ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS.
Tests of relative age of volcanic rocks--Test by superposition and
intrusion--Dike of Quarrington Hill, Durham--Test by alteration of
rocks in contact--Test by organic remains--Test of age by mineral
character--Test by included fragments--Volcanic rocks of the
Post-Pliocene period--Basalt of Bay of Trezza in Sicily--Post-Pliocene
volcanic rocks near Naples--Dikes of Somma--Igneous formations of the
Newer Pliocene period--Val di Noto in Sicily.
Having referred the sedimentary strata to a long succession of geological
periods, we have next to consider how far the volcanic formations can be
classed in a similar chronological order. The tests of relative age in this
class of rocks are four:--1st, superposition and intrusion, with or without
alteration of the rocks in contact; 2d, organic remains; 3d, mineral
character; 4th, included fragments of older rocks.
[Illustration: Fig. 461. Cross section.]
_Tests by superposition, &c._--If a volcanic rock rests upon an aqueous
deposit, the former must be the newest of the two, but the like rule does
not hold good where the aqueous formation rests upon the volcanic, for
melted matter, rising from below, may penetrate a sedimentary mass without
reaching the surface, or may be forced in conformably between two strata,
as _b_ at D in the annexed figure (fig. 461.), after which it may cool down
and consolidate. Superposition, therefore, is not of the same value as a
test of age in the unstratified volcanic rocks as in fossiliferous
formations. We can only rely implicitly on this test where the volcanic
rocks are contemporaneous, not where they are intrusive. Now they are said
to be contemporaneous if produced by volcanic action, which was going on
simultaneously with the deposition of the strata with which they are
associated. Thus in the section at D (fig. 461.), we may perhaps ascertain
that the trap _b_ flowed over the fossiliferous bed _c_, and that, after
its consolidation, _a_ was deposited upon it, _a_ and _c_ both belonging to
the same geological period. But if the stratum _a_ be altered by _b_ at the
point of contact, we must then conclude the trap to have been intrusive, or
if, in pursuing _b_ for some distance, we find at length that it cuts
through the stratum _a_, and then overlies it as at E.
We may, however, be easily deceived in supposing a volcanic rock to be
intrusive, when in reality it is contemporaneous; for a sheet of lava, as
it spreads over the bottom of the sea, cannot rest everywhere upon the
same stratum, either because these have been denuded, or because, if newly
thrown down, they thin out in certain places, thus allowing the lava to
cross their edges. Besides, the heavy igneous fluid will often, as it moves
along, cut a channel into beds of soft mud and sand. Suppose the submarine
lava F to have come in contact in this manner with the strata _a_, _b_,
_c_, and that after its consolidation, the strata _d_, _e_, are thrown down
in a nearly horizontal position, yet so as to lie unconformably to F, the
appearance of subsequent intrusion will here be complete, although the trap
is in fact contemporaneous. We must not, therefore, hastily infer that the
rock F is intrusive, unless we find the strata _d_ or _e_ to have been
altered at their junction, as if by heat.
[Illustration: Fig. 462. Cross section.]
When trap dikes were described in the preceding chapter, they were shown to
be more modern than all the strata which they traverse. A basaltic dike at
Quarrington Hill, near Durham, passes through coal-measures, the strata of
which are inclined, and shifted so that those on the north side of the dike
are 24 feet above the level of the corresponding beds on the south side
(see section, fig. 463.). But the horizontal beds of overlying Red
Sandstone and Magnesian Limestone are not cut through by the dike. Now here
the coal-measures were not only deposited, but had subsequently been
disturbed, fissured, and shifted, before the fluid trap now forming the
dike was introduced into a rent. It is also clear that some of the upper
edges of the coal strata, together with the upper part of the dike, had
been subsequently removed by denudation before the lower New Red Sandstone
and Magnesian Limestone were superimposed. Even in this case, however,
although the date of the volcanic eruption is brought within narrow limits,
it cannot be defined with precision; it may have happened either at the
close of the Carboniferous period, or early in that of the Lower New Red
Sandstone, or between these two periods, when the state of the animate
creation and the physical geography of Europe were gradually changing from
the type of the Carboniferous era to that of the Permian.
[Illustration: Fig. 463. Section at Quarrington Hill, east of
Durham. (Sedgwick.)
