The Student's Elements of Geology
by
Sir Charles Lyell
Part 10 out of 14
The Tremadoc slates of Sedgwick are more than 1000 feet in thickness, and
consist of dark earthy slates occurring near the little town of Tremadoc,
situated on the north side of Cardigan Bay, in Carnarvonshire. These slates were
first examined by Sedgwick in 1831, and were re-examined by him and described in
1846 (Quarterly Geological Journal volume 3 page 156.), after some fossils had
been found in the underlying Lingula flags by Mr. Davis. The inferiority in
position of these Lingula flags to the Tremadoc beds was at the same time
established. The overlying Tremadoc beds were traced by their pisolitic ore from
Tremadoc to Dolgelly. No fossils proper to the Tremadoc slates were then
observed, but subsequently, thirty-six species of all classes have been found in
them, thanks to the researches of Messrs. Salter, Homfray, and Ash. We have
already seen that in the Arenig or Stiper-Stones group, where the species are
distinct, the genera agree with Silurian types; but in these Tremadoc slates,
where the species are also peculiar, there is about an equal admixture of
Silurian types with those which Barrande has termed "primordial." Here,
therefore, it may truly be said that we are entering upon a new domain of life
in our retrospective survey of the past. The trilobites of new species, but of
Lower Silurian genera, belong to Ogygia, Asaphus, and Cheirurus; whereas those
belonging to primordial types, or Barrande's first fauna as well as to the
Lingula flags of Wales, comprise Dikelocephalus, Conocoryphe (for genera see
Figures 577 and 581 (This genus has been substituted for Barrande's
Conocephalus, as the latter term had been preoccupied by the entomologists.)),
Olenus, and Angelina. In the Tremadoc slates are found Bellerophon, Orthoceras,
and Cyrtoceras, all specifically distinct from Lower Silurian fossils of the
same genera: the Pteropods Theca (Figure 568) and Conularia range throughout
these slates; there are no Graptolites. The Lingula (Lingulella) Davisii ranges
from the top to the bottom of the formation, and links it with the zone next to
be described. The Tremadoc slates are very local, and seem to be confined to a
small part of North Wales; and Professor Ramsay supposes them to lie
unconformably on the Lingula flags, and that a long interval of time elapsed
between these formations. Cephalopoda have not yet been found lower than this
group, but it will be observed that they occur here associated with genera of
Trilobites considered by Barrande as characteristically Primordial, some of
which belong to all the divisions of the British Cambrian about to be mentioned.
This renders the absence of cephalopoda of less importance as bearing on the
theory of development.
LINGULA FLAGS.
(FIGURES 569 to 571. "Lingula flags" of Dolgelly, and Ffestiniog; N. Wales.
(FIGURE 569. Hymenocaris vermicauda, Salter. A phyllopod crustacean. One-half
natural size.)
(FIGURE 570. Lingulella Davisii, M'Coy.
a. One-half natural size.
b. Distorted by cleavage.)
(FIGURE 571. Olenus micrurus, Salter. One-half natural size.))
Next below the Tremadoc slates in North Wales lie micaceous flagstones and
slates, in which, in 1846, Mr. E. Davis discovered the Lingula (Lingulella),
Figure 570, named after him, and from which was derived the name of Lingula
flags. These beds, which are palaeontologically the equivalents of Barrande's
primordial zone, are represented by more than 5000 feet of strata, and have been
studied chiefly in the neighbourhood of Dolgelly, Ffestiniog, and Portmadoc in
North Wales, and at St. David's in South Wales. They have yielded about forty
species of fossils, of which six only are common to the overlying Tremadoc
rocks, but the two formations are closely allied by having several
characteristic "primordial" genera in common. Dikelocephalus, Olenus (Figure
571), and Conocoryphe are prominent forms, as is also Hymenocaris (Figure 569),
a genus of phyllopod crustacean entirely confined to the Lingula Flags.
According to Mr. Belt, who has devoted much attention to these beds, there are
already palaeontological data for subdividing the Lingula Flags into three
sections. (Geological Magazine volume 4.)
In Merionethshire, according to Professor Ramsay, the Lingula Flags attain their
greatest development; in Carnarvonshire they thin out so as to have lost two-
thirds of their thickness in eleven miles, while in Anglesea and on the Menai
Straits both they and the Tremadoc beds are entirely absent, and the Lower
Silurian rests directly on Lower Cambrian strata.
LOWER CAMBRIAN.
MENEVIAN BEDS.
(FIGURE 572. Paradoxides Davidis, Salter. One-tenth natural size. Menevian beds.
St. David's and Dolgelly.)
Immediately beneath the Lingula Flags there occurs a series of dark grey and
black flags and slates alternating at the upper part with some beds of
sandstone, the whole reaching a thickness of from 500 to 600 feet. These beds
were formerly classed, on purely lithological grounds, as the base of the
Lingula Flags, but Messrs. Hicks and Salter, to whose exertions we owe almost
all our knowledge of the fossils, have pointed out that the most characteristic
genera found in them are quite unknown in the Lingula Flags, while they possess
many of the strictly Lower Cambrian genera, such as Microdiscus and Paradoxides.
(British Association Report 1865, 1866, 1868 and Quarterly Geological Journal
volumes 21, 25.) They therefore proposed to place them, and it seems to me with
good reason, at the top of the Lower Cambrian under the term "Menevian," Menevia
being the classical name of St. David's. The beds are well exhibited in the
neighbourhood of St. David's in South Wales, and near Dolgelly and Maentwrog in
North Wales. They are the equivalents of the lowest part of Barrande's
Primordial Zone (Etage C). More than forty species have been found in them, and
the group is altogether very rich in fossils for so early a period. The
trilobites are of large size; Paradoxides Davidis (see Figure 572), the largest
trilobite known in England, 22 inches or nearly two feet long, is peculiar to
the Menevian Beds. By referring to the Bohemian trilobite of the same genus
(Figure 576), the reader will at once see how these fossils (though of such
different dimensions) resemble each other in Bohemia and Wales, and other
closely allied species from the two regions might be added, besides some which
are common to both countries. The Swedish fauna, presently to be mentioned, will
be found to be still more nearly connected with the Welsh Menevian. In all these
countries there is an equally marked difference between the Cambrian fossils and
those of the Upper and Lower Silurian rocks. The trilobite with the largest
number of rings, Erinnys venulosa, occurs here in conjunction with Agnostus and
Microdiscus, the genera with the smallest number. Blind trilobites are also
found as well as those which have the largest eyes, such as Microdiscus on the
one hand, and Anoplenus on the other.
