nal frequency.

Climatic conditions may determine the locus of search for certain
surface minerals. Bauxite and lateritic iron ores, for instance, are
known to favor tropical climates. In exploration for these minerals, the
climatic factor must be applied in connection with the topographic
considerations already mentioned, and both, in turn, in connection with
the character of the country rock as determined by general geologic
surveys. A combination of climatic, topographic, and other physiographic
conditions may be used also in exploration for certain types of residual
clays.

SIZE AND DEPTH OF ORE BODIES AS DETERMINED FROM OUTCROP

Where the ore body is harder than the surrounding rock, it stands out in
conspicuous outcrops and is likely to show a narrowing below. Where it
is softer than the surrounding rocks, and outcrops in a topographic
depression, it is perhaps more likely to show widening below. These
features are due to the general facts that, where the ore body is hard
and resistant, the downward progress of erosion is likely to be arrested
where the adjacent rocks occupy the larger part of the surface, that is,
where the ore body is narrower. This principle is often vaguely
recognized in the assumption that an exceptionally large outcrop of an
ore vein may be "too good to last." Again, such a generalization must be
applied to a specific case with much caution.

Attempts to forecast the depth of veins from their extent at the surface
meet with only partial success. In a very general way great persistence
horizontally suggests persistence in depth, on the ground that the
section exposed on the surface is as likely to be a section of average
dimensions as one along vertical lines.

Faith is the first article of the prospector's creed, and it is hard to
shake his conviction that every ore outcrop must widen and improve
below. As expressed by the French-Canadian prospector in the Cobalt
district, the "vein calcite can't go up, she must go down." While the
scientist may have grounds to doubt this reasoning, he is not often in a
position to offer definite negative evidence.

THE USE OF PLACERS IN TRACING MINERAL OUTCROPS

Outcrops of ore-bearing rock may occasionally be located by tracing a
placer deposit back to its source, or by following up ore fragments in
the "wash" on mountain sides to the place of origin, or by noting ore
fragments in glacial deposits. The presence of an ore mineral in a
placer nat