_a._ Magnesian Limestone (Permian).
_b._ Lower New Red Sandstone.
_c._ Coal strata.]
The test of age by superposition is strictly applicable to all stratified
volcanic tuffs, according to the rules already explained in the case of
other sedimentary deposits. (See p. 96.)
_Test of age by organic remains._--We have seen how, in the vicinity of
active volcanos, scoriae, pumice, fine sand, and fragments of rock are
thrown up into the air, and then showered down upon the land, or into
neighbouring lakes or seas. In the tuffs so formed shells, corals, or
any other durable organic bodies which may happen to be strewed over the
bottom of a lake or sea will be imbedded, and thus continue as permanent
memorials of the geological period when the volcanic eruption occurred.
Tufaceous strata thus formed in the neighbourhood of Vesuvius, Etna,
Stromboli, and other volcanos now active in islands or near the sea, may
give information of the relative age of these tuffs at some remote
future period when the fires of these mountains are extinguished. By
such evidence we can distinctly establish the coincidence in age of
volcanic rocks, and the different primary, secondary, and tertiary
fossiliferous strata already considered.
The tuffs now alluded to are not exclusively marine, but include, in some
places, freshwater shells; in others, the bones of terrestrial quadrupeds.
The diversity of organic remains in formations of this nature is perfectly
intelligible, if we reflect on the wide dispersion of ejected matter during
late eruptions, such as that of the volcano of Coseguina, in the province
of Nicaragua, January 19. 1835. Hot cinders and fine scoriae were then cast
up to a vast height, and covered the ground as they fell to the depth of
more than 10 feet, and for a distance of 8 leagues from the crater in a
southerly direction. Birds, cattle, and wild animals were scorched to death
in great numbers, and buried in these ashes. Some volcanic dust fell at
Chiapa, upwards of 1200 miles to windward of the volcano, a striking proof
of a counter current in the upper region of the atmosphere; and some on
Jamaica, about 700 miles distant to the north-east. In the sea, also, at
the distance of 1100 miles from the point of eruption, Captain Eden of the
Conway sailed 40 miles through floating pumice, among which were some
pieces of considerable size.[399-A]
_Test of age by mineral composition._--As sediment of homogeneous
composition, when discharged from the mouth of a large river, is often
deposited simultaneously over a wide space, so a particular kind of lava,
flowing from a crater during one eruption, may spread over an extensive
area; as in Iceland in 1783, when the melted matter, pouring from Skaptar
Jokul, flowed in streams in opposite directions, and caused a continuous
mass, the extreme points of which were 90 miles distant from each other.
This enormous current of lava varied in thickness from 100 feet to 600
feet, and in breadth from that of a narrow river gorge to 15 miles.[399-B]
Now, if such a mass should afterwards be divided into separate fragments by
denudation, we might still perhaps identify the detached portions by their
similarity in mineral composition. Nevertheless, this test will not always
avail the geologist; for, although there is usually a prevailing character
in lava emitted during the same eruption, and even in the successive
currents flowing from the same volcano, still, in many cases, the different
parts even of one lava-stream, or, as before stated, of one continuous mass
of trap, vary so much in mineral composition and texture as to render these
characters of minor importance when compared to their value in the
chronology of the fossiliferous rocks.
It will, however, be seen in the description which follows, of the European
trap rocks of different ages, that they had often a peculiar lithological
character, resembling the differences before remarked as existing between
the modern lavas of Vesuvius, Etna, and Chili. (See p. 378.)
