scale only sufficiently refined for map-making purposes, and will not be
directly useful for geodetic purposes in determining the figure of the
earth. The hypsometric work is based upon the railroad levels of the
country. Throughout the greater part of the country, there is a system of
railroad lines, constituting a net-work. The levels or profiles of these
roads have been established with reasonable accuracy, and as they cross
each other at a multiplicity of points, a system of checks is afforded, so
that the railroad surface of the country can be determined therefrom with
all the accuracy necessary for the most refined and elaborate topographic
maps. From such a hypsometric basis the reliefs for the whole country are
determined, by running lines of levels, by trigonometric construction, and
in mountainous regions by barometric observation.

The primary triangulation having been made, the topography is executed by
a variety of methods, adapted to the peculiar conditions found in various
portions of the country. To a large extent the plane-table is used. In the
hands of the topographers of the Geological Survey, the plane-table is not
simply a portable draughting table for the field; it is practically an
instrument of triangulation, and all minor positions of the details of
topography are determined through its use by trigonometric construction.

The scale on which the map is made is variable. In some portions of the
prairie region, and in the region of the great plains, the topography and
the geology alike are simple, and maps on a comparatively small scale are
sufficient for practical purposes. For these districts it is proposed to
construct the sheets of the map on a scale of 1-250,000, or about four
miles to the inch. In the mountain regions of the West the geology is more
complex, and the topography more intricate; but to a large extent these
regions are uninhabited, and to a more limited extent uninhabitable. It
would therefore not be wise to make a topographic or geologic survey of