re so covered than are exposed, and it is no exaggeration to
say that by far the greater part of the mineral wealth of the earth may
never be found. Where a mineral-bearing horizon is exposed by erosion at
the surface, underground operations may follow this horizon a long way
below the capping rocks; but, after all, such operations are
geographically small as compared with the vast areas over which the
covering rocks give no clue as to what is beneath. One of the principal
problems of economic geology for the future is to develop means for
exploration in territories of this sort. A beginning has been made in
various districts by the use of reconnaissance drilling, combined with
interpretation of all the geologic and structural features. The
discovery of one of the largest nickel deposits in the Sudbury district
of Canada was made by reconnaissance drilling to ascertain the general
geologic features, in an area so deeply covered as to give little
suggestion as to the proper location for attack.

SOME ILLUSTRATIVE CASES

The use of outcrops in oil exploration has been noted on other pages
(pp. 146-147).

Outcrops of coal seams may be found in folded or deeply eroded areas.
For the most part, however, and especially in areas of flat-lying rocks,
the presence of coal is inferred from stratigraphic evidence and from
the general nature of the geologic section--which has been determined by
outcrops of associated rocks or by information available at some distant
point. The structural mapping of coal beds on the basis of outcrops and
drill holes has been referred to (pp. 126-127).

Iron ores are very resistant to solution. Where hard and compact they
tend to form conspicuous outcrops, and where soft they may be pretty
well covered by clay and soil. In glaciated areas, like the Lake
Superior region, outcrops of iron ore are much less numerous because of
the drift covering. Certain of the harder iron ores of the Marquette,
Gogebic and Menominee districts of Michigan and of the Vermilion
district of Minnesota project in places through the glacial drift, and
these ores were the first and most easily found. Much the greater
number of iron ore deposits of Lake Superior, including the great soft
deposits of the Mesabi range of Minnesota, fail to outcrop. On the other
hand the _iron formation_, or mother rock of the ore, is hard and
resistant and outcrops are numerous. The hematite ores of Brazil have
many features in common with