its and by group uses. Some mineral
commodities have so many different kinds of origin in different regions
that to distribute them among several genetic groups in description
would make it impossible to preserve the unity necessary for
consideration of the economic features.

While in the descriptive chapters many references are made to origin, it
may be difficult for the reader to assemble them in perspective; for
this reason we summarize at the outset some of the salient features of
origin of mineral deposits and of their geologic classification.

To the layman the reason for emphasis on origin is often not clear. The
"practical" man frequently regards this phase of the subject as merely
incidental to the immediate economic questions--a playground for
harmless theorists. The answer of the economic geologist is that in no
other way than by a knowledge of origin is it possible to arrive at an
understanding of conditions which so well enables one to answer many
practical questions. In the exploration for mineral deposits, it is
obvious that an understanding of the kinds of geologic conditions and
processes under which a given type of deposit is known to develop
results in the elimination of much unpromising territory, and the
concentration of work on favorable localities. In forming any estimate
of mineral deposits beyond the ground immediately opened up,--for
instance, in estimating depth, form, change in values, mineralogical
character, or interruptions due to faulting,--it is difficult to form
any intelligent conception of the probabilities unless the history of
the deposit is understood. If, for instance, the ore is known to be
formed by hot waters, associated with the cooling of igneous rocks,
different conditions are to be expected below the zone of observation
than if the ore is formed by surface waters. If the ore body is formed
as a single episode under simple geologic conditions, the interpretation
of the possibilities in the situation may be quite different from the
interpretation applied where the history has been more complex. If the
surface conditions suggest possibilities of secondary enrichment of the
ores, the interpretation of the conditions underground will be different
from those applied where there is no evidence of such enrichment.

Where a mineral deposit is completely opened up in three dimensions, it
is often possible to work out economic questions of tonnage, grade,
shape, and values, without