LONGMYND GROUP.
Older than the Menevian Beds are a thick series of olive green, purple, red and
grey grits and conglomerates found in North and South Wales, Shropshire, and
parts of Ireland and Scotland. They have been called by Professor Sedgwick the
Longmynd or Bangor Group, comprising, first, the Harlech and Barmouth
sandstones; and secondly, the Llanberis slates.
HARLECH GRITS.
(FIGURE 573. Histioderma Hibernica, Kinahan. Oldhamia beds. Bray Head, Ireland.
1. Showing opening of burrow, and tube with wrinklings or crossing ridges,
probably produced by a tentacled sea worm or annelid.
2. Lower and curved extremity of tube with five transverse lines.)
The sandstones of this period attain in the Longmynd hills a thickness of no
less than 6000 feet without any interposition of volcanic matter; in some places
in Merionethshire they are still thicker. Until recently these rocks possessed
but a very scanty fauna.
With the exception of five species of annelids (see Figure 460) brought to light
by Mr. Salter in Shropshire, and Dr. Kinahan in Wicklow, and an obscure
crustacean form, Palaeopyge Ramsayi, they were supposed to be barren of organic
remains. Now, however, through the labours of Mr. Hicks, they have yielded at
St. David's a rich fauna of trilobites, brachiopods, phyllopods, and pteropods,
showing, together with other fossils, a by no means low state of organisation at
this early period. (British Association Report 1868.) Already the fauna amounts
to 20 species referred to 17 genera.
A new genus of trilobite called Plutonia Sedgwickii, not yet figured and
described, has been met with in the Harlech grits. It is comparable in size to
the large Paradoxides Davidis before mentioned, has well-developed eyes, and is
covered all over with tubercles. In the same strata occur other genera of
trilobites, namely, Conocoryphe, Paradoxides, Microdiscus, and the Pteropod
Theca (Figure 568), all represented by species peculiar to the Harlech grits.
The sands of this formation are often rippled, and were evidently left dry at
low tides, so that the surface was dried by the sun and made to shrink and
present sun-cracks. There are also distinct impressions of rain-drops on many
surfaces, like those in Figures 444 and 445.
LANBERIS SLATES.
(FIGURE 574. Oldhamia radiata, Forbes. Wicklow, Ireland.)
(FIGURE 575. Oldhamia antiqua, Forbes. Wicklow, Ireland.)
The slates of Llanberis and Penrhyn in Carnarvonshire, with their associated
sandy strata, attain a great thickness, sometimes about 3000 feet. They are
perhaps not more ancient than the Harlech and Barmouth beds last mentioned, for
they may represent the deposits of fine mud thrown down in the same sea, on the
borders of which the sands above-mentioned were accumulating. In some of these
slaty rocks in Ireland, immediately opposite Anglesea and Carnarvon, two species
of fossils have been found, to which the late Professor E. Forbes gave the name
of Oldhamia. The nature of these organisms is still a matter of discussion among
naturalists.
CAMBRIAN ROCKS OF BOHEMIA (PRIMORDIAL ZONE OF BARRANDE).
In the year 1846, as before stated, M. Joachim Barrande, after ten years'
exploration of Bohemia, and after collecting more than a thousand species of
fossils, had ascertained the existence in that country of three distinct faunas
below the Devonian. To his first fauna, which was older than any then known in
this country, he gave the name of Etage C; his two first stages A and B
consisting of crystalline and metamorphic rocks and unfossiliferous schists.
This Etage C or primordial zone proved afterwards to be the equivalent of those
subdivisions of the Cambrian groups which have been above described under the
names of Menevian and Lingula Flags. The second fauna tallies with Murchison's
Lower Silurian, as originally defined by him when no fossils had been discovered
below the Stiper-Stones. The third fauna agrees with the Upper Silurian of the
same author. Barrande, without government assistance, had undertaken single-
handed the geological survey of Bohemia, the fossils previously obtained from
that country having scarcely exceeded 20 in number, whereas he had already
acquired, in 1850, no less than 1100 species, namely, 250 crustaceans (chiefly
Trilobites), 250 Cephalopods, 160 gasteropods and pteropods, 130 acephalous
mollusks, 210 brachiopods, and 110 corals and other fossils. These numbers have
since been almost doubled by subsequent investigations in the same country.
(Figures 576 to 580. Fossils of the lowest Fossiliferous Beds in Bohemia, or
"Primordial Zone" of Barrande.
(FIGURE 576. Paradoxides Bohemicus, Barr. About one-half natural size.)
(FIGURE 577. Conocoryphe striata. Syn. Conocephalus striatus, Emmrich. One-half
natural size. Ginetz and Skrey.)
(FIGURE 578. Agnostus integer, Beyrich. Natural size and magnified.)
(FIGURE 579. Agnostus Rex, Barr. Natural size, Skrey.)
(FIGURE 580. Sao hirsuta, Barrande, in its various stages of growth. The small
lines beneath indicate the true size. In the youngest state,
a, no segments are visible; as the metamorphosis progresses,
b, c, the body segments begin to be developed: in the stage
d the eyes are introduced, but the facial sutures are not completed; at
e the full-grown animal, half its true size, is shown.))
In the primordial zone C, he discovered trilobites of the genera Paradoxides,
Conocoryphe, Ellipsocephalus, Sao, Arionellus, Hydrocephalus, and Agnostus. M.
Barrande pointed out that these primordial trilobites have a peculiar facies of
their own dependent on the multiplication of their thoracic segments and the
diminution of their caudal shield or pygidium.
