62 EARTH FEATURES AND THEIR MEANING 



a relief map will erelong be obtainable of any part of the civilized 

 world, and may be purchased in separate sections. Nowhere is this 

 work being taken up with greater vigor than in the United States, 

 where a vast domain representing every type of topographic pecul- 

 iarity is being attacked from many centers. Here and elsewhere 

 the relief of the land is being expressed by so-called contours or 

 lines of equal altitude upon the earth's surface. It is as though 

 a series of horizontal planes, separated by uniform intervals of 20 

 or 40 or 100 feet, had been made to intersect the surface, and the 

 intersection curves, after consecutive numeration, had been dropped 

 into a single plane for printing. 



Where the slopes are steep, the contour lines in the topographic 

 map will appear crowded together and so produce a deep shade 

 upon the map ; whereas with relatively flat surfaces white patches 

 will stand out prominently upon the map. More and more the 

 topographic map is coming into use, and for the student of nature 

 in particular it is important to acquire facility in interpreting the 

 relief from the topographic map. To further this end, a special 

 model has been devised, and its use is described in appendix C. 

 Usually before any satisfactory geological map can be prepared, 

 a contoured topographic map of the district to be studied must 

 be available. 



The field map and the areal geological map. As the atlas of 

 topographic maps is the physiographic gazetteer, so geological 

 maps together constitute the reference dictionary of descriptive 

 geology. Not only are topographic maps of many districts now 

 generally available, but more and more it has become the policy 

 of governments to supply geological maps in the same quadrangle 

 form which is -the unit of the topographic map. The geological 

 map is, however, a complex of so many conventional symbols, 

 that without some practical experience in the actual preparation 

 of one, it is exceedingly difficult for the student to comprehend 

 its significance. A modern geological map is usually a rectangular 

 sheet printed in color, upon which are many irregular areas of in- 

 dividual hue joined to each other like the parts of a child's pic- 

 ture puzzle. 



The colored areas upon the geological map ate each supposed 

 to indicate where a certain rock type or formation lies immediately 

 below the surface, and this distribution represents the best judg- 



