GEOGRAPHIC WORK OF GENERAL' GOVERNMENT 331 



may be classed in three groups, viz: water features, including 

 the sea, lakes, ponds, rivers and other natural streams, and canals 

 and irrigation ditches ; land features, including mountains, hills, 

 and valleys ; and cultural features, or the works of man, such as 

 towns and cities, roads, railroads, boundaries, and names. 



Water features. — All water features are shown in blue, the 

 smaller streams and canals in full blue lines, and the larger 

 streams, lakes, and the sea by wavy blue lining. Certain streams, 

 however, flow only a part of the year, being dry at other times, 

 and such streams are shown not by full lines, but by dotted blue 

 lines. Fresh-water marshes and swamps are shown by broken 

 horizontal lining, interspersed with tufts of blue. Salt-water 

 marshes are shown simply by horizontal blue lining. 



Culture. — The works of man are shown on the map in black, 

 in which color also is printed the lettering. They are enumer- 

 ated, and the characters used to represent them are given in what 

 is called the legend at the side of the map. 



Land features. — The land features, commonly called the relief, 

 include all the variations of the surface, the alternation of moun- 

 tain and valley, plateau and canyon, hill and plain. These 

 features are represented by means of contour lines, or lines of 

 equal elevation above the, level of the sea. The line of sea-coast 

 itself is a contour line— the line at zero elevation. The contour 

 line at, say, 20 feet above sea-level is the line which would be the 

 sea-coast', if the sea were to rise or the land to sink 20 feet. Such 

 a line would run back up the valleys and forward around the 

 points of hills and spurs. On a gentle slope this 20-foot contour 

 line would be far from the present sea-level, while on a steep 

 slope it would be very close to it. So a succession of these con- 

 tour lines, one above another, with equal vertical spaces between 

 them, would, if they were far apart on the map, indicate a gentle 

 slope; if they were close together, a steep slope; and if they 

 were run into a single line, as if they were on top of one an- 

 other, they would indicate a cliff. The contour lines of any 

 region, when represented on a map, show the elevation of any 

 part of the map above the sea. They also show the slopes of 

 the ground and the forms of the mountains, hills, and valleys ; 

 in short, of all the relief features. These contour lines are 

 printed in brown. 



The geological work proper of the Survey consists in a study 

 of the rock formations and in the mapping of their extent and 

 form. The results are published in annual reports, in mono- 

 graphs, and in geological folios. 



