VOLCANIC ROCKS--_continued_.
Trap dikes--sometimes project--sometimes leave fissures vacant by
decomposition--Branches and veins of trap--Dikes more crystalline in
the centre--Foreign fragments of rock imbedded--Strata altered at or
near the contact--Obliteration of organic remains--Conversion of chalk
into marble--and of coal into coke--Inequality in the modifying
influence of dikes--Trap interposed between strata--Columnar and
globular structure--Relation of trappean rocks to the products of
active volcanos--Submarine lava and ejected matter corresponds
generally to ancient trap--Structure and physical features of Palma
and some other extinct volcanos.
Having in the last chapter spoken of the composition and mineral characters
of volcanic rocks, I shall next describe the manner and position in which
they occur in the earth's crust, and their external forms. Now the leading
varieties, such as basalt, greenstone, trachyte, porphyry, and the rest,
are found sometimes in dikes penetrating stratified and unstratified
formations, sometimes in shapeless masses protruding through or overlying
them, or in horizontal sheets intercalated between strata.
_Volcanic dikes._--Fissures have already been spoken of as occurring in
all kinds of rocks, some a few feet, others many yards in width, and
often filled up with earth or angular pieces of stone, or with sand and
pebbles. Instead of such materials, suppose a quantity of melted stone
to be driven or injected into an open rent, and there consolidated, we
have then a tabular mass resembling a wall, and called a trap dike. It
is not uncommon to find such dikes passing through strata of soft
materials, such as tuff or shale, which, being more perishable than the
trap, are often washed away by the sea, rivers, or rain, in which case
the dike stands prominently out in the face of precipices, or on the
level surface of a country. (See the annexed figure.[378-A])
[Illustration: Fig. 439. Dike in inland valley, near the Brazen
Head, Madeira.]
In the islands of Arran, Skye, and other parts of Scotland, where
sandstone, conglomerate, and other hard rocks are traversed by dikes of
trap, the converse of the above phenomenon is seen. The dike having
decomposed more rapidly than the containing rock, has once more left open
the original fissure, often for a distance of many yards inland from the
sea-coast, as represented in the annexed view (fig. 440.). In these
instances, the greenstone of the dike is usually more tough and hard than
the sandstone; but chemical action, and chiefly the oxidation of the iron,
has given rise to the more rapid decay.
[Illustration: Fig. 440. Fissures left vacant by decomposed trap.
Strathaird, Skye. (MacCulloch.)]
There is yet another case, by no means uncommon in Arran and other parts of
Scotland, where the strata in contact with the dike, and for a certain
distance from it, have been hardened, so as to resist the action of the
weather more than the dike itself, or the surrounding rocks. When this
happens, two parallel walls of indurated strata are seen protruding above
the general level of the country, and following the course of the dike.
[Illustration: Fig. 441. Trap veins in Airdnamurchan.]
As fissures sometimes send off branches, or divide into two or more
fissures of equal size, so also we find trap dikes bifurcating and
ramifying, and sometimes they are so tortuous as to be called veins, though
this is more common in granite than in trap. The accompanying sketch (fig.
441.) by Dr. MacCulloch represents part of a sea-cliff in Argyleshire,
where an overlying mass of trap, _b_, sends out some veins which terminate
downwards. Another trap vein, _a a_, cuts through both the limestone, _c_,
and the trap, _b_.
In fig. 442., a ground plan is given of a ramifying dike of greenstone,
which I observed cutting through sandstone on the beach near Kildonan
Castle, in Arran. The larger branch varies from 5 to 7 feet in width, which
will afford a scale of measurement for the whole.
[Illustration: Fig. 442. Ground plan of greenstone dike traversing
sandstone. Arran.]
In the Hebrides and other countries, the same masses of trap which
occupy the surface of the country far and wide, concealing the subjacent
stratified rocks, are seen also in the sea cliffs, prolonged downwards
in veins or dikes, which probably unite with other masses of igneous
rock at a greater depth. The largest of the dikes represented in the
annexed diagram, and which are seen in part of the coast of Skye, is no
less than 100 feet in width.
[Illustration: Fig. 443. Trap dividing and covering sandstone near
Suishnish in Skye. (MacCulloch.)]
Every variety of trap-rock is sometimes found in these dikes, as basalt,
greenstone, felspar-porphyry, and more rarely trachyte. The amygdaloidal
traps also occur, and even tuff and breccia, for the materials of these
last may be washed down into open fissures at the bottom of the sea, or
during eruptions on the land may be showered into them from the air.
Some dikes of trap may be followed for leagues uninterruptedly in nearly a
straight direction, as in the north of England, showing that the fissures
which they fill must have been of extraordinary length.
_Dikes more crystalline in the centre._--In many cases trap at the edges or
sides of a dike is less crystalline or more earthy than in the centre, in
consequence of the melted matter having cooled more rapidly by coming in
contact with the cold sides of the fissure; whereas, in the centre, the
matter of the dike being kept long in a fluid or soft state, the crystals
are slowly formed. In the ancient part of Vesuvius, called Somma, a thin
band of half-vitreous lava is found at the edge of some dikes. At the
junction of greenstone dikes with limestone, a _sahlband_, or selvage, of
serpentine is occasionally observed.
