64 BION AIM, (Co SECIS Pi by 
netic north, the declination of the needle should also be shown 
(the average for the district being taken) or stated verbally. 
Projection.— In maps where scale or area is sufficiently large 
to render the information of value, the system of projection 
should be stated; or, better, it should be shown graphically by 
introducing a number of meridians of longitude and parallels of 
latitude. As noted under the discussion, in a previous paragraph, 
of the scale of the map, these meridians and parallels will serve 
a double purpose. 
IV. CHOICE OF SCALE, CONVENTIONS, ETC. 
Area of map and coloring.—The area covered by the map 
should not be larger than is necessary to illustrate the point 
under discussion. Whenever possible, the colors should be car- 
ried to the border of the map. Political divisions are rarely 
natural divisions; and it is therefore rarely advisable to stop 
the coloring abruptly at a political boundary, especially if the 
geologic value of the map can be increased by carrying the 
mapping into an adjoining political division. 
Scale.—As the scale adopted directly affects the size of the 
map, the selection of the scale will frequently be affected by 
consideration of expense. Commonly, however, there will be 
room for choice, within certain limits; and in this case, or in 
rare instances where the question of expense may be disregarded, 
the selection of the scale will be subject to the following rules: 
The scale should be sufficiently large to show all the geo- 
logic and geographic detail necessary. Curiously enough, this 
rule is rarely disregarded ; the fault commonly committed being 
the adoption of too large a scale. 
The scale should not be larger than is permitted by the 
degree of accuracy of the geologic field work, or by the geologic 
detail which must be shown. The degree of accuracy of the 
geologic field work will usually have been fixed by the scale of 
the field map on which the geologic observations were first 
plotted ; and the published map, therefore, should not in gen- 
eral be on a larger scale than was this original field map, 
