shown on the map are represented. 
All these conventions are self-explanatory and are readily under- 
stood and interpreted by the layman, except, perhaps, the brown 
" contour " lines. 
These contours are lines of equal elevation— lines along which the 
ground would be touched by the border of a water surface (of the 
ocean, for instance) if it were repeated!} 7 raised by a given amount. 
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Fig. 1. — Ideal view and corresponding contour map. 
Contour lines express three features of relief — (1) elevation, (2) hori- 
zontal form, and (3) grade or slope. To explain more clearly the 
manner in which the contours shown on the maps of the Geological 
Survey delineate height, form, and slope, the accompanying contour 
map (fig. 1) has been prepared from the ideal view shown above it. 
It may be interpreted as follows: 
1. A contour indicates a certain height above sea level. In this 
illustration the contour interval is 50 feet; therefore the contours are 
drawn at 50, 100, 150, and 200 feet, and so on, above mean sea level. 
Along the 250-foot contour lie all points of the surface 250 feet above 
sea; along the 200-foot contour, all points that are 200 feet above sea, 