[Illustration: Fig. 444. Syenitic greenstone dike of Naesodden, Christiania.
_b._ imbedded fragment of crystalline schist surrounded by a band
of greenstone.]
On the left shore of the fiord of Christiania, in Norway, I examined, in
company with Professor Keilhau, a remarkable dike of syenitic greenstone,
which is traced through Silurian strata, until at length, in the promontory
of Naesodden, it enters mica-schist. Fig. 444. represents a ground plan,
where the dike appears 8 paces in width. In the middle it is highly
crystalline and granitiform, of a purplish colour, and containing a few
crystals of mica, and strongly contrasted with the whitish mica-schist,
between which and the syenitic rock there is usually on each side a
distinct black band, 18 inches wide, of dark greenstone. When first seen,
these bands have the appearance of two accompanying dikes; yet they are, in
fact, only the different form which the syenitic materials have assumed
where near to or in contact with the mica-schist. At one point, _a_, one
of the sahlbands terminates for a space; but near this there is a large
detached block, _b_, having a gneiss-like structure, consisting of
hornblende and felspar, which is included in the midst of the dike. Round
this a smaller encircling zone is seen, of dark basalt, or fine-grained
greenstone, nearly corresponding to the larger ones which border the dike,
but only 1 inch wide.
It seems, therefore, evident that the fragment, _b_, has acted on the
matter of the dike, probably by causing it to cool more rapidly, in the
same manner as the walls of the fissure have acted on a larger scale. The
facts, also, illustrate the facility with which a granitiform syenite may
pass into ordinary rocks of the volcanic family.
[Illustration: Fig. 445. Greenstone dike, with fragments of gneiss.
Sorgenfri, Christiania.]
The fact above alluded to, of a foreign fragment, such as _b_, fig.
444., included in the midst of the trap, as if torn off from some
subjacent rock or the walls of a fissure, is by no means uncommon. A
fine example is seen in another dike of greenstone, 10 feet wide, in the
northern suburbs of Christiania, in Norway, of which the annexed figure
is a ground plan. The dike passes through shale, known by its fossils to
belong to the Silurian series. In the black base of greenstone are
angular and roundish pieces of gneiss, some white, others of a light
flesh-colour, some without lamination, like granite, others with laminae,
which, by their various and often opposite directions, show that they
have been scattered at random through the matrix. These imbedded pieces
of gneiss measure from 1 to about 8 inches in diameter.
_Rocks altered by volcanic dikes._--After these remarks on the form and
composition of dikes themselves, I shall describe the alterations which
they sometimes produce in the rocks in contact with them. The changes are
usually such as the intense heat of melted matter and the entangled gases
might be expected to cause.
_Plas-Newydd._--A striking example, near Plas-Newydd, in Anglesea, has
been described by Professor Henslow.[381-A] The dike is 134 feet wide,
and consists of a rock which is a compound of felspar and augite
(dolerite of some authors). Strata of shale and argillaceous limestone,
through which it cuts perpendicularly, are altered to a distance of 30,
or even, in some places, to 35 feet from the edge of the dike. The
shale, as it approaches the trap, becomes gradually more compact, and is
most indurated where nearest the junction. Here it loses part of its
schistose structure, but the separation into parallel layers is still
discernible. In several places the shale is converted into hard
porcellanous jasper. In the most hardened part of the mass the fossil
shells, principally _Producti_, are nearly obliterated; yet even here
their impressions may frequently be traced. The argillaceous limestone
undergoes analogous mutations, losing its earthy texture as it
approaches the dike, and becoming granular and crystalline. But the most
extraordinary phenomenon is the appearance in the shale of numerous
crystals of analcime and garnet, which are distinctly confined to those
portions of the rock affected by the dike.[382-A] Some garnets contain
as much as 20 per cent. of lime, which they may have derived from the
decomposition of the fossil shells or Producti. The same mineral has
been observed, under very analogous circumstances, in High Teesdale,
by Professor Sedgwick, where it also occurs in shale and limestone,
altered by basalt.[382-B]
_Antrim._--In several parts of the county of Antrim, in the north of
Ireland, chalk with flints is traversed by basaltic dikes. The chalk is
there converted into granular marble near the basalt, the change sometimes
extending 8 or 10 feet from the wall of the dike, being greatest near the
point of contact, and thence gradually decreasing till it becomes
evanescent. "The extreme effect," says Dr. Berger, "presents a dark brown
crystalline limestone, the crystals running in flakes as large as those of
coarse primitive (_metamorphic_) limestone; the next state is saccharine,
then fine grained and arenaceous; a compact variety, having a porcellanous
aspect and a bluish-grey colour, succeeds: this, towards the outer edge,
becomes yellowish-white, and insensibly graduates into the unaltered chalk.
The flints in the altered chalk usually assume a grey yellowish
colour."[382-C] All traces of organic remains are effaced in that part of
the limestone which is most crystalline.