It has been remarked that in Auvergne, the Eifel, and other countries where
trachyte and basalt are both present, the trachytic rocks are for the most
part older than the basaltic. These rocks do, indeed, sometimes alternate
partially, as in the volcano of Mont Dor, in Auvergne; but the great mass
of trachyte occupies in general an inferior position, and is cut through
and overflowed by basalt. It can by no means be inferred that trachyte
predominated greatly at one period of the earth's history and basalt at
another, for we know that trachytic lavas have been formed at many
successive periods, and are still emitted from many active craters; but it
seems that in each region, where a long series of eruptions have occurred,
the more felspathic lavas have been first emitted, and the escape of the
more augitic kinds has followed. The hypothesis suggested by Mr. Scrope
may, perhaps, afford a solution of this problem. The minerals, he observes,
which abound in basalt are of greater specific gravity than those composing
the felspathic lavas; thus, for example, hornblende, augite, and olivine
are each more than three times the weight of water; whereas common felspar,
albite, and Labrador felspar, have each scarcely more than 2-1/2 times the
specific gravity of water; and the difference is increased in consequence
of there being much more iron in a metallic state in basalt and greenstone
than in trachyte and other felspathic lavas and traps. If, therefore, a
large quantity of rock be melted up in the bowels of the earth by volcanic
heat, the denser ingredients of the boiling fluid may sink to the bottom,
and the lighter remaining above would in that case be first propelled
upwards to the surface by the expansive power of gases. Those materials,
therefore, which occupied the lowest place in the subterranean reservoir
will always be emitted last, and take the uppermost place on the exterior
of the earth's crust.
_Test by included fragments._--We may sometimes discover the relative age
of two trap rocks, or of an aqueous deposit and the trap on which it rests,
by finding fragments of one included in the other, in cases such as those
before alluded to, where the evidence of superposition alone would be
insufficient. It is also not uncommon to find conglomerates almost
exclusively composed of rolled pebbles of trap, associated with stratified
rocks in the neighbourhood of masses of intrusive trap. If the pebbles
agree generally in mineral character with the latter, we are then enabled
to determine the age of the intrusive rock by knowing that of the
fossiliferous strata associated with the conglomerate. The origin of such
conglomerates is explained by observing the shingle beaches composed of
trap pebbles in modern volcanic islands, or at the base of Etna.
_Post-Pliocene Period (including the Recent)._--I shall now select examples
of contemporaneous volcanic rocks of successive geological periods, to show
that igneous causes have been in activity in all past ages of the world,
and that they have been ever shifting the places where they have broken out
at the earth's surface.
One portion of the lavas, tuffs, and trap dikes of Etna, Vesuvius, and
the Island of Ischia, has been produced within the historical era;
another, and a far more considerable part, originated at times
immediately antecedent, when the waters of the Mediterranean were
already inhabited by the existing species of testacea. The southern and
eastern flanks of Etna are skirted by a fringe of alternating
sedimentary and volcanic deposits, of submarine origin, as at Aderno,
Trezza, and other places. Of sixty-five species of fossil shells which I
procured in 1828 from this formation, near Trezza, it was impossible to
distinguish any from species now living in the neighbouring sea.
[Illustration: Fig. 464. View of the Isle of Cyclops in the Bay
of Trezza.[401-A]]
The Cyclopian Islands, called by the Sicilians Dei Faraglioni, in the
sea cliffs of which these beds of clay, tuff, and associated lava are
laid open to view, are situated in the Bay of Trezza, and may be
regarded as the extremity of a promontory severed from the main land.
Here numerous proofs are seen of submarine eruptions, by which the
argillaceous and sandy strata were invaded and cut through, and
tufaceous breccias formed. Inclosed in these breccias are many angular
and hardened fragments of laminated clay in different states of
alteration by heat, and intermixed with volcanic sands.
The loftiest of the Cyclopian islets, or rather rocks, is about 200 feet in
height, the summit being formed of a mass of stratified clay, the laminae of
which are occasionally subdivided by thin arenaceous layers. These strata
dip to the N.W., and rest on a mass of columnar lava (see fig. 464.) in
which the tops of the pillars are weathered, and so rounded as to be often
hemispherical. In some places in the adjoining and largest islet of the
group, which lies to the north-eastward of that represented in the drawing
(fig. 464.), the overlying clay has been greatly altered, and hardened by
the igneous rock, and occasionally contorted in the most extraordinary
manner; yet the lamination has not been obliterated, but, on the contrary,
rendered much more conspicuous, by the indurating process.
[Illustration: Fig. 465. Contortions of strata in the largest of
the Cyclopian Islands.]
In the annexed woodcut (fig. 465.) I have represented a portion of the
altered rock, a few feet square, where the alternating thin laminae of sand
and clay have put on the appearance which we often observe in some of the
most contorted of the metamorphic schists.
A great fissure, running from east to west, nearly divides this larger
island into two parts, and lays open its internal structure. In the section
thus exhibited, a dike of lava is seen, first cutting through an older mass
of lava, and then penetrating the superincumbent tertiary strata. In one
place the lava ramifies and terminates in thin veins, from a few feet to a
few inches in thickness. (See fig. 466.)
