60 ISIDQNIUN. (Cs SH OSI ay b 
grade of accuracy, the fault is generally in the field work, and 
not in the map-drafting. Many geologists fail to realize in 
practice, though they may admit in theory, that geologic map- 
ping on an atlas sheet is very different from mapping on a post- 
route map. It might seem hardly necessary to say that the 
geologic boundaries, when a contoured base is used, should be 
accurately fitted to the topography; but unfortunately evident 
violations of this rule are frequent. In one very recent instance 
a map, whose mechanical execution is admirable, is accompanied 
by structure sections which show that the geologic. boundaries 
on the contoured map are in physically impossible positions. 
Throughout much the greater part of the United States geo- 
logic mapping must be carried on in areas which have not as yet 
been mapped with any approach to accuracy. County, land- 
office, or post-route maps will be the only bases obtainable. In 
any inhabited area, roads are surveyed and mapped with toler- 
able accuracy long before any attempt is made to show drainage 
or relief. For this reason, in geologic mapping in a base known 
to be geographically inaccurate, boundaries and outcrops should 
be, when possible, referenced with respect to roads and road 
intersections, and not with regard to natural features. If thus 
referenced, these boundaries may be transferred to another base 
at any time, without necessitating the revision of the field work. 
The accuracy of the field work is not a matter which inter- 
ests the cartographer so much as its precision. If the field 
geologist has erred in determining the age or general relations 
of a certain formation, the cartographer cannot question the 
determination. He has, however, a right to ask that boundaries 
be traced and locations made with a certain degree of precision, 
a degree which will vary with the character of the base map 
used ; and that the various elements of the manuscript map 
agree iter se (e. g., that the areal mapping is not contradicted 
by the structure sections). 
II. REQUISITE GEOLOGIC DATA. 
Title.—The title of the map, omission of which is rarely per- 
missible, should explain the character of the mapping (geologic, 