[Illustration: Fig. 446. Basaltic dikes in chalk in island of
Rathlin, Antrim. Ground plan, as seen on the beach. (Conybeare
and Buckland.[382-D])]
The annexed drawing (fig. 446.) represents three basaltic dikes
traversing the chalk, all within the distance of 90 feet. The chalk
contiguous to the two outer dikes is converted into a finely granular
marble, _m m_, as are the whole of the masses between the outer dikes
and the central one. The entire contrast in the composition and colour
of the intrusive and invaded rocks, in these cases, renders the
phenomena peculiarly clear and interesting.
Another of the dikes of the north-east of Ireland has converted a mass
of red sandstone into hornstone.[382-E] By another, the slate clay of
the coal measures has been indurated, and has assumed the character of
flinty slate[383-A]; and in another place the slate clay of the lias
has been changed into flinty slate, which still retains numerous
impressions of ammonites.[383-B]
It might have been anticipated that beds of coal would, from their
combustible nature, be effected in an extraordinary degree by the
contact of melted rock. Accordingly, one of the greenstone dikes of
Antrim, on passing through a bed of coal, reduces it to a cinder for the
space of 9 feet on each side.[383-C]
At Cockfield Fell, in the north of England, a similar change is observed.
Specimens taken at the distance of about 30 yards from the trap are not
distinguishable from ordinary pit coal; those nearer the dike are like
cinders, and have all the character of coke; while those close to it are
converted into a substance resembling soot.[383-D]
As examples might be multiplied without end, I shall merely select one
or two others, and then conclude. The rock of Stirling Castle is a
calcareous sandstone, fractured and forcibly displaced by a mass of
greenstone which has evidently invaded the strata in a melted state. The
sandstone has been indurated, and has assumed a texture approaching to
hornstone near the junction. In Arthur's Seat and Salisbury Craig, near
Edinburgh, a sandstone which comes in contact with greenstone is
converted into a jaspideous rock.[383-E]
The secondary sandstones in Skye are converted into solid quartz in
several places, where they come in contact with veins or masses of trap;
and a bed of quartz, says Dr. MacCulloch, found near a mass of trap,
among the coal strata of Fife, was in all probability a stratum of
ordinary sandstone, having been subsequently indurated and turned into
quartzite by the action of heat.[383-F]
But although strata in the neighbourhood of dikes are thus altered in a
variety of cases, shale being turned into flinty slate or jasper, limestone
into crystalline marble, sandstone into quartz, coal into coke, and the
fossil remains of all such strata wholly and in part obliterated, it is by
no means uncommon to meet with the same rocks, even in the same districts,
absolutely unchanged in the proximity of volcanic dikes.
This great inequality in the effects of the igneous rocks may often arise
from an original difference in their temperature, and in that of the
entangled gases, such as is ascertained to prevail in different lavas, or
in the same lava near its source and at a distance from it. The power also
of the invaded rocks to conduct heat may vary, according to their
composition, structure, and the fractures which they may have experienced,
and perhaps, also, according to the quantity of water (so capable of being
heated) which they contain. It must happen in some cases that the component
materials are mixed in such proportions as prepare them readily to enter
into chemical union, and form new minerals; while in other cases the mass
may be more homogeneous, or the proportions less adapted for such union.
We must also take into consideration, that one fissure may be simply filled
with lava, which may begin to cool from the first; whereas in other cases
the fissure may give passage to a current of melted matter, which may
ascend for days or months, feeding streams which are overflowing the
country above, or are ejected in the shape of scoriae from some crater. If
the walls of a rent, moreover, are heated by hot vapour before the lava
rises, as we know may happen on the flanks of a volcano, the additional
caloric supplied by the dike and its gases will act more powerfully.
[Illustration: Fig. 447. Trap interposed between displaced beds
of limestone and shale, at White Force, High Teesdale, Durham.
(Sedgwick.[384-A])]
_Intrusion of trap between strata._--In proof of the mechanical force
which the fluid trap has sometimes exerted on the rocks into which it
has intruded itself, I may refer to the Whin-Sill, where a mass of
basalt, from 60 to 80 feet in height, represented by _a_, fig. 447., is
in part wedged in between the rocks of limestone, _b_, and shale, _c_,
which have been separated from the great mass of limestone and shale,
_d_, with which they were united.
The shale in this place is indurated; and the limestone, which at a
distance from the trap is blue, and contains fossil corals, is here
converted into granular marble without fossils.
Masses of trap are not unfrequently met with intercalated between strata,
and maintaining their parallelism to the planes of stratification
throughout large areas. They must in some places have forced their way
laterally between the divisions of the strata, a direction in which there
would be the least resistance to an advancing fluid, if no vertical rents
communicated with the surface, and a powerful hydrostatic pressure was
caused by gases propelling the lava upwards.
_Columnar and globular structure._--One of the characteristic forms of
volcanic rocks, especially of basalt, is the columnar, where large masses
are divided into regular prisms, sometimes easily separable, but in other
cases adhering firmly together. The columns vary in the number of angles,
from three to twelve; but they have most commonly from five to seven sides.
They are often divided transversely, at nearly equal distances, like the
joints in a vertebral column, as in the Giant's Causeway, in Ireland. They
vary exceedingly in respect to length and diameter. Dr. MacCulloch
mentions some in Skye which are about 400 feet long; others, in Morven, not
exceeding an inch. In regard to diameter, those of Ailsa measure 9 feet,
and those of Morven an inch or less.[385-A] They are usually straight, but
sometimes curved; and examples of both these occur in the island of Staffa.
