THE ATMOSPHERE AS A GEOLOGICAL AGENT. 



31 



of all parts of the area mapped, to delineate the horizontal outline, or contour, 

 of all slopes, and to indicate their grade or degree of steepness. This is done by 

 lines connecting points of equal elevation above mean sea-level, the lines being 

 drawn at regular vertical intervals. These lines are called contours, and the 

 uniform vertical space between each two contours is called the contour interval. 



Fig. 15. — Sketch and map of the same area to illustrate the representation of 

 topography by means of contour lines (U. S. Geol. Surv.) 



On the maps of the United States Geological Survey the contours and eleva- 

 tions are printed in brown (see Plate II). 



The manner in which contours express elevation, form, and grade is shown 

 in the following sketch and corresponding contour map. Fig. 15. 



The sketch represents a river valley between two hills. In the foreground 

 is the sea, with a bay which is partly closed by a hooked sand bar. On each 

 side of the valley is a terrace. From the terrace on the right a hill rises gradually, 

 while from that on the left the ground ascends steeply in a precipice. Contrasted 

 with this precipice is the gentle descent of the slope at the left. In the map 

 each of these features is indicated, directly beneath its position in the sketch, by 

 contours. The following explanation may make clearer the manner in which 

 contours delineate elevation, form, and grade: 



