the Lake Superior ores in age and
occurrence, but they have not been covered with glacial deposits.
Outcrops of the iron ore are large and conspicuous, and the surface in
this territory gives one some idea of what the Lake Superior region may
have looked like before the glaciers came along. Certain of the soft
iron ores of the lateritic type, as in Cuba, outcrop over great areas
where their topographic situation is such that erosion has not swept
them off. On erosion slopes they are seldom found. The Clinton iron ores
of the southeastern United States outcrop freely.

Some of the lead and zinc deposits of the Mississippi Valley outcrop at
the grass roots as varying mixtures of iron oxide, galena, chert, and
clay, though they seldom project above the general surface. The old lead
ranges of Wisconsin and Illinois, found at the surface a century ago by
the early explorers and traders, have served as starting points for
deeper exploration which has located the zinc deposits. Erosion channels
have freely exposed these ore bodies, and in the Wisconsin-Illinois
deposits most of the ores thus far found are confined to the vicinity of
these channels. The greater number of the lead and zinc deposits of the
Mississippi Valley, however, are covered with weathered material or with
outliers of overlying sediments, with the result that underground
exploration is necessary to locate them.

Sulphide deposits in general, including those carrying gold, silver,
copper, lead, zinc, and other metals, have many common features of
outcrop. The iron sulphide commonly present in these ore bodies is
oxidized to limonite at the surface, with the result that prospectors
look for iron-stained rocks. These iron-stained rocks are variously
called the "gossan," the "iron capping," the "colorado," or the
"eiserner Hut" (iron hat). The gossan is likely to resist erosion and to
be conspicuous at the surface,--though this depends largely on the
relative resistance of the wall rocks, and on whether the gangue is a
hard material like quartz, or some material which weathers more rapidly
like limestone or igneous rock. The gossan does not often carry much
value, though it may show traces of minerals which suggest what may be
found below. Gold, silver, and lead are not easily leached out of the
surface outcrops. Copper and zinc are much more readily leached, and in
the outcrop may disclose their existence only by traces of staining. It
happens not infrequently