One of the "primordial" or Upper Cambrian Trilobites of the genus Sao, a form
not found as yet elsewhere in the world, afforded M. Barrande a fine
illustration of the metamorphosis of these creatures, for he traced them through
no less than twenty stages of their development. A few of these changes have
been selected for representation in Figure 580, that the reader may learn the
gradual manner in which different segments of the body and the eyes make their
appearance.
In Bohemia the primordial fauna of Barrande derived its importance exclusively
from its numerous and peculiar trilobites. Besides these, however, the same
ancient schists have yielded two genera of brachiopods, Orthis and Orbicula, a
Pteropod of the genus Theca, and four echinoderms of the cystidean family.
CAMBRIAN OF SWEDEN AND NORWAY.
The Cambrian beds of Wales are represented in Sweden by strata the fossils of
which have been described by a most able naturalist, M. Angelin, in his
"Palaeontologica Suecica" (1852-4). The "alum-schists," as they are called in
Sweden, are horizontal argillaceous rocks which underlie conformably certain
Lower Silurian strata in the mountain called Kinnekulle, south of the great
Wener Lake in Sweden. These schists contain trilobites belonging to the genera
Paradoxides, Olenus, Agnostus, and others, some of which present rudimentary
forms, like the genus last mentioned, without eyes, and with the body segments
scarcely developed, and others, again, have the number of segments excessively
multiplied, as in Paradoxides. Such peculiarities agree with the characters of
the crustaceans met with in the Cambrian strata of Wales; and Dr. Torell has
recently found in Sweden the Paradoxides Hicksii, a well-known Lower Cambrian
fossil.
At the base of the Cambrian strata in Sweden, which in the neighbourhood of Lake
Wener are perfectly horizontal, lie ripple-marked quartzose sandstones with
worm-tracks and annelid borings, like some of those found in the Harlech grits
of the Longmynd. Among these are some which have been referred doubtfully to
plants. These sandstones have been called in Sweden "fucoid sandstones." The
whole thickness of the Cambrian rocks of Sweden does not exceed 300 feet from
the equivalents of the Tremadoc beds to these sandstones, which last seem to
correspond with the Longmynd, and are regarded by Torell as older than any
fossiliferous primordial rocks in Bohemia.
CAMBRIAN OF THE UNITED STATES AND CANADA (POTSDAM SANDSTONE).
(FIGURE 581. Dikelocephalus Minnesotensis. Dale Owen. One-third diameter. A
large crustacean of the Olenoid group. Potsdam sandstone. Falls of St. Croix, on
the Upper Mississippi.)
This formation, as we learn from Sir W. Logan, is 700 feet thick in Canada; the
upper part consists of sandstone containing fucoids, and perforated by small
vertical holes, which are very characteristic of the rock, and appear to have
been made by annelids (Scolithus linearis). The lower portion is a conglomerate
with quartz pebbles. I have seen the Potsdam sandstone on the banks of the St.
Lawrence, and on the borders of Lake Champlain, where, as at Keesville, it is a
white quartzose fine-grained grit, almost passing into quartzite. It is divided
into horizontal ripple-marked beds, very like those of the Lingula Flags of
Britain, and replete with a small round-shaped Obolella, in such numbers as to
divide the rock into parallel planes, in the same manner as do the scales of
mica in some micaceous sandstones. Among the shells of this formation in
Wisconsin are species of Lingula and Orthis, and several trilobites of the
primordial genus Dikelocephalus (Figure 581). On the banks of the St. Lawrence,
near Beauharnois and elsewhere, many fossil footprints have been observed on the
surface of the rippled layers. They are supposed by Professor Owen to be the
trails of more than one species of articulate animal, probably allied to the
King Crab, or Limulus.
Recent investigations by the naturalists of the Canadian survey have rendered it
certain that below the level of the Potsdam Sandstone there are slates and
schists extending from New York to Newfoundland, occupied by a series of
trilobitic forms similar in genera, though not in species, to those found in the
European Upper Cambrian strata.
HURONIAN SERIES.
Next below the Upper Cambrian occur strata called the Huronian by Sir W. Logan,
which are of vast thickness, consisting chiefly of quartzite, with great masses
of greenish chloritic slate, which sometimes include pebbles of crystalline
rocks derived from the Laurentian formation, next to be described. Limestones
are rare in this series, but one band of 300 feet in thickness has been traced
for considerable distances to the north of Lake Huron. Beds of greenstone are
intercalated conformably with the quartzose and argillaceous members of this
series. No organic remains have yet been found in any of the beds, which are
about 18,000 feet thick, and rest unconformably on the Laurentian rocks.
LAURENTIAN GROUP.
In the course of the geological survey carried on under the direction of Sir
W.E. Logan, it has been shown that, northward of the river St. Lawrence, there
is a vast series of crystalline rocks of gneiss, mica-schist, quartzite, and
limestone, more than 30,000 feet in thickness, which have been called
Laurentian, and which are already known to occupy an area of about 200,000
square miles. They are not only more ancient than the fossiliferous Cambrian
formations above described, but are older than the Huronian last mentioned, and
had undergone great disturbing movements before the Potsdam sandstone and the
other "primordial" or Cambrian rocks were formed. The older half of this
Laurentian series is unconformable to the newer portion of the same.
UPPER LAURENTIAN OR LABRADOR SERIES.
The Upper Group, more than 10,000 feet thick, consists of stratified crystalline
rocks in which no organic remains have yet been found. They consist in great
part of feldspars, which vary in composition from anorthite to andesine, or from
those kinds in which there is less than one per cent of potash and soda to those
in which there is more than seven per cent of these alkalies, the soda
preponderating greatly. These feldsparites sometimes form mountain masses almost
without any admixture of other minerals; but at other times they include augite,
which passes into hypersthene. They are often granitoid in structure. One of the
varieties is the same as the apolescent labradorite rock of Labrador. The
Adirondack Mountains in the State of New York are referred to the same series,
and it is conjectured that the hypersthene rocks of Skye, which resemble this
formation in mineral character, may be of the same geological age.
