the aid of geology. Also, where conditions
are comparatively simple and uniform throughout a district, the local
knowledge of other mines may be a sufficient basis for answering these
questions for any new property developed. Empirical methods may suffice.
However, it is seldom that the conditions are so simple that some
geological inference is not necessary. Even where problems are settled
without calling in the geologist, geological inferences are required in
the interpretation of, and projection from, the known facts. It is often
the case that the practical man has in his mind a rather elaborate
assortment of geologic hypotheses, based on his individual experience,
which make the so-called theories of the geologist seem conservative in
comparison. The geologist comes to the particular problem with a
background of established geologic principles and observations, and his
first thought is to ascertain all the local conditions which will aid in
deciphering the complete history of the mineral deposit. There is no
fact bearing on the history, however remote from practical questions,
which may not be potentially valuable.

With this digression to explain the geologist's emphasis on origin of
mineral products, we may return to a consideration of a few of the
principles of rock and mineral genesis which have been found to be
significant in the study of mineral products.

In the preceding chapters it has been indicated that mineral deposits
are mere incidents in the mass of common rocks; that they are made by
the same processes which make common rocks, that none of the processes
affecting mineral deposits are unique for these minerals, and that most
common rocks are on occasion themselves used as mineral resources. These
facts are emphasized in order to make it clear that the study of mineral
deposits cannot be dissociated from the study of rocks, and that the
study of the latter is essential to bring mineral deposits into their
proper perspective. Absorption in the details of a mineral deposit makes
it easy for the investigator to forget or minimize these relations.

Nevertheless, in the study of mineral deposits, and especially deposits
of the metallic minerals, certain geologic features stand out
conspicuously against the common background indicated above. Our
discussion of these features will follow the order of rock genesis
indicated in the description of the metamorphic cycle.

NAMES

Any classification of miner