102 THE UNITED STATES GEOLOGICAL SURVEY. [bull. 227. 
attention. If the explanation appearing in the legend is not clour the 
map is worthless. The legend must be precise and yet concise, as the 
space allotted to it is limited. Besides naming and describing the 
formations, it must show their age and their relation to one another 
as far as practicable. 
When engraved the map and legend are transferred to stone and the 
combined proof is carefully compared with the manuscript map to see 
that the boundaries fit the contours properly. The colors and patterns 
which are to represent the geologic formations are next selected. The 
colors for the water-laid sedimentary rocks are predetermined by the 
color scheme and are the same throughout the United States for rocks 
of the same age. Formations of the same age are distinguished from 
one another by their patterns. No set rules are followed in selecting 
these, but the distinctions are better if dense and light patterns are 
alternated. It is desirable also to give the larger area the lighter pat- 
terns and the smaller ones the denser patterns. This practice can not 
always be followed, however, because the pattern selected for a for-, 
mation in one area is used for that formation wherever it occurs, 
although relations may be different. The direction of the lines forming 
the line patterns is also a means of distinction, but though the same 
direction is maintained for a considerable area, yet a change is made 
to conform with some distinct change in sedimentation, such as an 
unconformity, if possible. For the igneous and the surficial sedimen- 
tary rocks there is greater freedom in selecting the colors, and it is 
on the skillful use of this libert}^ that variety of color in the map and | 
pleasing effect largely depend. 
After the colors to be used are determined, sheets are prepared for j 
the guidance of the engraver, each sheet representing a stone from which 
a color is to be printed. On each sheet are shown in water colors the 
areas which are to be printed from this particular stone, different \ 
colors being used for the various patterns to be employed. If the 
areas are complicated the sheets are copied on a combined or check I 
sheet in order to avoid omissions or duplications, as any change in 
pattern or area requires the modification of the stone — a difficult and 
often impossible task. 
The color stones are then prepared and the proof in colors is read 
and compared with the sheets previously made. It is often found 
desirable to change some of the colors, to bring out distinctions or to 
harmonize harsh combinations which appear in the color proof. The 
precision of register, or fitting of the colors, on this map is little 
less than marvelous when one realizes the possibilities of error. On 
some maps as many as 27 colors are printed, and a slip in one of the 
printings would spoil the sheet. Among the sources of faults in 
register, change in the size of sheets of paper due to humidity is 
one of the most trying to overcome, as in a month — the time it takes 