LOWER LAURENTIAN.
This series, about 20,000 feet in thickness, is, as before stated, unconformable
to that last mentioned; it consists in great part of gneiss of a reddish tint
with orthoclase feldspar. Beds of nearly pure quartz, from 400 to 600 feet
thick, occur in some places. Hornblendic and micaceous schists are often
interstratified, and beds of limestone, usually crystalline. Beds of plumbago
also occur. That this pure carbon may have been of organic origin before
metamorphism has naturally been conjectured.
(FIGURES 582 and 583. Eozoon Canadense, Daw. (after Carpenter). Oldest known
organic body.
(FIGURE 582. Eozoon Canadense, Daw. (after Carpenter). Oldest known organic
body.
a. Chambers of lower tier communicating at +, and separated from adjoining
chambers at o by an intervening septum, traversed by passages.
b. Chambers of an upper tier.
c. Walls of the chambers traversed by fine tubules. (These tubules pass with
uniform parallelism from the inner to the outer surface, opening at regular
distances from each other.)
d. Intermediate skeleton, composed of homogeneous shell substance, traversed by
f. Stoloniferous passages connecting the chambers of the two tiers.
e. Canal system in intermediate skeleton, showing the arborescent saceodic
prolongations.
(Figure 583 shows these bodies in a decalcified state.))
(FIGURE 583. Eozoon Canadense, Daw. (after Carpenter). Oldest known organic
body.
Decalcified portion of natural rock, showing CANAL SYSTEM and the several
layers; the acuteness of the planes prevents more than one or two parallel tiers
being observed. Natural size.))
There are several of these limestones which have been traced to great distances,
and one of them is from 700 to 1500 feet thick. In the most massive of them Sir
W. Logan observed, in 1859, what he considered to be an organic body much
resembling the Silurian fossil called Stromatopora rugosa. It had been obtained
the year before by Mr. J. MacMullen at the Grand Calumet, on the river Ottawa.
This fossil was examined in 1864 by Dr. Dawson of Montreal, who detected in it,
by aid of the microscope, the distinct structure of a Rhizopod or Foraminifer.
Dr. Carpenter and Professor T. Rupert Jones have since confirmed this opinion,
comparing the structure to that of the well-known nummulite. It appears to have
grown one layer over another, and to have formed reefs of limestone as do the
living coral-building polyp animals. Parts of the original skeleton, consisting
of carbonate of lime, are still preserved; while certain inter-spaces in the
calcareous fossil have been filled up with serpentine and white augite. On this
oldest of known organic remains Dr. Dawson has conferred the name of Eozoon
Canadense (see Figures 582, 583); its antiquity is such that the distance of
time which separated it from the Upper Cambrian period, or that of the Potsdam
sandstone, may, says Sir W. Logan, be equal to the time which elapsed between
the Potsdam sandstone and the nummulitic limestones of the Tertiary period. The
Laurentian and Huronian rocks united are about 50,000 feet in thickness, and the
Lower Laurentian was disturbed before the newer series was deposited. We may
naturally expect the other proofs of unconformability will hereafter be detected
at more than one point in so vast a succession of strata.
The mineral character of the Upper Laurentian differs, as we have seen, from
that of the Lower, and the pebbles of gneiss in the Huronian conglomerates are
thought to prove that the Laurentian strata were already in a metamorphic state
before they were broken up to supply materials for the Huronian. Even if we had
not discovered the Eozoon, we might fairly have inferred from analogy that as
the quartzites were once beds of sand, and the gneiss and mica-schist derived
from shales and argillaceous sandstones, so the calcareous masses, from 400 to
1000 feet and more in thickness, were originally of organic origin. This is now
generally believed to have been the case with the Silurian, Devonian,
Carboniferous, Oolitic, and Cretaceous limestones and those nummulitic rocks of
tertiary date which bear the closest affinity to the Eozoon reefs of the Lower
Laurentian. The oldest stratified rock in Scotland is that called by Sir R.
Murchison "the fundamental gneiss," which is found in the north-west of Ross-
shire, and in Sutherlandshire (see Figure 82), and forms the whole of the
adjoining island of Lewis, in the Hebrides. It has a strike from north-west to
south-east, nearly at right angles to the metamorphic strata of the Grampians.
On this Laurentian gneiss, in parts of the western Highlands, the Lower Cambrian
and various metamorphic rocks rest unconformably. It seems highly probable that
this ancient gneiss of Scotland may correspond in date with part of the great
Laurentian group of North America.
CHAPTER XXVIII.
VOLCANIC ROCKS.
External Form, Structure, and Origin of Volcanic Mountains.
Cones and Craters.
Hypothesis of "Elevation Craters" considered.
Trap Rocks.
Name whence derived.
Minerals most abundant in Volcanic Rocks.
Table of the Analysis of Minerals in the Volcanic and Hypogene Rocks.
Similar Minerals in Meteorites.
Theory of Isomorphism.
Basaltic Rocks.
Trachytic Rocks.
Special Forms of Structure.
The columnar and globular Forms.
Trap Dikes and Veins.
Alteration of Rocks by volcanic Dikes.
Conversion of Chalk into Marble.
Intrusion of Trap between Strata.
Relation of trappean Rocks to the Products of active Volcanoes.
(FIGURE 584. Section through formations from a, low, to c, high.
a. Hypogene formations, stratified and unstratified.
b. Aqueous formations.
c. Volcanic rocks.)
The aqueous or fossiliferous rocks having now been described, we have next to
examine those which may be called volcanic, in the most extended sense of that
term. In the diagram (Figure 584) suppose a, a to represent the crystalline
formations, such as the granitic and metamorphic; b, b the fossiliferous strata;
and c, c the volcanic rocks. These last are sometimes found, as was explained in
the first chapter, breaking through a and b, sometimes overlying both, and
occasionally alternating with the strata b, b.
EXTERNAL FORM, STRUCTURE, AND ORIGIN OF VOLCANIC MOUNTAINS.
