National Geographic : 1898 Jul
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.