In a horizontal bed or sheet of trap the columns are vertical; in a
vertical dike they are horizontal. Among other examples of the
last-mentioned phenomenon is the mass of basalt, called the Chimney, in St.
Helena (see fig. 448.), a pile of hexagonal prisms, 64 feet high, evidently
the remainder of a narrow dike, the walls of rock which the dike originally
traversed having been removed down to the level of the sea. In fig. 449. a
small portion of this dike is represented on a less reduced scale.[385-B]
[Illustration: Fig. 448. Volcanic dike composed of horizontal prisms.
St. Helena.]
[Illustration: Fig. 449. Small portion of the dyke in Fig. 448.]
[Illustration: Fig. 450. Lava of La Coupe d'Ayzac, near Antraigue, in the
province of Ardeche.]
It being assumed that columnar trap has consolidated from a fluid state,
the prisms are said to be always at right angles to the _cooling surfaces_.
If these surfaces, therefore, instead of being either perpendicular, or
horizontal, are curved, the columns ought to be inclined at every angle to
the horizon; and there is a beautiful exemplification of this phenomenon in
one of the valleys of the Vivarais, a mountainous district in the South of
France, where, in the midst of a region of gneiss, a geologist encounters
unexpectedly several volcanic cones of loose sand and scoriae. From the
crater of one of these cones called La Coupe d'Ayzac, a stream of lava
descends and occupies the bottom of a narrow valley, except at those points
where the river Volant, or the torrents which join it, have cut away
portions of the solid lava. The accompanying sketch (fig. 450.) represents
the remnant of the lava at one of the points where a lateral torrent joins
the main valley of the Volant. It is clear that the lava once filled the
whole valley up to the dotted line _d a_; but the river has gradually swept
away all below that line, while the tributary torrent has laid open a
transverse section; by which we perceive, in the first place, that the lava
is composed, as usual in this country, of three parts: the uppermost, at
_a_, being scoriaceous; the second, _b_, presenting irregular prisms; and
the third, _c_, with regular columns, which are vertical on the banks of
the Volant, where they rest on a horizontal base of gneiss, but which are
inclined at an angle of 45 deg. at _g_, and then horizontal at _f_, their
position having been every where determined, according to the law before
mentioned, by the concave form of the original valley.
[Illustration: Fig 451. Columnar basalt in the Vicentin. (Fortis.)]
In the annexed figure (451.) a view is given of some of the inclined and
curved columns which present themselves on the sides of the valleys in the
hilly region north of Vicenza, in Italy, and at the foot of the higher
Alps.[386-A] Unlike those of the Vivarais, last mentioned, the basalt of
this country was evidently submarine, and the present valleys have since
been hollowed out by denudation.
The columnar structure is by no means peculiar to the trap rocks in which
hornblende or augite predominate; it is also observed in clinkstone,
trachyte, and other felspathic rocks of the igneous class, although in
these it is rarely exhibited in such regular polygonal forms.
[Illustration: Fig. 452. Basaltic pillars of the Kaesegrotte,
Bertrich-Baden, half way between Treves and Coblentz. Height of
grotto, from 7 to 8 feet.]
It has been already stated that basaltic columns are often divided by
cross joints. Sometimes each segment, instead of an angular, assumes a
spheroidal form, so that a pillar is made up of a pile of balls, usually
flattened, as in the Cheese-grotto at Bertrich-Baden, in the Eifel, near
the Moselle (fig. 452.). The basalt, there, is part of a small stream of
lava, from 30 to 40 feet thick, which has proceeded from one of several
volcanic craters, still extant, on the neighbouring heights. The position
of the lava bordering the river in this valley might be represented by a
section like that already given at fig. 450. p. 385., if we merely supposed
inclined strata of slate and the argillaceous sandstone called greywacke to
be substituted for gneiss.
In some masses of decomposing greenstone, basalt, and other trap rocks, the
globular structure is so conspicuous that the rock has the appearance of a
heap of large cannon balls.
[Illustration: Fig. 453. Globiform pitchstone. Chiaja di Luna, Isle
of Ponza. (Scrope.)]
A striking example of this structure occurs in a resinous trachyte or
pitchstone-porphyry in one of the Ponza islands, which rise from the
Mediterranean, off the coast of Terracina and Gaeta. The globes vary from a
few inches to three feet in diameter, and are of an ellipsoidal form (see
fig. 453.). The whole rock is in a state of decomposition, "and when the
balls," says Mr. Scrope, "have been exposed a short time to the weather,
they scale off at a touch into numerous concentric coats, like those of a
bulbous root, inclosing a compact nucleus. The laminae of this nucleus have
not been so much loosened by decomposition; but the application of a ruder
blow will produce a still further exfoliation."[387-A]
A fissile texture is occasionally assumed by clinkstone and other trap
rocks, so that they have been used for roofing houses. Sometimes the
prismatic and slaty structure is found in the same mass. The causes which
give rise to such arrangements are very obscure, but are supposed to be
connected with changes of temperature during the cooling of the mass, as
will be pointed out in the sequel. (See Chaps. XXXV. and XXXVI.)
