The volcanic rocks consist of materials ejected from volcanoes. They
are, however, ejected in very _different states_; sometimes as dust,
sand, angular fragments of rock, cinders, &c., and sometimes as lava
streams. In some instances, the lava has so little fluidity that it
accumulates in a dome-shaped mass over the orifice of eruption, and
perhaps in a few instances it has been thrust upward in a solid state.
There are _two principal varieties_ of lava, the trachytic, consisting
mostly of felspar, and the basaltic, consisting of hornblende. When both
kinds are products of the same eruption, the trachytic lava is thrown
out first, and the basaltic last. The reason of this is, that felspar
is lighter than hornblende, and probably rises to the surface of the
lava mass at the volcanic focus, and the basaltic lava is therefore
reserved till the trachytic has been thrown off.
These, like other rocks, have been produced at different epochs. There
is, however, great difficulty in determining their age; There are some
differences of structure and composition observed, in comparing the
older and newer lavas; but the only method that can be relied on to
determine their age is their relation to other rocks. When they occur
between strata whose age is determined by imbedded fossils, they must be
of intermediate age between the inferior and superior strata.
1. _Modern Volcanic Rocks._--Some of the volcanic rocks are of modern
origin, and are produced by volcanoes now active. The total amount of
these, and of all the other volcanic rocks, is probably less than that
of either of the other principal divisions of rocks; yet they form no
inconsiderable part of the earth's crust. The number of active volcanoes
is not far from three hundred, and the number of eruptions annually is
estimated at about twenty. In some cases, the lava consists of only a
single stream, of but a few hundred yards in extent. It extends,
however, not unfrequently twenty miles in length, and two or three
hundred yards in breadth. The eruption of Mount Loa, on the island of
Hawaii, in 1840, from the crater of Kilauea, covered an area of fifteen
square miles to the depth of twelve feet; and another eruption of the
same mountain, in 1843, covered an area of at least fifty square miles.
The eruption in Iceland, in 1783, continued in almost incessant activity
for a year, and sent off two streams in opposite directions, which
reached a distance of fifty miles in one case, and of forty in the
other, with a width varying from three to fifteen miles, and with an
average depth of more than a hundred feet. The size of some of the
volcanic mountains will also assist in forming an idea of the amount of
volcanic rocks. Monte Nuovo, near Naples, which is a mile and a half in
circumference and four hundred and forty feet high, was thrown up in a
single day. Ætna, which is eleven thousand feet high, and eighty-seven
miles in circumference at its base, has probably been produced wholly
by its own eruptions. A large part of the chain of the Andes consists of
volcanic rock, but the proportion we have not the means of estimating.
2. _Tertiary Lavas._--There is another class of volcanic products, which
are so situated with reference to the tertiary strata that they must be
referred to that period. The principal localities of these lavas, so far
as yet known, are Italy, Spain, Central France, Hungary, and Germany.
They are also found in South America. Those of Central France have been
studied with the most care. They occur in several groups, but they were
the seats of volcanic activity during the same epoch, and formed parts
of one extensive volcanic region. Each of these minor areas, embracing a
circle of twenty or thirty miles in diameter, is covered with hills two
or three thousand feet in height, which are composed entirely of
volcanic products, like the cone of Ætna. On many of them there are
perfectly-formed craters still remaining. Numerous streams of lava have
flowed from these craters, some of which can now be traced, throughout
their whole extent, with as much certainty as if they were eruptions of
the present century. Some of the lavas have accumulated around the
orifices of eruption, forming rounded, dome-shaped eminences. These
lavas generally consist of trachyte, and have therefore a low specific
gravity, and imperfect fluidity. The basaltic lavas have often spread
out over broad areas, and, when they have been confined in valleys, have
reached a distance of fifteen miles or more from their source. There
still remain indications of a current of lava which was thirty miles
long, six broad, and in a part of its course from four to six hundred
feet deep. The above sketch (Fig. 5) will give some idea of the highly
volcanic aspect which the district of Auvergne, in France, presents.
[Illustration: Fig. 5.]
