PREPARATION OF A GEOLOGIC MAP 63 
Annual Report, have indicated actual outcrops, but the practice 
has not been general. It would seem that this distinction might 
profitably be made on maps included in the geologic folios, at 
least in those cases in which the geology of the area covered is 
complicated. 
III. REQUISITE GEOGRAPHIC DATA. 
Acknowledgments— The geographic base should be credited 
and described, if possible, on the map itself; if the necessary 
description be too long for the space available, it should be con- 
tained in the textual discussion of the map. The value of a 
geologic map is greatly increased if the accuracy of its geogra- 
phy and topography can be estimated: and this is generally 
possible if the source whence they were derived be noted. 
Scale.— Except in the case discussed in the next paragraph 
the scale should always be indicated diagrammatically on the 
map, by means of the familiar bar scale. The scale may, in 
addition, be indicated fractionally or verbally ; allowance being 
made for reduction (if any) during reproduction. The shrink- 
age and expansion of paper due to changes in moisture are so 
great that these arithmetical or verbal statements of the scale are 
of little value; and they should never be used without a diagram. 
In maps whose scale or area is so large that the effect of the 
earth’s sphericity is appreciable, several meridians of longitude 
and parallels of latitude szust be introduced and properly num- 
bered, in order that the scale in different parts of the map be 
determined. 
Contours.— If the base shows topography by means of con- 
tours, the datum plane and contour interval should be stated on 
the map. 
Orientation. When possible, the map should be so oriented 
with respect to the sheet on which it is printed that the north 
will be at the top. If this is not possible—and invariably if no 
parallels or meridians are introduced—the direction of a true 
north and south line should be indicated by the conventional 
arrow. In casethe strike readings are quoted from the mag- 
