83. Mention has already been made of the fact, that the heated matters
ejected from volcanoes, or forcibly intruded into cracks, crevices, &c.,
occasionally _alter_ the rocks with which they come in contact. When
this alteration has proceeded so far as to induce a crystalline or
semi-crystalline character, the rock so altered is said to be
metamorphosed. Metamorphism has likewise been produced by the chemical
action of percolating water, which frequently dissolves out certain
minerals, and replaces these with others having often a very different
chemical composition. But metamorphism on the large scale--that is to
say, metamorphism which has affected wide areas, such as the northern
Highlands of Scotland and wide regions in Scandinavia, or the still
vaster areas in North America--has most probably been effected both by
the agency of heat and chemical action, at considerable depths, and
under great pressure. When we observe what effect can be produced by
heat upon rocks, under little or no pressure, and how water percolating
from above gradually changes the composition of some rock-masses, we may
readily believe that at great depths, where the heat is excessive, such
metamorphic action must often be intensified. Thus, for example,
limestone heated in the usual way gives off its carbonic acid gas, and
is reduced to quicklime; but, under sufficient pressure, this gas is not
evolved, the limestone becoming converted into a crystalline marble.
Some crystalline limestones, indeed, have all the appearance of having
at one time been actually melted and squirted under great pressure into
seams and cracks of the surrounding strata. Heated water would appear to
have been the agent to which much of the metamorphism which affects the
rocky strata must be attributed. But the mode or modes in which it has
acted are still somewhat obscure; as may be readily understood when it
is remembered how difficult, and often how impossible it is to realise
or reproduce in our laboratories the conditions under which deep-seated
metamorphic action must frequently have taken place. In foliated rocks,
the minerals are chiefly quartz, felspar, and mica, talc, or chlorite.
The ingredients of these minerals undoubtedly existed in a diffused
state in the original rocks, and heated water charged with alkaline
carbonates, as it percolated through the strata, either along the
layers of bedding or lines of cleavage, slowly acted upon these,
dissolving and redepositing them, and thus inducing segregation. There
is every kind of gradation in metamorphism. Thus, we find certain rocks
which are but slightly altered--their original character being still
quite apparent; while, in other cases, the original character is so
entirely effaced that we can only conjecture what that may have been.
When we have a considerable thickness of metamorphic rocks which still
exhibit more or less distinct traces of bedding, like the successive
beds of gneiss, mica-schist, and quartz rock of the Scottish Highlands,
we can hardly doubt that the now crystalline masses are merely highly
altered aqueous strata. But there are cases where even the bedding
becomes obliterated, and it is then much more difficult to determine the
origin of the rocks. Thus, we find bedded gneiss passes often, by
insensible gradations, into true amorphous granite. There has been much
difference of opinion as to the origin of granite--some holding it to be
an igneous rock, others maintaining its metamorphic origin. It is
probably both igneous and metamorphic, however. If we conceive of
certain aqueous rocks becoming metamorphosed into gneiss, we may surely
conceive of the metamorphism being still further continued until the
mass is reduced to a semi-fluid or pasty condition, when all trace of
foliation and bedding might readily disappear, and the weight of the
superincumbent strata would be sufficient to force portions of the
softened mass into cracks and crevices of the still solid rocks above
and around it. Hence we might expect to find the same mass of granite
passing gradually in some places into gneiss, and in other places
protruding as _veins_ and _dykes_ into the surrounding rocks; and this
is precisely what occurs in nature.
84. _Mineral veins_ have, as a rule, been formed by water depositing
along the walls of fissures the various matters which they held in
solution, but certain kinds of veins (such as quartz veins in granite)
probably owe their origin to chemical action which has induced the
quartz to segregate from the rock mass. Some have maintained that the
metallic substances met with in many veins owe their deposition to the
action of currents of voltaic electricity; while others have attributed
their presence to sublimation from below, the metals having been
deposited in the fissures very much as lead is deposited in the chimney
of a leadmill. But in many cases there seems little reason to doubt that
the ores have merely been extracted from the rocks, and re-deposited in
fissures, by water, in the same way as the other minerals with which
they are associated.