28 THE UNITED STATES GEOLOGICAL SURVEY. [bull. 227. 
things needful to the development of a region. In connection with 
structure sections, which usually accompany such a map, it is often 
possible to indicate very precisely not only the localities but the extent 
of beds of commercial importance and many of the conditions which 
influence their availability in mining. 
The geologic map of the United States which is being made by the 
Survey is in large part on scales of 1 and 2 miles to the inch. Such 
a map of the whole country on the scale of 1 mile to the inch would 
require paper dimensions of about 240 by 180 feet — about the area of 
half a city block. This would be too large for general use, so the map 
is being made piece by piece, each sheet on the scales named represent- 
ing one sixteenth or one-quarter of a square degree of the earth's surface 
(see pp. 60-61). The individual sheets as they are completed are bound 
up with a brief descriptive text in folios (see pp. 99-100). Thus the 
information relating to each area is available at once, and it is not 
necessary to wait for the completion of the whole. 
Of the 3,025,000 square miles of area of the United States, exclud- 
ing Alaska and island possessions, the geologic map of 100,000 square 
miles has now been surve}^ed and published. A large additional area 
has been covered by preliminary or reconnaissance surveys, and the 
field parties are at work in widely scattered areas, as shown by PI. I. 
The base map used by the geologist is prepared by the topographic 
branch, though occasionally the geologist works with the topographer, 
or even at times does preliminary work in advance of topographic 
mapping. In such cases he must measure his own distances, usually 
by pacing, and determine altitudes b}" means of the barometer. 
In order to make his map, the geologist needs to determine the 
character and distribution of the individual rock masses and their 
relations to one another. To do this it is necessary to travel over the 
whole area and carefully plot all outcrops and ledges of rock. The 
dips of the rocks must be measured in order to determine the a., unt 
of deformation they have undergone, and since in places they have been 
repeatedly- folded and broken it requires many careful observ lions to 
determine the position of the individual beds beneath the surface. 
In some districts, as in much of the Lake Superior region, magnetic 
surveys are made in connection with the geologic work. In that case 
observations are made with a dipping needle at many points, and the 
results are plotted to show the underground distribution of the ores. 
Unfortunately, very few ores are magnetic, so that this method can 
be used but rarely. 
Many rocks do not show their true character in the outcrop, where 
they have been changed by the process of weathering. It is also dif- 
ficult, if not impossible, to determine by the eye the nature of many 
igneous rocks. Samples are accordingly taken and, when necessary, 
chemical analyses of these are made. More commonly the nature of a 
rock may be determined by studying it with the microscope, and this 