_Relation of Trappean Rocks to the products of active Volcanos._
When we reflect on the changes above described in the strata near their
contact with trap dikes, and consider how great is the analogy in
composition and structure of the rocks called trappean and the lavas of
active volcanos, it seems difficult at first to understand how so much
doubt could have prevailed for half a century as to whether trap was of
igneous or aqueous origin. To a certain extent, however, there was a real
distinction between the trappean formations and those to which the term
volcanic was almost exclusively confined. The trappean rocks first studied
in the north of Germany, and in Norway, France, Scotland, and other
countries, were either such as had been formed entirely under deep water,
or had been injected into fissures and intruded between strata, and which
had never flowed out in the air, or over the bottom of a shallow sea. When
these products, therefore, of submarine or subterranean igneous action were
contrasted with loose cones of scoriae, tuff, and lava, or with narrow
streams of lava in great part scoriaceous and porous, such as were observed
to have proceeded from Vesuvius and Etna, the resemblance seemed remote and
equivocal. It was, in truth, like comparing the roots of a tree with its
leaves and branches, which, although they belong to the same plant, differ
in form, texture, colour, mode of growth, and position. The external cone,
with its loose ashes and porous lava, may be likened to the light foliage
and branches, and the rocks concealed far below, to the roots. But it is
not enough to say of the volcano,
"quantum vertice in auras
AEtherias, tantum radice in Tartara tendit,"
for its roots do literally reach downwards to Tartarus, or to the regions
of subterranean fire; and what is concealed far below, is probably always
more important in volume and extent than what is visible above ground.
[Illustration: Fig. 454. Strata intersected by a trap dike, and
covered with alluvium.]
We have already stated how frequently dense masses of strata have been
removed by denudation from wide areas (see Chap. VI.); and this fact
prepares us to expect a similar destruction of whatever may once have
formed the uppermost part of ancient submarine or subaerial volcanos, more
especially as those superficial parts are always of the lightest and most
perishable materials. The abrupt manner in which dikes of trap usually
terminate at the surface (see fig. 454.), and the water-worn pebbles of
trap in the alluvium which covers the dike, prove incontestably that
whatever was uppermost in these formations has been swept away. It is easy,
therefore, to conceive that what is gone in regions of trap may have
corresponded to what is now visible in active volcanos.
It will be seen in the following chapters, that in the earth's crust there
are volcanic tuffs of all ages, containing marine shells, which bear
witness to eruptions at many successive geological periods. These tuffs,
and the associated trappean rocks, must not be compared to lava and scoriae
which had cooled in the open air. Their counterparts must be sought in the
products of modern submarine volcanic eruptions. If it be objected that we
have no opportunity of studying these last, it may be answered, that
subterranean movements have caused, almost everywhere in regions of active
volcanos, great changes in the relative level of land and sea, in times
comparatively modern, so as to expose to view the effects of volcanic
operations at the bottom of the sea.
Thus, for example, the recent examination of the igneous rocks of Sicily,
especially those of the Val di Noto, has proved that all the more ordinary
varieties of European trap have been there produced under the waters of the
sea, at a modern period; that is to say, since the Mediterranean has been
inhabited by a great proportion of the existing species of testacea.
These igneous rocks of the Val di Noto, and the more ancient trappean rocks
of Scotland and other countries, differ from subaerial volcanic formations
in being more compact and heavy, and in forming sometimes extensive sheets
of matter intercalated between marine strata, and sometimes stratified
conglomerates, of which the rounded pebbles are all trap. They differ also
in the absence of regular cones and craters, and in the want of conformity
of the lava to the lowest levels of existing valleys.
It is highly probable, however, that insular cones did exist in some
parts of the Val di Noto: and that they were removed by the waves, in
the same manner as the cone of Graham island, in the Mediterranean, was
swept away in 1831, and that of Nyoee, off Iceland, in 1783.[389-A] All
that would remain in such cases, after the bed of the sea has been
upheaved and laid dry, would be dikes and shapeless masses of igneous
rock, cutting through sheets of lava which may have spread over the
level bottom of the sea, and strata of tuff, formed of materials first
scattered far and wide by the winds and waves, and then deposited. Trap
conglomerates also, to which the action of the waves must give rise
during the denudation of such volcanic islands, will emerge from the
deep whenever the bottom of the sea becomes land.
The proportion of volcanic matter which is originally submarine must
always be very great, as those volcanic vents which are not entirely
beneath the sea, are almost all of them in islands, or, if on
continents, near the shore. This may explain why extended sheets of trap
so often occur, instead of narrow threads, like lava streams. For, a
multitude of causes tend, near the land, to reduce the bottom of the sea
to a nearly uniform level,--the sediment of rivers,--materials
transported by the waves and currents of the sea from wasting
cliffs,--showers of sand and scoriae ejected by volcanos, and scattered
by the wind and waves. When, therefore, lava is poured out on such a
surface, it will spread far and wide in every direction in a liquid
sheet, which may afterwards, when raised up, form the tabular capping
of the land.