The arenaceous laminae are much hardened at the point of contact, and the
clays are converted into siliceous schist. In this island the altered rocks
assume a honeycombed structure on their weathered surface, singularly
contrasted with the smooth and even outline which the same beds present in
their usual soft and yielding state.
The pores of the lava are sometimes coated, or entirely filled, with
carbonate of lime, and with a zeolite resembling analcime, which has been
called cyclopite. The latter mineral has also been found in small fissures
traversing the altered marl, showing that the same cause which introduced
the minerals into the cavities of the lava, whether we suppose sublimation
or aqueous infiltration, conveyed it also into the open rents of the
contiguous sedimentary strata.
[Illustration: Fig. 466. Post-Pliocene strata invaded by lava, Isle of
Cyclops (horizontal section).
_a._ Lava.
_b._ Laminated clay and sand.
_c._ The same altered.]
_Post-Pliocene formations near Naples._--I have traced in the "Principles
of Geology" the history of the changes which the volcanic region of
Campania is known to have undergone during the last 2000 years. The
aggregate effect of igneous operations during that period is far from
insignificant, comprising as it does the formation of the modern cone of
Vesuvius since the year 79, and the production of several minor cones in
Ischia, together with that of Monte Nuovo in the year 1538. Lava-currents
have also flowed upon the land and along the bottom of the sea--volcanic
sand, pumice, and scoriae have been showered down so abundantly, that whole
cities were buried--tracts of the sea have been filled up or converted into
shoals--and tufaceous sediment has been transported by rivers and
land-floods to the sea. There are also proofs, during the same recent
period, of a permanent alteration of the relative levels of the land and
sea in several places, and of the same tract having, near Puzzuoli, been
alternately upheaved and depressed to the amount of more than 20 feet. In
connection with these convulsions, there are found, on the shores of the
Bay of Baiae, recent tufaceous strata, filled with articles fabricated by
the hands of man, and mingled with marine shells.
It was also stated in this work (p. 113.), that when we examine this same
region, it is found to consist largely of tufaceous strata, of a date
anterior to human history or tradition, which are of such thickness as to
constitute hills from 500 to more than 2000 feet in height. These
post-pliocene strata, containing recent marine shells, alternate with
distinct currents and sheets of lava which were of contemporaneous origin;
and we find that in Vesuvius itself, the ancient cone called Somma is of
far greater volume than the modern cone, and is intersected by a far
greater number of dikes. In contrasting this ancient part of the mountain
with that of modern date, one principal point of difference is observed;
namely, the greater frequency in the older cone of fragments of altered
sedimentary rocks ejected during eruptions. We may easily conceive that the
first explosions would act with the greatest violence, rending and
shattering whatever solid masses obstructed the escape of lava and the
accompanying gases, so that great heaps of ejected pieces of rock would
naturally occur in the tufaceous breccias formed by the earliest eruptions.
But when a passage had once been opened, and an habitual vent established,
the materials thrown out would consist of liquid lava, which would take the
form of sand and scoriae, or of angular fragments of such solid lavas as may
have choked up the vent.
Among the fragments which abound in the tufaceous breccias of Somma, none
are more common than a saccharoid dolomite, supposed to have been derived
from an ordinary limestone altered by heat and volcanic vapours.
Carbonate of lime enters into the composition of so many of the simple
minerals found in Somma, that M. Mitscherlich, with much probability,
ascribes their great variety to the action of the volcanic heat on
subjacent masses of limestone.
_Dikes of Somma._--The dikes seen in the great escarpment which Somma
presents towards the modern cone of Vesuvius are very numerous. They are
for the most part vertical, and traverse at right angles the beds of
lava, scoriae, volcanic breccia, and sand, of which the ancient cone is
composed. They project in relief several inches, or sometimes feet, from
the face of the cliff, being extremely compact, and less destructible
than the intersected tuffs and porous lavas. In vertical extent they
vary from a few yards to 500 feet, and in breadth from 1 to 12 feet.