The origin of volcanic cones with crater-shaped summits has been explained in
the "Principles of Geology" (Chapters 23 to 27), where Vesuvius, Etna, Santorin,
and Barren Island are 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 volcanoes of
still higher antiquity; and these last have evidently been due to a complicated
series of operations, varied in kind according to circumstances; as, for
example, whether the accumulation took place above or below the level of the
sea, whether the lava issued from one or several contiguous vents, and, lastly,
whether the rocks reduced to fusion in the subterranean regions happened to have
contained more or less silica, potash, soda, lime, iron, and other ingredients.
We are best acquainted with the effects of eruptions above water, or those
called subaerial or supramarine; yet the products even of these are arranged in
so many ways that their interpretation has given rise to a variety of
contradictory opinions, some of which will have to be considered in this
chapter.
CONES AND CRATERS.
(FIGURE 585. Part of the chain of extinct volcanoes called the Monts Dome,
Auvergne. (Scrope.))
In regions where the eruption of volcanic matter has taken place in the open
air, and where the surface has never since been subjected to great aqueous
denudation, cones and craters constitute the most striking peculiarity of this
class of formations. Many hundreds of these cones are seen in central France, in
the ancient provinces of Auvergne, Velay, and Vivarais, where they observe, for
the most part, a linear arrangement, and form chains of hills. Although none of
the eruptions have happened within the historical era, the streams of lava may
still be traced distinctly descending from many of the craters, and following
the lowest levels of the existing valleys. The origin of the cone and crater-
shaped hill is well understood, the growth of many having been watched during
volcanic eruptions. A chasm or fissure first opens in the earth, from which
great volumes of steam are evolved. The explosions are so violent as to hurl up
into the air fragments of broken stone, parts of which are shivered into minute
atoms. At the same time melted stone or LAVA usually ascends through the chimney
or vent by which the gases make their escape. Although extremely heavy, this
lava is forced up by the expansive power of entangled gaseous fluids, chiefly
steam or aqueous vapour, exactly in the same manner as water is made to boil
over the edge of a vessel when steam has been generated at the bottom by heat.
Large quantities of the lava are also shot up into the air, where it separates
into fragments, and acquires a spongy texture by the sudden enlargement of the
included gases, and thus forms SCORIAE, other portions being reduced to an
impalpable powder or dust. The showering down of the various ejected materials
round the orifice of eruption gives rise to a conical mound, in which the
successive envelopes of sand and scoriae form layers, dipping on all sides from
a central axis. In the mean time a hollow, called a CRATER, has been kept open
in the middle of the mound by the continued passage upward of steam and other
gaseous fluids. The lava sometimes flows over the edge of the crater, and thus
thickens and strengthens the sides of the cone; but sometimes it breaks down the
cone on one side (see Figure 585), and often it flows out from a fissure at the
base of the hill, or at some distance from its base.
Some geologists had erroneously supposed, from observations made on recent cones
of eruption, that lava which consolidates on steep slopes is always of a
scoriaceous or vesicular structure, and never of that compact texture which we
find in those rocks which are usually termed "trappean." Misled by this theory,
they have gone so far as to believe that if melted matter has originally
descended a slope at an angle exceeding four or five degrees, it never, on
cooling, acquires a stony compact texture. Consequently, whenever they found in
a volcanic mountain sheets of stony materials inclined at angles of from 5
degrees to 20 degrees or even more than 30 degrees, they thought themselves
warranted in assuming that such rocks had been originally horizontal, or very
slightly inclined, and had acquired their high inclination by subsequent
upheaval. To such dome-shaped mountains with a cavity in the middle, and with
the inclined beds having what was called a quaquaversal dip or a slope outward
on all sides, they gave the name of "Elevation craters."
As the late Leopold Von Buch, the author of this theory, had selected the Isle
of Palma, one of the Canaries, as a typical illustration of this form of
volcanic mountain, I visited that island in 1854, in company with my friend Mr.
Hartung, and I satisfied myself that it owes its origin to a series of eruptions
of the same nature as those which formed the minor cones, already alluded to. In
some of the more ancient or Miocene volcanic mountains, such as Mont Dor and
Cantal in central France, the mode of origin by upheaval as above described is
attributed to those dome-shaped masses, whether they possess or not a great
central cavity, as in Palma. Where this cavity is present, it has probably been
due to one or more great explosions similar to that which destroyed a great part
of ancient Vesuvius in the time of Pliny. Similar paroxysmal catastrophes have
caused in historical times the truncation on a grand scale of some large cones
in Java and elsewhere. (Principles volume 2 pages 56 and 145.)
Among the objections which may be considered as fatal to Von Buch's doctrine of
upheaval in these cases, I may state that a series of volcanic formations
extending over an area six or seven miles in its shortest diameter, as in Palma,
could not be accumulated in the form of lavas, tuffs, and volcanic breccias or
agglomerates without producing a mountain as lofty as that which they now
constitute. But assuming that they were first horizontal, and then lifted up by
a force acting most powerfully in the centre and tilting the beds on all sides,
a central crater having been formed by explosion or by a chasm opening in the
middle, where the continuity of the rocks was interrupted, we should have a
right to expect that the chief ravines or valleys would open towards the central
cavity, instead of which the rim of the great crater in Palma and other similar
ancient volcanoes is entire for more than three parts of the whole
circumference.
If dikes are seen in the precipices surrounding such craters or central
cavities, they certainly imply rents which were filled up with liquid matter.