As to the absence of porosity in the trappean formations, the appearances
are in a great degree deceptive, for all amygdaloids are, as already
explained, porous rocks, into the cells of which mineral matter, such as
silex, carbonate of lime, and other ingredients, have been subsequently
introduced (see p. 373.); sometimes, perhaps, by secretion during the
cooling and consolidation of lavas.
In the Little Cumbray, one of the Western Islands, near Arran, the
amygdaloid sometimes contains elongated cavities filled with brown spar;
and when the nodules have been washed out, the interior of the cavities is
glazed with the vitreous varnish so characteristic of the pores of slaggy
lavas. Even in some parts of this rock which are excluded from air and
water, the cells are empty, and seem to have always remained in this state,
and are therefore undistinguishable from some modern lavas.[390-A]
Dr. MacCulloch, after examining with great attention these and the other
igneous rocks of Scotland, observes, "that it is a mere dispute about
terms, to refuse to the ancient eruptions of trap the name of submarine
volcanos; for they are such in every essential point, although they no
longer eject fire and smoke."[390-B] The same author also considers it not
improbable that some of the volcanic rocks of the same country may have
been poured out in the open air.[390-C]
Although the principal component minerals of subaerial lavas are the same
as those of intrusive trap, and both the columnar and globular structure
are common to both, there are, nevertheless, some volcanic rocks which
never occur as lava, such as greenstone, clinkstone, the more crystalline
porphyries, and those traps in which quartz and mica appear as constituent
parts. In short, the intrusive trap rocks, forming the intermediate step
between lava and the plutonic rocks, depart in their characters from lava
in proportion as they approximate to granite.
These views respecting the relations of the volcanic and trap rocks will be
better understood when the reader has studied, in the 33d chapter, what is
said of the plutonic formations.
FORM, STRUCTURE, AND ORIGIN OF VOLCANIC MOUNTAINS.
The origin of volcanic cones with crater-shaped summits has been alluded to
in the last chapter (p. 368.), and more fully explained in the "Principles
of Geology" (chaps. xxiv. to xxvii.), where Vesuvius, Etna, Santorin, and
Barren Island were described. The more ancient portions of those mountains
or islands, formed long before the times of history, exhibit the same
external features and internal structure which belong to most of the
extinct volcanos of still higher antiquity.
The island of Palma, for example, one of the Canaries, offers an excellent
illustration of what, in common with many others, I regard as the ruins of
a large dome-shaped mass formed by a series of eruptions proceeding from a
crater at the summit, this crater having been since replaced by a larger
cavity, the origin of which has afforded geologists an ample field for
discussion and speculation.
[Illustration: Fig. 455. View of the Isle of Palma, and of the entrance
into the central cavity or Caldera. From Von Buch's "Canary Islands."]
[Illustration: Fig. 456. Map of the Caldera of Palma and the great
ravine, called "Barranco de las Angustias." From Survey of Capt.
Vidal, R.N., 1837.]
Von Buch, in his excellent account of the Canaries, has given us a
graphic picture of this island, which consists chiefly of a single
mountain (fig. 455.). This mountain has the general form of a great
truncated cone, with a huge and deep cavity in the middle, about six
miles in diameter, called by the inhabitants "the Caldera," or cauldron.
The range of precipices surrounding the Caldera are no less than 4000
feet in their average height; at one point, where they are highest, they
are 7730 feet above the level of the sea. The external flanks of the
cone incline gently in every direction towards the base of the island,
and are in part cultivated; but the walls and bottom of the Caldera
present on all sides rugged and uncultivated rocks, almost completely
devoid of vegetation. So steep are these walls, that there is no part by
which they can be descended, and the only entrance is by a great ravine,
or Barranco, as it is called (see _b b'_, map, fig. 456.), which extends
from the sea to the interior of the great cavity, and by its jagged,
broken, and precipitous sides, exhibits to the geologist a transverse
section of the rocks of which the whole mountain is composed. By this
means, we learn that the cone is made up of a great number of sloping
beds, which dip outwards in every direction from the centre of the void
space, or from the hollow axis of the cone. The beds consist chiefly of
sheets of basalt, alternating with conglomerates; the materials of the
latter being in part rounded, as if rolled by water in motion. The
inclination of all the beds corresponds to that of the external slope of
the island, being greatest towards the Caldera, and least steep when
they are nearest the sea. There are a great number of tortuous veins,
and many dikes of lava or trap, chiefly basaltic, and most of them
vertical, which cut through the sloping beds laid open to view in the
great gorge or Barranco. These dikes and veins are more and more
abundant as we approach the Caldera, being therefore most numerous where
the slope of the beds is greatest.
Assuming the cone to be a pile of volcanic materials ejected by a long
succession of eruptions (a point on which all geologists are agreed), we
have to account for the Caldera and the great Barranco. I conceive that the
cone itself may be explained, in accordance with what we know of the
ordinary growth of volcanos[392-A], by supposing most of the eruptions to
have taken place from one or more central vents, at or near the summit of
the cone, before it was truncated. From this culminating point, sheets of
lava flowed down one after the other, and showers of ashes or ejected
stones. The volcano may, in the earlier stages of its growth, have been in
great part submerged, like Stromboli, in the sea; and, therefore, some of
the fragments of rock cast out of its crater may not only have been rolled
by torrents sweeping down the mountain's side, but have also been rounded
by the waves of the sea, as we see happen on the beach near Catania, on
which the modern lavas of Etna are broken up. The increased number of
dykes, as we approach the axis of the cone, agrees well with the hypothesis
of the eruptions having been most frequent towards the centre.
