acter and heat-giving qualities
(whence comes its value), quantity, thickness, depth, and other
conditions that effect the cost of its extraction. Metalliferous mineral
lands are considered in relation to general geology, country rock,
intrusions and metamorphism, structure, outcrops and float of lodes,
prospects and mines, samples, and history of the region.

Classifications of this kind have often proved useful to large holders
of land as a basis for intelligent handling of problems of sale,
taxation, and the granting of rights to explorers. Because of the lack
of this elementary information, there has been in some quarters timidity
about dealing with large holdings, for fear of parting with possible
future mineral wealth,--with the result that such tracts are carried at
large expense and practically removed from the field of exploration. To
the same cause may be attributed some of the long delays on the part of
the government in opening lands for mineral entry or in issuing patents
on land grants.

OUTCROPS OF MINERAL DEPOSITS

Many mineral deposits have been found because they outcrop at the
surface; the discoveries may have been by accident or they may have been
aided by consideration of geologic factors. There are still vast
unexplored areas in which mineral deposits are likely to be found
standing out at the surface. For much of the world, however, the surface
has been so thoroughly examined that the easy surface discoveries have
been made, and the future is likely to see a larger application of
scientific methods to ground where the outcrops do not tell an obvious
story. Mineral deposits may fail to outcrop because of covering by
weathered rock or soil, by glacial deposits, or by younger formations
(surface igneous flows or sediments), or the outcrop of a deposit may be
so altered by weathering as to give little clue to the uninitiated as to
what is beneath. Mineral deposits formed in older geologic periods have
in most cases been deeply covered by later sediments and igneous rocks.
Such deposits are in reach of exploration from the surface only in
places where erosion has partly or wholly removed the later covering. An
illustration of this condition is furnished in the Great Basin district
of Nevada, where ore bodies have been covered by later lava flows. The
ore-bearing districts are merely islands exposed by erosion in a vast
sea of lava and surface sediments. Beyond reasonable doubt many more
deposits a