But none of the dislocations producing such rents can have belonged to the
supposed period of terminal and paroxysmal upheaval, for had a great central
crater been already formed before they originated, or at the time when they took
place, the melted matter, instead of filling the narrow vents, would have flowed
down into the bottom of the cavity, and would have obliterated it to a certain
extent. Making due allowance for the quantity of matter removed by subaerial
denudation in volcanic mountains of high antiquity, and for the grand explosions
which are known to have caused truncation in active volcanoes, there is no
reason for calling in the violent hypothesis of elevation craters to explain the
structure of such mountains as Teneriffe, the Grand Canary, Palma, or those of
central France, Etna, or Vesuvius, all of which I have examined. With regard to
Etna, I have shown, from observations made by me in 1857, that modern lavas,
several of them of known date, have formed continuous beds of compact stone even
on slopes of 15, 36, and 38 degrees, and, in the case of the lava of 1852, more
than 40 degrees. The thickness of these tabular layers varies from 1 1/2 foot to
26 feet. And their planes of stratification are parallel to those of the
overlying and underlying scoriae which form part of the same currents. (Memoir
on Mount Etna Philosophical Transactions 1858.)
NOMENCLATURE OF TRAPPEAN ROCKS.
When geologists first began to examine attentively the structure of the northern
and western parts of Europe, they were almost entirely ignorant of the phenomena
of existing volcanoes. They found certain rocks, for the most part without
stratification, and of a peculiar mineral composition, to which they gave
different names, such as basalt, greenstone, porphyry, trap tuff, and
amygdaloid. All these, which were recognised as belonging to one family, were
called "trap" by Bergmann, from trappa, Swedish for a flight of steps-- a name
since adopted very generally into the nomenclature of the science; for it was
observed that many rocks of this class occurred in great tabular masses of
unequal extent, so as to form a succession of terraces or steps. It was also
felt that some general term was indispensable, because these rocks, although
very diversified in form and composition, evidently belonged to one group,
distinguishable from the Plutonic as well as from the non-volcanic fossiliferous
rocks.
By degrees familiarity with the products of active volcanoes convinced
geologists more and more that they were identical with the trappean rocks. In
every stream of modern lava there is some variation in character and
composition, and even where no important difference can be recognised in the
proportions of silica, alumina, lime, potash, iron, and other elementary
materials, the resulting materials are often not the same, for reasons which we
are as yet unable to explain. The difference also of the lavas poured out from
the same mountain at two distinct periods, especially in the quantity of silica
which they contain, is often so great as to give rise to rocks which are
regarded as forming distinct families, although there may be every intermediate
gradation between the two extremes, and although some rocks, forming a
transition from the one class to the other, may often be so abundant as to
demand special names. These species might be multiplied indefinitely, and I can
only afford space to name a few of the principal ones, about the composition and
aspect of which there is the least discordance of opinion.
MINERALS MOST ABUNDANT IN VOLCANIC ROCKS.
TABLE 28.1. ANALYSIS OF MINERALS MOST ABUNDANT IN THE VOLCANIC AND HYPOGENE
ROCKS.
COLUMN 1: SILICA.
COLUMN 2: ALUMINA.
COLUMN 3: SESQUIOXIDE OF IRON.
COLUMN 4: PROTOXIDES OF IRON AND MANGANESE.
COLUMN 5: LIME.
COLUMN 6: MAGNESIA.
COLUMN 7: POTASH.
COLUMN 8: SODA.
COLUMN 9: OTHER CONSTITUENTS.
In this column the following signs are used:
F. Fluorine;
Li. Lithia;
W. Loss on igniting the mineral, in most instances only Water.
COLUMN 10: SPECIFIC GRAVITY.
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
-- -- -- -- -- -- -- -- -- -- -
THE QUARTZ GROUP:
1 2 3 4 5 6 7 8 9 10.
Quartz:
100.0 2.6.
Tridymite:
100.0 2.3.
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
-- -- -- -- -- -- -- -- -- -- -
THE FELDSPAR GROUP:
1 2 3 4 5 6 7 8 9 10.
Orthoclase. Carlsbad, in granite (Bulk):
65.23 18.26 0.27 .... trace .... 14.66 1.45 .... 2.55.
Orthoclase. Sanadine, Drachenfels in trachyte (Rammelsberg).
65.87 18.53 .... .... 0.95 0.30 10.32 3.42 W 0.44 2.55.
Albite. Arendal, in granite (G. Rose).
68.46 19.30 .... 0.28 0.68 .... .... 11.27 .... 2.61.
Oligoclase. Ytterby, in granite (Berzelius).
61.55 23.80 .... .... 3.18 0.80 0.38 9.67 .... 2.65.
Oligoclase. Teneriffe, in trachyte (Deville).
61.55 22.03 .... .... 2.81 0.47 3.44 7.74 .... 2.59.
Labradorite. Hitteroe, in Labrador-Rock (Waage).
51.39 29.42 2.90 .... 9.44 0.37 1.10 5.03 W 0.71 2.72.
Labradorite. Iceland, in volcanic (Damour).
52.17 29.22 1.90 .... 13.11 .... .... 3.40 .... 2.71.
Anorthite. Harzburg, in diorite (Streng).
45.37 34.81 0.59 .... 16.52 0.83 0.40 1.45 W 0.87 2.74.
Anorthite. Hecla, in volcanic (Waltershausen).
45.14 32.10 2.03 0.78 18.32 .... 0.22 1.06 .... 2.74.
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
-- -- -- -- -- -- -- -- -- -- -
Leucite. Vesuvius, 1811, in lava (Rammelsberg).
56.10 23.22 .... .... .... .... 20.59 0.57 .... 2.48.
Nepheline. Miask, in Miascite (Scheerer).
44.30 33.25 0.82 .... 0.32 0.07 5.82 16.02 .... 2.59.
Nepheline. Vesuvius, in volcanic (Arfvedson).
44.11 33.73 .... .... .... .... .... 20.46 W 0.62 2.60.
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
-- -- -- -- -- -- -- -- -- -- -
THE MICA GROUP:
1 2 3 4 5 6 7 8 9 10.
Muscovite. Finland, in granite (Rose).
46.36 36.80 4.53 .... .... .... 9.22 .... F 0.67 2.90.
W 1.84.
Lepidolite. Cornwall, in granite (Regnault).
52.40 26.80 .... 1.50 .... .... 9.14 .... F 4.18 2.90.
Li 4.85.
Biotite. Bodennais (V. Kobell).
40.86 15.13 13.00 .... .... 22.00 8.83 .... W 0.44 2.70.
