rom below.

In the early days of economic geology there was relatively more emphasis
on the possible effectiveness of ground-waters in concentrating ores of
this type. With the recognition of evidence of a deeper source related
to magmas, the emphasis has swung rapidly to the other extreme. While
the evidence is sound that the magmatic process has been an important
one, it is difficult to see how and to just what extent this process may
have been related to the action of ground-waters,--which were probably
present in a heated condition near the contact. It may never be possible
to discriminate closely between these two agencies. It seems likely that
at some stages the two were so intimately associated that the net result
of deposition cannot be specifically assigned either to one or to the
other.

ZONAL ARRANGEMENT OF MINERALS RELATED TO IGNEOUS ROCKS

Evidence is accumulating in many mining districts that ore deposits of
these igneous associations were deposited with a rough zonal arrangement
about the igneous rock. At Bingham, Tintic, and Butte (pp. 204, 208,
235), copper ores are on the whole closest to the igneous rock, and the
lead, zinc, and silver ores are farther away. Furthermore, the quartz
gangue near the igneous rock is likely to contain minerals
characteristic of hot solutions, while farther away such minerals as
dolomite and calcite appear in the gangue, suggestive of cooler
conditions. In Cornwall (p. 262), tin ores occur close to the
intrusives, and lead-silver ores farther away. The gradations are by no
means uniform; shoots of one class of ore may locally cut abruptly
across or through those of another class.

The existence of zones horizontally or areally arranged about intrusives
suggests also the possibility of a vertical zonal arrangement with
reference to the deep sources of the solutions. Of course when secondary
concentration from the surface, described later, is taken into account,
there may be a marked zonal distribution in a vertical direction, but
this is not primary zoning. A few veins and districts show evidence of
vertical zoning apparently related to primary deposition; for the most
part, however, in any one mine or camp there is yet little evidence of
primary vertical zoning. On the other hand, certain groups of minerals
are characteristic of intense conditions of heat and pressure, as
indicated by the coarse recrystallization and high degree of
metamorphism of the rocks with whic