at broadly, the deposition
of ores at igneous contacts under contact metamorphism is a mere
incident in the much more widespread and extensive alterations of this
kind. Hence it is that the subject of contact metamorphism is of
interest to economic geologists. The minerals here formed which do not
constitute ores throw much light on the nature of the ore-bearing
solutions, the conditions of temperature and pressure, and the processes
which locally and incidentally develop the ore bodies. The subject,
however, is a complex one, the full discussion of which belongs in
treatises on metamorphism.[7] We may note only a few salient features.

For many hundreds of yards the rocks adjacent to the intrusions may be
metamorphosed almost beyond recognition. This is especially true of the
limestone, which may be changed completely to solid masses of quartz and
silicates. The shales and sandstones are ordinarily less vitally
affected. The shales become dense, highly crystalline rocks of a
"hornstone" type, with porphyritic developments of silicate minerals.
The sands and sandstones become highly crystalline quartzites, spotted
with porphyritic developments of silicates. Occasionally even these
rocks may be extensively replaced by other minerals, as in the Coeur
d'Alene district, where quartzites adjacent to the ore veins may be
completely replaced by iron carbonate.

A question of special interest to economic geologists is the source of
the materials for the new minerals in these extensively altered zones.
In some cases the minerals are known to be the result of
recrystallization of materials already in the rock, after the
elimination of certain substances such as carbon dioxide and lime, under
the pressures and temperatures of the contact conditions. In such cases
there has obviously been large reduction in volume to close the voids
created by the elimination of substances. In the majority of cases, the
new substances or minerals are clearly introduced from the igneous
source, replacing the wall rock volume for volume so precisely that such
original textures and structures as bedding are not destroyed. In many
cases the result is clearly due to a combination of recrystallization of
materials already present and introduction of minerals by magmatic
solutions from without. So obvious is the evidence of the introduction
of materials from without, that there has been a tendency in some
quarters to overlook the extensive recrystalliza