100 THE UNITED STATES GEOLOGICAL SURVEY. [bull. 227. 
the distribution of the water-bearing strata, the areas where flowing 
water may be obtained, and the depths to the water horizon are shown on 
a special sheet. For active mining regions a map on a scale sufficiently 
large to show important details of the geology, mines, and ore deposits 
is inserted. For coal regions of economic importance special methods 
are used to show the structure or lay of the coal beds — of so much im 
portance to the mine worker. This is done by means of contour lines 
which show the elevation above sea of the coal bed throughout its 
occurrence in the area, and thus furnish the data for determining the 
depth of the coal below the surface at any point. Sections of coal 
beds which have been opened or worked are shown on a large scale and 
their relation to one another is indicated. 
The folio maps are generally published on the standard scale of 
1:125000, or nearly 2 miles to the inch, a scale used for most of the 
area of the United States. In certain localities, chiefly about cities, the 
scale of 1:62500, or nearly 1 mile to the inch, is employed; while in a 
few cases the scale of 1 : 250000, or nearly 1 miles to the inch, is adopted 
The folios are of uniform size, 19 by 22 inches, and consequently those 
on the 1-mile scale represent an area one-fourth of that shown on the 
2-mile scale and one-sixteenth of that shown on the 1-mile scale. 
In the preparation of the geologic map the black or "culture" plate 
of the topographic map is electrotyped, and on the electrotype are 
engraved the geologic boundaries, faults, letter symbols, mines, and 
explanation — that is, everything that is to appear in black on the 
geologic map. 
In the classification adopted in 1901, which is essentially that 
adopted in 1889 with minor modifications, there are recognized three 
kinds of rocks — sedimentary, igneous, and metamorphic. These 
classes are distinguished on the maps by colors and patterns. Of the 
sedimentary rocks, those which were deposited under water are repre- 
sented by patterns of parallel straight lines. Eleven such patterns 
are in use, and other variations are produced by inclining the lines al 
different angles — horizontal, vertical, or oblique. Glacial, alluvial, and 
eolian deposits are represented by patterns of dots and circles, com 
prising 17 designs of different density or size. The igneous rocks arej 
represented by patterns of crossed lines forming triangles and rhombs, 
of which 11 designs are in use. For metamorphic rocks of unknowr 
origin, intricate designs of short dashes or hachures are used, of 
which 8 patterns have been made; and for schistose rocks these dashes 
may be made parallel and wavy, simulating the structure. Metamor 
phic rocks known to have been derived from sedimentary or from 
igneous rocks are indicated by suitable combination patterns. 
The igneous rocks are represented by bright colors of any hue 
which contrast with those used to denote surrounding formations, anc 
indicate their intruded or extraneous character. The surficial anc 