There are three known causes or modes of operation, which may have
conduced towards the vast size of the Caldera. First, the summit of a
conical mountain may have fallen in, as happened in the case of
Capacurcu, one of the Andes, according to tradition, in the year 1462,
and of many other volcanic mountains.[393-A] Sections seem wanting, to
supply us with all the data required for judging fairly of the
tenability of this hypothesis. It appears, however, from Captain Vidal's
survey (see fig. 456.), that a hill of considerable height rises up from
the bottom of the Caldera, the structure of which, if it be any where
laid open, might doubtless throw much light on this subject. Secondly,
an original crater may have been enlarged by a vast gaseous explosion,
never followed by any subsequent eruption. A serious objection to this
theory arises from our not finding that the exterior of the cone
supports a mass of ruins, such as ought to cover it, had so enormous a
volume of matter, partly made up of the solid contents of the dikes,
been blown out into the air. In that case, an extensive bed of angular
fragments of stone, and of fine dust, might be looked for, enveloping
the entire exterior of the mountain up to the very rim of the Caldera,
and ought nowhere to be intersected by a dike. The absence of such a
formation has induced Von Buch to suppose that the missing portion
of the cone was engulphed. It should, however, be remembered, that
in existing volcanos, large craters, two or three miles in diameter,
are sometimes formed by explosions, or by the discharge of great
volumes of steam.
There is yet another cause to which the extraordinary dimensions of the
Caldera may, in part at least, be owing; namely, aqueous denudation. Von
Buch has observed, that the existence of a single deep ravine, like the
Great Barranco, is a phenomenon common to many extinct volcanos, as well as
to some active ones. Now, it will be seen by Captain Vidal's map (fig. 456.
p. 391.), that the sea-cliff at Point Juan Graje, 780 feet high, now
constituting the coast at the entrance of the great ravine, is continuous
with an inland cliff which bounds the same ravine on its north-western
side. No one will dispute that the precipice, at the base of which the
waves are now beating, owes its origin to the undermining power of the sea.
It is natural, therefore, to attribute the extension of the same cliff to
the former action of the waves, exerted at a time when the relative level
of the island and the ocean were different from what they are now. But if
the waves and tides had power to remove the rocks once filling a great
gorge which is 7 miles long, and, in its upper part, 2000 feet deep, can we
doubt that the same power may have cleared out much of the solid mass now
missing in the Great Caldera?
The theory advanced to account for the configuration of Palma, commonly
called the "elevation crater theory," is this. All the alternating masses
of basalt and conglomerate, intersected in the Barranco, or abruptly cut
off in the escarpment or walls of the Caldera, were at first disposed in
horizontal masses on the level floor of the ocean, and traversed, when in
that position, by all the basaltic dikes which now cut through them. At
length they were suddenly uplifted by the explosive force of elastic
vapours, which raised the mass bodily, so as to tilt the beds on all sides
away from the centre of elevation, causing at the same time an opening at
the culminating point. Besides many other objections which may be urged
against this hypothesis, it leaves unexplained the unbroken continuity of
the rim of the Caldera, which is uninterrupted in all places save
one[394-A], namely, that where the great gorge or Barranco occurs.
As a more natural way of explaining the phenomenon, the following series of
events may be imagined. The principal vent, from which a large part of the
materials of the cone were poured or thrown out, was left empty after the
last escape of vapour, when the volcano became extinct. We learn from Mr.
Dana's valuable work on the geology of the United States' Exploring
Expedition, published in 1849, that two of the principal volcanos of the
Sandwich Islands, Mounts Loa and Kea in Owyhee, are huge flattened volcanic
cones, 15,000 feet high (see fig. 457.), each equalling two and a half
Etnas in their dimensions.
[Illustration: Fig. 457. Mount Loa, in the Sandwich Islands. (Dana)
_a._ Crater at the summit.
_b._ The lateral crater of Kilauea.
The dotted lines indicate a supposed column of solid rock caused by the
lava consolidating after eruptions.]
From the summits of these lofty though featureless hills, and from vents
not far below their summits, successive streams of lava, often 2 miles or
more in width, and sometimes 26 miles long, have flowed. They have been
poured out one after the other, some of them in recent times, in every
direction from the apex of the cone, down slopes varying on an average from
4 degrees to 8 degrees; but at some places considerably steeper.[394-B]
Sometimes deep rents open on the sides of these cones, which are filled by
streams of lava passing over them, the liquid matter in such cases probably
uniting in the fissure with other lava melted in subterranean reservoirs
below, and thus explaining the origin of one great class of lateral dikes,
on Etna, Palma, and other cones.
