THE WORK OF THE ATMOSPHERE 93 



relief (inequalities of surface), and that any given line runs through 

 points of the same elevation above sea level. This will be under- 

 stood readily by reference to Figures 84 and 85. Figure 84 shows a 

 model of an ideal landscape viewed from above, on which lines 

 have been drawn connecting places of equal elevation. In Figure 85 

 the above lines are shown alone ; this is a contour map of the 

 region represented by the model. By comparison of the model and 

 map it will be seen that where the slopes of the former are steep, 



FIG. 83. Stratification in a sand dune near the head of Lake Michigan. 



(Bastin.) 



the lines of the latter are close together, and vice versa. The vertical 

 distance between two adjacent contour lines is the contour interval. 

 The contour interval varies on different maps. In regions of low 

 relief an interval of 10 or 20 feet is used frequently ; in mountainous 

 areas an interval of 500 or more feet sometimes has to be used in 

 order to avoid having the lines too close together to be read. In the 

 map of Figure 85 the interval is 20 feet, the exact value of the 100 

 and 200 foot lines being indicated. By counting the lines it will be 

 seen that the top of the hill to the left of the river is over 260 feet 

 above the level of the ocean in the foreground. It cannot be 280 

 feet high, however, for no 280 foot line is drawn. A comparison of 

 the model and map will show also how valley depressions are shown 

 by contours. 



The features shown upon the three-color contour maps are of 

 three general classes : (1) elevations and irregularities of the sur- 

 face, shown by brown contours ; (2) water, including streams, 

 ponds, lakes, etc., represented in blue ; (3) artificial features, such 



