GEOLOGICAL WORK OF THE ATMOSPHERE 95 



and the uniform vertical space between each two contours is called the con- 

 tour interval. On the maps of the United States Geological Survey the 

 contours and elevations are printed in brown (see PL I). 



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

 shown in the following sketch and corresponding contour map, Fig. 58. 





Fig. 58. Sketch and map of the same area, to illustrate the representation 

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



"The sketch represents a river valley between two hills. In the fore- 

 ground 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: 



