e stocks
the ores are much more abundant near the top or apex of a stock than
lower down.[6] In parts of the region where erosion has removed all but
the deeper portions of the stocks, ore bodies are less abundant. It will
be of interest to follow the testing of this generalization in other
parts of the world.

The scientist is constantly groping for underlying simple truth. Such
glimpses of order and symmetry in the distribution of ore around igneous
rocks as are afforded by the facts above stated, tempt the imagination
to a conception of a simple type or pattern of ore distribution around
intrusions. For this reason we should not lose sight of the fact that,
in the present state of knowledge, the common and obvious case is one of
irregular and heterogeneous distribution, and that there are many
variations and contradictions even to the simplest generalization that
can be made. The observer is repeatedly struck by the freakish
distribution of ores about igneous masses, as compared with their
regularity of arrangement under sedimentary processes to be discussed
later. It is yet unexplained how an intrusive like the Butte granite can
produce so many different types of ores at different places along its
periphery or within its mass, and yet all apparently under much the same
general conditions and range of time. It is difficult also to discern
the laws under which successive migrations of magma, from what seems to
be a single deep-seated source or melting-pot, may carry widely
contrasting mineral solutions. Far below the surface, beyond our range
of observation, it is clear that there is a wonderful laboratory for the
compounding and refinement of ores, but as to its precise location and
the nature of its processes we can only guess.

Other features of distribution of minerals associated with igneous rocks
are indicated by their grouping in metallogenic provinces and epochs
(see pp. 308-309).

THE RELATION OF CONTACT METAMORPHISM TO ORE BODIES OF THE FOREGOING CLASS.

The deposition of ores of igneous source in the country rock into which
the igneous rocks are intruded is a phase of contact metamorphism.
Ordinarily where this deposition occurs there are further extensive
replacements and alterations of the country rock, resulting in the
development of great masses of quartz, garnet, pyroxene, amphibole, and
other silicates, and in some cases of calcite, dolomite, siderite,
barite, alunite, and other minerals. Looked