Many of them cut all the inclined beds in the escarpment of Somma from
top to bottom, others stop short before they ascend above half way, and
a few terminate at both ends, either in a point or abruptly. In mineral
composition they scarcely differ from the lavas of Somma, the rock
consisting of a base of leucite and augite, through which large crystals
of augite and some of leucite are scattered.[404-A] Examples are not
rare of one dike cutting through another, and in one instance a shift or
fault is seen at the point of intersection.
In some cases, however, the rents seem to have been filled laterally, when
the walls of the crater had been broken by star-shaped cracks, as seen in
the accompanying woodcut (fig. 467.). But the shape of these rents is an
exception to the general rule; for nothing is more remarkable than the
usual parallelism of the opposite sides of the dikes, which correspond
almost as regularly as the two opposite faces of a wall of masonry. This
character appears at first the more inexplicable, when we consider how
jagged and uneven are the rents caused by earthquakes in masses of
heterogeneous composition, like those composing the cone of Somma. In
explanation of this phenomenon, M. Necker refers us to Sir W. Hamilton's
account of an eruption of Vesuvius in the year 1779, who records the
following facts:--"The lavas, when they either boiled over the crater, or
broke out from the conical parts of the volcano, constantly formed channels
as regular as if they had been cut by art down the steep part of the
mountain; and, whilst in a state of perfect fusion, continued their course
in those channels, which were sometimes full to the brim, and at other
times more or less so, according to the quantity of matter in motion.
[Illustration: Fig. 467. Dikes or veins at the Punta del Nasone on
Somma. (Necker.[405-A])]
"These channels, upon examination after an eruption, I have found to be in
general from two to five or six feet wide, and seven or eight feet deep.
They were often hid from the sight by a quantity of scoriae that had formed
a crust over them; and the lava, having been conveyed in a covered way for
some yards, came out fresh again into an open channel. After an eruption, I
have walked in some of those subterraneous or covered galleries, which were
exceedingly curious, the sides, top, and bottom _being worn perfectly
smooth and even_ in most parts, by the violence of the currents of the
red-hot lavas which they had conveyed for many weeks successively."[405-B]
Now, the walls of a vertical fissure, through which lava has ascended in
its way to a volcanic vent, must have been exposed to the same erosion as
the sides of the channels before adverted to. The prolonged and uniform
friction of the heavy fluid, as it is forced and made to flow upwards,
cannot fail to wear and smooth down the surfaces on which it rubs, and the
intense heat must melt all such masses as project and obstruct the passage
of the incandescent fluid.
The texture of the Vesuvian dikes is different at the edges and in the
middle. Towards the centre, observes M. Necker, the rock is larger
grained, the component elements being in a far more crystalline state;
while at the edge the lava is sometimes vitreous, and always finer
grained. A thin parting band, approaching in its character to
pitchstone, occasionally intervenes, on the contact of the vertical dike
and intersected beds. M. Necker mentions one of these at the place
called Primo Monte, in the Atrio del Cavallo; and when on Somma, in
1828, I saw three or four others in different parts of the great
escarpment. These phenomena are in perfect harmony with the results of
the experiments of Sir James Hall and Mr. Gregory Watt, which have shown
that a glassy texture is the effect of sudden cooling, and that, on the
contrary, a crystalline grain is produced where fused minerals are
allowed to consolidate slowly and tranquilly under high pressure.
It is evident that the central portion of the lava in a fissure would,
during consolidation, part with its heat more slowly than the sides,
although the contrast of circumstances would not be so great as when we
compare the lava at the bottom and at the surface of a current flowing in
the open air. In this case the uppermost part, where it has been in contact
with the atmosphere, and where refrigeration has been most rapid, is always
found to consist of scoriform, vitreous, and porous lava; while at a
greater depth the mass assumes a more lithoidal structure, and then becomes
more and more stony as we descend, until at length we are able to recognize
with a magnifying glass the simple minerals of which the rock is composed.
On penetrating still deeper, we can detect the constituent parts by the
naked eye, and in the Vesuvian currents distinct crystals of augite and
leucite become apparent.
The same phenomenon, observes M. Necker, may readily be exhibited on a
smaller scale, if we detach a piece of liquid lava from a moving current.
The fragment cools instantly, and we find the surface covered with a
vitreous coat; while the interior, although extremely fine-grained, has a
more stony appearance.