Biotite. Vesuvius, in volcanic (Chodnef).
40.91 17.71 11.02 .... 0.30 19.04 9.96 .... .... 2.75.
Phlogopite. New York, in metamorphic limestone (Rammelsberg).
41.96 13.47 .... 2.67 0.34 27.12 9.37 .... F 2.93 2.81.
W 0.60.
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
-- -- -- -- -- -- -- -- -- -- --
Margarite. Nexos (Smith).
30.02 49.52 1.65 .... 10.82 0.48 1.25 W 5.55 2.99.
Chlorite. Dauphiny (Marignac).
26.88 17.52 29.76 .... .... 13.84 .... .... W 11.33 2.87.
Rapidolite. Pyrenees (Delesse).
32.10 18.50 .... 0.06 .... 36.70 .... .... W 12.10 2.61.
Talc. Zillerthal (Delesse).
63.00 .... .... trace .... 33.60 .... .... W 3.10 2.78.
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
-- -- -- -- -- -- -- -- -- -- --
THE AMPHIBOLE AND PYROXENE GROUP.
1 2 3 4 5 6 7 8 9 10.
Tremolite. St. Gothard (Rammelsberg)
58.55 .... .... .... 13.90 26.63 .... .... FW 0.34 2.93.
Actinolite. Arendal, in granite (Rammelsberg).
56.77 0.97 .... 5.88 13.56 21.48 .... .... W 2.20 3.02.
Hornblende. Faymont, in diorite (Deville).
41.99 11.66 .... 22.22 9.55 12.59 .... 1.02 W 1.47 3.20.
Hornblende Etna, in volcanic (Waltershausen).
40.91 13.68 .... 17.49 13.44 13.19 .... .... W 0.85 3.01.
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
-- -- -- -- -- -- -- -- -- -- --
Uralite. Ural (Rammelsberg)
50.75 5.65 .... 17.27 11.59 12.28 .... .... W 1.80 3.14.
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
-- -- -- -- -- -- -- -- -- -- --
Augite. Bohemia, in dolerite (Rammelsberg).
51.12 3.38 0.95 8.08 23.54 12.82 .... .... .... 3.35.
Augite. Vesuvius, in lava of 1858 (Rammelsberg).
49.61 4.42 .... 9.08 22.83 14.22 .... .... .... 3.25.
Diallage. Harz, in Gabbro (Rammelsberg).
52.00 3.10 .... 9.36 16.29 18.51 .... .... W 1.10 3.23.
Hypersthene. Labrador, in Labrador-Rock (Damour).
51.36 0.37 .... 22.59 3.09 21.31 .... .... .... 3.39.
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
-- -- -- -- -- -- -- -- -- -- --
THE OLIVINE GROUP.
1 2 3 4 5 6 7 8 9 10
Bronzite. Greenland (V. Kobell).
58.00 1.33 11.14 .... .... 29.66 .... .... .... 3.20.
Olivine. Carlsbad, in basalt (Rammelsberg).
39.34 .... .... 14.85 .... 45.81 .... .... .... 3.40.
Olivine. Mount Somma, in volcanic (Walmstedt).
40.08 0.18 .... 15.74 .... 44.22 .... .... .... 3.33.
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
-- -- -- -- -- -- -- -- -- -- --
The minerals which form the chief constituents of these igneous rocks are few in
number. Next to quartz, which is nearly pure silica or silicic acid, the most
important are those silicates commonly classed under the several heads of
feldspar, mica, hornblende or augite, and olivine. In Table 28.1, in drawing up
which I have received the able assistance of Mr. David Forbes, the chemical
analysis of these minerals and their varieties is shown, and he has added the
specific gravity of the different mineral species, the geological application of
which in determining the rocks formed by these minerals will be explained in the
sequel.
From Table 28.1 it will be observed that many minerals are omitted which, even
if they are of common occurrence, are more to be regarded as accessory than as
essential components of the rocks in which they are found. (For analyses of
these minerals see the Mineralogies of Dana and Bristow.) Such are, for example,
Garnet, Epidote, Tourmaline, Idocrase, Andalusite, Scapolite, the various
Zeolites, and several other silicates of somewhat rarer occurrence. Magnetite,
Titanoferrite, and Iron-pyrites also occur as normal constituents of various
igneous rocks, although in very small amount, as also Apatite, or phosphate of
lime. The other salts of lime, including its carbonate or calcite, although
often met with, are invariably products of secondary chemical action.
The Zeolites, above mentioned, so named from the manner in which they froth up
under the blow-pipe and melt into a glass, differ in their chemical composition
from all the other mineral constituents of volcanic rocks, since they are
hydrated silicates containing from 10 to 25 per cent of water. They abound in
some trappean rocks and ancient lavas, where they fill up vesicular cavities and
interstices in the substance of the rocks, but are rarely found in any quantity
in recent lavas; in most cases they are to be regarded as secondary products
formed by the action of water on the other constituents of the rocks. Among them
the species Analcime, Stilbite, Natrolite, and Chabazite may be mentioned as of
most common occurrence.
QUARTZ GROUP.
The microscope has shown that pure quartz is oftener present in lavas than was
formerly supposed. It had been argued that the quartz in granite having a
specific gravity of 2.6, was not of purely igneous origin, because the silica
resulting from fusion in the laboratory has only a specific gravity of 2.3. But
Mr. David Forbes has ascertained that the free quartz in trachytes, which are
known to have flowed as lava, has the same specific gravity as the ordinary
quartz of granite; and the recent researches of Von Rath and others prove that
the mineral Tridymite, which is crystallised silica of specific gravity 2.3 (see
Table 28.1), is of common occurrence in the volcanic rocks of Mexico, Auvergne,
the Rhine, and elsewhere, although hitherto entirely overlooked.
FELDSPAR GROUP.