If the flattened domes, such as those here alluded to in the Sandwich
Islands, instead of being inland, and above water, were situated in
mid-ocean, like the Island of St. Paul, and for the most part submerged
(see figs. 458, 459, 460.), and if a gradual upheaval of such a dome should
then take place, the denuding power of the sea could scarcely fail to play
an important part in modifying the form of the volcanic mountain as it
rose. The crater will almost invariably have one side much lower than all
the others, namely, that side towards which the prevailing winds never
blow, and to which, therefore, showers of dust and scoriae are rarely
carried during eruptions. There will also be one point on this windward or
lowest side more depressed than all the rest, by which the sea may enter as
often as the tide rises, or as often as the wind blows from that quarter.
For the same reason that the sea continues to keep open a single entrance
into the lagoon of an atoll or annular coral reef, it will not allow this
passage into the crater to be stopped up, but scour it out, at low tide, or
as often as the wind changes. The channel, therefore, will always be
deepened in proportion as the island rises above the level of the sea, at
the rate perhaps of a few feet or yards in a century.
[Illustration: Fig. 458. Map of the Island of St. Paul, in the Indian
Ocean, lat. 38 deg. 44' S., long. 77 deg. 37' E., surveyed by Capt. Blackwood,
R.N., 1842.]
[Illustration: Fig. 459. View of the Crater of the Island of St. Paul.]
The island of St. Paul may perhaps be motionless; but if, like many
other parts of the earth's crust, it should begin to undergo a gradual
upheaval, or if, as has happened to the shores of the Bay of Baiae, its
level should oscillate, with a tendency upon the whole to increased
elevation, the same power which has cut away part of the cone, and
caused the cliffs now seen on the north-east side of the island, would
have power to undermine the walls of the crater, and enlarge its
diameter, keeping open the channel, by which it enters into it. This
ravine might be excavated to the depth of 180 feet (the present depth of
the crater), and its length might be extended to many miles according to
the size of the submerged part of the cone. The crater is only a mile in
diameter, and the surrounding cliffs, where loftiest, only 800 feet
high, so that the size of this cone and crater is insignificant when
compared to those in the Sandwich Islands, and I have merely selected
it because it affords an example of a class of insular volcanos, into
the craters of which the sea now enters by a single passage. The crater
of Vesuvius in 1822 was 2000 feet deep; and if it were a half submerged
cone, like St. Paul, the excavating power of the ocean might in
conjunction with gaseous explosions and co-operating with a gradual
upheaving force, give rise to a caldera on as grand a scale as
that exhibited by Palma.
[Illustration: Fig. 460. Side view of the Island of St. Paul (N.E. side).
Nine-pin rocks two miles distant. (Captain Blackwood.)]
If, after the geographical changes above supposed, the volcanic fires
long dormant should recover their energy, they might, as in the case of
Teneriffe, Vesuvius, Santorin, and Barren Island, discharge from the old
central vent, long sealed up at the bottom of the caldera, new floods of
lava and clouds of elastic vapours. Should this happen, a new cone will
be built up in the middle of the cavity or circular bay, formed, partly
by explosion, partly perhaps by engulphment, and partly by aqueous
denudation. In the island of Palma this last phase of volcanic activity
has never occurred; but the subterranean heat is still in full operation
beneath the Canary Islands, so that we know not what future changes it
may be destined to undergo.
FOOTNOTES:
[378-A] I have been favoured with this drawing by Captain B. Hall.
[381-A] Cambridge Transactions, vol. i. p. 402.
[382-A] Cambridge Trans., vol. i. p. 410.
[382-B] Ibid. vol. ii. p. 175.
[382-C] Dr. Berger, Geol. Trans., 1st series, vol. iii. p. 172.
[382-D] Geol. Trans., 1st series, vol. iii. p. 210. and plate 10.
[382-E] Ibid. p. 201.
[383-A] Geol. Trans., 1st series, vol. iii. p. 205.
[383-B] Ibid. p. 213.; and Playfair, Illust. of Hutt. Theory, p. 253.
[383-C] Geol. Trans., 1st series, vol. iii. p. 206.
[383-D] Sedgwick, Camb. Trans. vol. ii. p. 37.
[383-E] Illust. of Hutt. Theory, Sec. 253. and 261. Dr. MacCulloch, Geol.
Trans., 1st series, vol. ii. p. 305.
[383-F] Syst. of Geol. vol. i. p. 206.
[384-A] Camb. Trans. vol. ii. p. 180.
[385-A] MacCul. Syst. of Geol. vol. ii. p. 137.
[385-B] Seale's Geognosy of St. Helena, plate 9.
[386-A] Fortis. Mem. sur l'Hist. Nat. de l'Italie, tom. i. p. 233. plate 7.
[387-A] Scrope, Geol. Trans. vol. ii. p. 205. 2d series.
[389-A] See Princ. of Geol., _Index_, "Graham Island," "Nyoee,"
"Conglomerates, volcanic," &c.
[390-A] MacCulloch, West. Isl., vol. ii. p. 487.
[390-B] Syst. of Geol., vol. ii. p. 114.
[390-C] Ibid.
[392-A] See Principles, chaps. xxiv-xxvii.
[393-A] See Principles, chaps. xxvi. and xxx.; 8th ed. p. 397-475.
[394-A] See Principles of Geol. ch. xxiv. (8th ed. p. 355.).
[394-B] See Lyell on Craters of Denudation, Quart. Geol. Journ.
vol. vi. p. 232.