It must, however, be observed, that although the lateral portions of the
dikes are finer grained than the central, yet the vitreous parting layer
before alluded to is rare in Vesuvius. This may, perhaps, be accounted for,
as the above-mentioned author suggests, by the great heat which the walls
of a fissure may acquire before the fluid mass begins to consolidate, in
which case the lava, even at the sides, would cool very slowly. Some
fissures, also, may be filled from above, as frequently happens in the
volcanos of the Sandwich Islands, according to the observations of Mr.
Dana; and in this case the refrigeration at the sides would be more rapid
than when the melted matter flowed upwards from the volcanic foci, in an
intensely heated state. Mr. Darwin informs me that in St. Helena almost
every dike has a vitreous selvage.
The rock composing the dikes both in the modern and ancient part of
Vesuvius is far more compact than that of ordinary lava, for the pressure
of a column of melted matter in a fissure greatly exceeds that in an
ordinary stream of lava; and pressure checks the expansion of those gases
which give rise to vesicles in lava.
There is a tendency in almost all the Vesuvian dikes to divide into
horizontal prisms, a phenomenon in accordance with the formation of
vertical columns in horizontal beds of lava; for in both cases the
divisions which give rise to the prismatic structure are at right angles to
the cooling surfaces.
_Newer Pliocene Period--Val di Noto._--I have already alluded (see p. 150.)
to the igneous rocks which are associated with a great marine formation of
limestone, sand, and marl, in the southern part of Sicily, as at Vizzini
and other places. In this formation, which was shown to belong to the Newer
Pliocene period, large beds of oysters and corals repose upon lava, and are
unaltered at the point of contact. In other places we find dikes of igneous
rock intersecting the fossiliferous beds, and converting the clays into
siliceous schist, the laminae being contorted and shivered into innumerable
fragments at the junction, as near the town of Vizzini.
The volcanic formations of the Val di Noto usually consist of the most
ordinary variety of basalt, with or without olivine. The rock is sometimes
compact, often very vesicular. The vesicles are occasionally empty, both in
dikes and currents, and are in some localities filled with calcareous spar,
arragonite, and zeolites. The structure is, in some places, spheroidal; in
others, though rarely, columnar. I found dikes of amygdaloid, wacke, and
prismatic basalt, intersecting the limestone at the bottom of the hollow
called Gozzo degli Martiri, below Melilli.
[2 Illustrations: Fig. 468. Fig. 469. Ground-plan of dikes near Palagonia.
_a._ Lava.
_b._ Peperino, consisting of volcanic sand, mixed with fragments of lava
and limestone.]
_Dikes._--Dikes of vesicular and amygdaloidal lava are also seen traversing
marine tuff or peperino, west of Palagonia, some of the pores of the lava
being empty, while others are filled with carbonate of lime. In such cases,
we may suppose the peperino to have resulted from showers of volcanic sand
and scoriae, together with fragments of limestone, thrown out by a submarine
explosion, similar to that which gave rise to Graham Island in 1831. When
the mass was, to a certain degree, consolidated, it may have been rent
open, so that the lava ascended through fissures, the walls of which were
perfectly even and parallel. After the melted matter that filled the rent
in fig. 468. had cooled down, it must have been fractured and shifted
horizontally by a lateral movement.
In the second figure (fig. 469.), the lava has more the appearance of a
vein which forced its way through the peperino. It is highly probable that
similar appearances would be seen, if we could examine the floor of the sea
in that part of the Mediterranean where the waves have recently washed away
the new volcanic island; for when a superincumbent mass of ejected
fragments has been removed by denudation, we may expect to see sections of
dikes traversing tuff, or, in other words, sections of the channels of
communication by which the subterranean lavas reached the surface.
FOOTNOTES:
[399-A] Caldcleugh, Phil. Trans. 1836. p. 27., and Official Documents
of Nicaragua.
[399-B] See Principles, _Index_, "Skaptar Jokul."
[401-A] This view of the Isle of Cyclops is from an original drawing by my
friend the late Captain Basil Hall, R. N.
[404-A] Consult the valuable memoir of M. L. A. Necker, Mem. de la Soc. de
Phys. et d'Hist. Nat. de Geneve, tom. ii. part i. Nov. 1822.
[405-A] From a drawing of M. Necker, in Mem. above cited.
[405-B] Phil. Trans., vol. lxx., 1780.