62 EDWINA GC TE CTGEL 
will almost invariably become separated from the volume or 
paper which it is supposed to accompany; and the legend of a 
map published in that shape should accordingly be as detailed 
as possible. A map inserted as a plate is somewhat less likely 
to be used separately and the legend may be less detailed ; while 
a map set in the text cannot well be separated from it and its 
legend may therefore be reduced to the briefest outline. 
Boundary lines.— Every formation should be bordered by a 
definite boundary, unless transition or intergradation is meant 
to be shown. Except in this relatively infrequent case, it is not 
permissible to allow two colors or conventions to meet without 
an intervening boundary line. When possible, it is advisable to 
distinguish different grades of precision by the use of different 
conventional boundaries. For example, where the boundary has 
been traced in the field with a certain degree of precision, the 
separating line may be continuous ; where the boundary is doubt- 
ful, or inferred, the line may be broken or dotted. Continuous 
lines indicating faults may be given double thickness, in order 
to distinguish them from continuous lines indicating precisely 
located boundaries, 
Distinction of outcrops —\n detailed maps whose scale is suffi- 
ciently large to permit such treatment, precision is gained by 
distinguishing actual outcrops from areas whose geology is 
merely inferred. This distinction may be made by using for the 
outcrops either a pattern or a hachure or cross-lining under the 
color. The best examples of the latter method with which the 
writer is acquainted are to be found in Monograph XXXVI of 
the United States Geological Survey (Ox the Crystal Falls Iron- 
Bearing District of Michigan). In this volume, the practice is 
entirely consistent on the point in question, all its detailed geolo- 
gic maps (Plates XVI, XVII, XVIII, XLIX, L, LI) distinguishing 
actual outcrops by means of cross-lining, disposed so as to show 
both location and size of these outcrops. Plate XXXI of Mon- 
ograph XXXIII (Geology of the Narragansett Basin) distinguishes 
outcrops by means of cross-marks. Earlier publications on the 
Lake Superior region, both in monographs and in papers in the 