The unimpaired state of some of the cones and craters, and of the lava
currents, would lead to the impression that these regions have been the
theatre of intense volcanic action within a very recent period. But
there is good reason to believe that this has not been the case. "The
high antiquity of the most modern of these volcanoes is indeed
sufficiently obvious. Had any of them been in a state of activity in the
age of Julius Cæsar, that general, who encamped upon the plains of
Auvergne and laid siege to its principal city, could hardly have failed
to notice them."
It is equally certain that the commencement of their activity was at a
late period in the history of the earth. Lava currents are frequently
found in France resting upon the early tertiary strata, but no lava
current is found below them. The later tertiary strata contain pebbles
of volcanic rocks, showing that lavas had been previously ejected, but
none are found in the older strata of this formation. We must,
therefore, conclude that these volcanic tracts assumed their volcanic
character at some intermediate point in the tertiary period.
When we find that their activity commenced at so late a period and
closed so long ago, we might be led to suppose that it was of very short
duration. But a great number of facts, in the present condition of the
country, require that we should assign to them a very prolonged
activity. A single instance will be sufficient to show the nature of the
evidence upon which this conclusion rests. The heavy line (Fig. 6)
represents the present form of one of the valleys. A bed of lava forms
the highest point of land represented, and a second bed is found in an
intermediate part of the slope. The position of the upper bed must have
been a valley, when the lava flowed there. We may represent this valley
by the line _a b c_. The slow operation of natural denuding causes at
length excavated the valley _d e h_, when another lava current flowed
through it, covering its bed of pebbles, as before. The same denuding
causes have at length produced the present valley, _f g h_. These
remnants of lava-currents, as they have formed a very imperishable rock,
have protected the subjacent strata from erosion, and furnish evidence
of the position of the valley at different periods. When we consider
with what extreme slowness denuding causes produce changes on the
surface, and what extensive changes they have here nevertheless effected
in the interval between the production of the different lava currents,
we are compelled to feel that that interval was a very prolonged one.
Yet this period, however long it may have been, was evidently less than
the period of activity of these volcanoes.
[Illustration: Fig. 6.]
3. _Volcanic rocks of an earlier date_ are also found, sometimes as
distinct lavas, though generally as volcanic grits. They occur
interstratified with the cretaceous rocks, and with every other
formation of the fossiliferous series, showing that, from the earliest
times, these rocks have been accumulating as they now are.
_The trappean rocks_ may, in a general classification, be considered as
volcanic. It will be shown, hereafter, that they are the lavas of
submarine volcanoes. They do not, however, occur in the form of lava
currents, but in great tabular masses, generally between stratified
rocks, or in the form of dikes. They are also entirely unconnected with
cones or craters.
The trappean rocks occur more or less abundantly in all countries. One
of the most noted localities of this rock is a region embracing the
north of Ireland, and several of the islands on the western coast of
Scotland. It contains the celebrated Giant's Causeway, which consists of
a mass of columnar trap; also Fingal's Cave, which is produced by a
portion of the trap being columnar, and thus disintegrating more rapidly
than the rest, by the action of the waves. An immense mass of greenstone
trap, which has generally been considered as a vast dike, though often a
mile in thickness, is found extending from New Haven to Northampton, on
the west side of the Connecticut river. It then crosses to the east
side, and continues in a northerly direction to the Massachusetts line.
Under different names, it constitutes a nearly continuous and
precipitous mountain range for about one hundred miles. Dr. Hitchcock
supposes this greenstone range to be, not an injected dike, but a
tabular mass of ancient lava, which was spread out on the bed of the
ocean during the period of the deposition of the Connecticut river
sandstone. It was subsequently covered with a deposit of strata of great
thickness, and then by subterranean forces thrown into its present
inclined position.
There is a mass of basaltic rock in the valley of the Columbia river, in
the Oregon Territory, which extends without interruption for a distance
of four hundred miles. Its breadth and thickness is not known, but in
some places the river has cut a channel in this rock to a depth of four
hundred feet. Its age has not been determined, and it will, perhaps, be
found to be a tertiary or modern production.