All rocks are not embraced in the sedimentary and eruptive divisions,
but there is a third grand division, which, although rarely mentioned or
recognized in the more comprehensive works on geology, it is deemed best
not to leave entirely unnoticed here. These are the _vein_ rocks. They
present an immense number of varieties, and yet, taken altogether, form
but a small fraction of the earth’s crust. They are, however, the great
repositories of the precious and other metals, and hence are objects of
far greater interest to the miner and practical man than the eruptive
rocks, or, in some parts of the world, even than the sedimentary rocks.
The vein rocks, like the eruptive rocks, occupy fissures in the earth’s
crust intersecting the stratified formations; but the fissures filled
with vein rocks are called veins, and not dikes. We will first notice
the mode of formation of a typical vein, and then examine its contents.
Geologists are agreed that water penetrates to a very great depth in the
earth’s crust. All minerals are more or less soluble in water; and we
may consider the water circulating through the rocks, especially at
considerable depths, as, in most cases, a saturated solution of the
various minerals of which they are composed. Very slight changes in the
conditions will cause saturated solutions to deposit part of their
mineral load. The water at great depths has a high temperature, and is
subjected to an enormous pressure; and both of these circumstances favor
solution. Suppose, now, that these hot subterranean waters enter a
fissure in the crust and flow upwards, perhaps issuing on the surface as
a warm mineral spring; as they approach the surface, the temperature and
pressure, and consequently their solvent power, are diminished; and a
portion of the dissolved minerals must be deposited on the walls of the
fissure, which thus becomes narrower, and in the course of time is
gradually filled up. The vein is then complete; and the mineral waters
are forced to seek a new outlet.
Veins have the same general forms as dikes, since the fissures are
formed in the same way for both; but the vein is of slow growth, and may
require ages for its completion, while the dike is formed in an hour or
a day. It is now generally believed that water is an important agent in
the formation of eruptive rocks; since they all contain water at the
time of their eruption; and since it has been demonstrated that, while
ordinary rocks require a temperature of 2000° to 3000° for their fusion
in the absence of water, they are liquified at temperatures below 1000°
in the presence of water. In other words, common rocks are very
infusible and insoluble bodies, and heat and water acting independently
have little effect upon them; but when fire and water are combined in
what is now known as aqueo-igneous fusion, they prove very efficient
agents of liquefaction.
If we adopt these views, then it can be shown that, in origin, veins and
dikes differ in degree only, and are not fundamentally unlike; and the
formation and relations of the three great classes of rocks may be
summarized as follows:—
The ocean and atmosphere, operating on the earth’s surface, have worked
over and stratified the crust, until the sedimentary rocks have now an
average thickness variously estimated at from ten to thirty or forty
miles. This entire thickness of stratified rocks, and a considerable
depth of the underlying unstratified crust, must be saturated with
water; and all but the more superficial portions of this water-soaked
crust must be very hot, the temperature increasing steadily downwards
from the surface. Both eruptive and vein rocks originate in this highly
heated, hydrated crust. Eruptive rocks are formed when the heat, aided
by more or less water, softens the rocks, either stratified or
unstratified, by aqueo-igneous fusion, and the plastic materials are
forced up through fissures to or toward the surface. Vein rocks are
formed when the water, aided by more or less heat, dissolves the rocks,
either stratified or unstratified, by what may be called igneo-aqueous
solution, and subsequently deposits the mineral matter in, _i.e._, on
the walls of, fissures leading up to or toward the surface. In the case
of the dike rocks, heat is the chief agent, and water merely an
auxiliary; while with the vein rocks it is just the reverse. But between
the two it is probably impossible to draw any sharp line.
The water circulating through the crust, and saturated with its various
mineral constituents, has been called the “juice” of the crust; and
veins are formed by the concentration of this earth-juice in fissures.
One of the most important characteristics of the vein rocks, as a class,
is the immense variety which they present; for nearly every known
mineral is embraced among their constituents; and these are combined in
all possible ways and proportions, so that the number of combinations is
almost endless. The solvent power of the subterranean waters varies for
different minerals; and appears often to be greatest for the rarer
species. In other words, there is a sort of selective action, whereby
many minerals which exist in stratified and eruptive rocks, so thinly
diffused as to entirely escape the most refined observation, are
concentrated in veins in masses of sensible size; and our lists of known
minerals and chemical elements are undoubtedly much longer than they
would be if these wonderful storehouses of fine minerals which we call
veins had never been explored. As a rule, the minerals in veins form
larger and more perfect crystals than we find in either of the other
great classes of rocks. This is simply because the conditions are more
favorable for crystallization in veins than in dikes or sedimentary
strata. In both dike and stratified rocks, the growing crystals are
surrounded on all sides by solid or semi-solid matter; and, being thus
hampered, it is simply impossible that they should become either large
or perfect. In the vein, on the other hand, there are usually no such
obstacles to be overcome; but the crystals, starting from the walls of
the fissure, grow toward its centre, their growing ends projecting into
a free space, where they have freedom to develop their normal forms and
to attain a size limited only, in many cases, by the breadth of the
fissure. With, possibly, some rare exceptions, all the large and perfect
crystals of quartz, feldspar, mica, beryl, apatite, fluorite, and of
minerals generally, which we see in mineralogical cabinets, have
originated in veins. Those fissures which become the seats of mineral
veins are really Nature’s laboratories for the production of rare and
beautiful mineral specimens; and hence the vein rocks are the chief
resort of the mineralogist, to whom they are of far greater interest
than all the eruptive and stratified rocks combined.
The leading characteristics, then, of the vein rocks may be summarized
as follows: (1) They contain nearly all known minerals, including many
rare species and elements which are unknown outside of this class of
rocks. (2) These mineral constituents, occurring singly and combined,
give rise to a number of varieties of rocks so vast as to baffle
detailed description. (3) They exceed all other rocks in the coarseness
of their crystallization, and in the perfection and beauty of the single
crystals which they afford.