In the Feldspar group (Table 28.1) the five mineral species most commonly met
with as rock constituents are: 1. Orthoclase, often called common or potash-
feldspar. 2. Albite, or soda-feldspar, a mineral which plays a more subordinate
part than was formerly supposed, this name having been given to much which has
since been proved to be Oligoclase. 3. Oligoclase, or soda-lime feldspar, in
which soda is present in much larger proportion than lime, and of which mineral
andesite are andesine, is considered to be a variety. 4. Labradorite, or lime-
soda-feldspar, in which the proportions of lime and soda are the reverse to what
they are in Oligoclase. 5. Anorthite or lime-feldspar. The two latter feldspars
are rarely if ever found to enter into the composition of rocks containing
quartz.
In employing such terms as potash-feldspar, etc., it must, however, always be
borne in mind that it is only intended to direct attention to the predominant
alkali or alkaline earth in the mineral, not to assert the absence of the
others, which in most cases will be found to be present in minor quantity. Thus
potash-feldspar (orthoclase) almost always contains a little soda, and often
traces of lime or magnesia; and in like manner with the others. The terms
"glassy" and "compact" feldspars only refer to structure, and not to species or
composition; the student should be prepared to meet with any of the above
feldspars in either of these conditions: the glassy state being apparently due
to quick cooling, and the compact to conditions unfavourable to crystallisation;
the so-called "compact feldspar" is also very commonly found to be an admixture
of more than one feldspar species, and frequently also contains quartz and other
extraneous mineral matter only to be detected by the microscope.
Feldspars when arranged according to their system of crystallisation are
MONOCLINIC, having one axis obliquely inclined; or TRICLINIC, having the three
axes all obliquely inclined to each other. If arranged with reference to their
cleavage they are ORTHOCLASTIC, the fracture taking place always at a right
angle; or PLAGIOCLASTIC, in which the cleavages are oblique to one another.
Orthoclase is orthoclastic and monoclinic; all the other feldspars are
plagioclastic and triclinic.
MINERALS IN METEORITES.
That variety of the Feldspar Group which is called Anorthite has been shown by
Rammelsberg to occur in a meteoric stone, and his analysis proves it to be
almost identical in its chemical proportions to the same mineral in the lavas of
modern volcanoes. So also Bronzite (Enstatite) and Olivine have been met with in
meteorites shown by analysis to come remarkably near to these minerals in
ordinary rocks.
MICA GROUP.
With regard to the micas, the four principal species (Table 28.1) all contain
potash in nearly the same proportion, but differ greatly in the proportion and
nature of their other ingredients. Muscovite is often called common or potash
mica; Lepidolite is characterised by containing lithia in addition; Biotite
contains a large amount of magnesia and oxide of iron; whilst Phlogopite
contains still more of the former substance. In rocks containing quartz,
muscovite or lepidolite are most common. The mica in recent volcanic rocks,
gabbros, and diorites is usually Biotite, while that so common in metamorphic
limestones is usually, if not always, Phlogopite.
AMPHIBOLE AND PYROXENE GROUP.
The minerals included in Table 28.1 under the Amphibole and Pyroxene Group
differ somewhat in their crystallisation form, though they all belong to the
monoclinic system. Amphibole is a general name for all the different varieties
of Hornblende, Actinolite, Tremolite, etc., while Pyroxene includes Augite,
Diallage, Malacolite, Sahlite, etc. The two divisions are so much allied in
chemical composition and crystallographic characters, and blend so completely
one into the other in Uralite, that it is perhaps best to unite them in one
group.
THEORY OF ISOMORPHISM.
The history of the changes of opinion on this point is curious and instructive.
Werner first distinguished augite from hornblende; and his proposal to separate
them obtained afterwards the sanction of Hauy, Mohs, and other celebrated
mineralogists. It was agreed that the form of the crystals of the two species
was different, and also their structure, as shown by CLEAVAGE-- that is to say,
by breaking or cleaving the mineral with a chisel, or a blow of the hammer, in
the direction in which it yields most readily. It was also found by analysis
that augite usually contained more lime, less alumina, and no fluoric acid;
which last, though not always found in hornblende, often enters into its
composition in minute quantity. In addition to these characters, it was remarked
as a geological fact, that augite and hornblende are very rarely associated
together in the same rock. It was also remarked that in the crystalline slags of
furnaces augitic forms were frequent, the hornblendic entirely absent; hence it
was conjectured that hornblende might be the result of slow, and augite of rapid
cooling. This view was confirmed by the fact that Mitscherlich and Berthier were
able to make augite artificially, but could never succeed in forming hornblende.
Lastly, Gustavus Rose fused a mass of hornblende in a porcelain furnace, and
found that it did not, on cooling, assume its previous shape, but invariably
took that of augite. The same mineralogist observed certain crystals called
Uralite (see Table 28.1) in rocks from Siberia, which possessed the cleavage and
chemical composition of hornblende, while they had the external form of augite.
If, from these data, it is inferred that the same substance may assume the
crystalline forms of hornblende or augite indifferently, according to the more
or less rapid cooling of the melted mass, it is nevertheless certain that the
variety commonly called augite, and recognised by a peculiar crystalline form,
has usually more lime in it, and less alumina, than that called hornblende,
although the quantities of these elements do not seem to be always the same.
Unquestionably the facts and experiments above mentioned show the very near
affinity of hornblende and augite; but even the convertibility of one into the
other, by melting and recrystallising, does not perhaps demonstrate their
absolute identity. For there is often some portion of the materials in a crystal
which are not in perfect chemical combination with the rest. Carbonate of lime,
for example, sometimes carries with it a considerable quantity of silex into its
own form of crystal, the silex being mechanically mixed as sand, and yet not
preventing the carbonate of lime from assuming the form proper to it. This is an
extreme case, but in many others some one or more of the ingredients in a
crystal may be excluded from perfect chemical union; and after fusion, when the
mass recrystallises, the same elements may combine perfectly or in new
proportions, and thus a new mineral may be produced. Or some one of the gaseous
elements of the atmosphere, the oxygen for example, may, when the melted matter
reconsolidates, combine with some one of the component elements.
The different quantity of the impurities or the refuse above alluded to, which
may occur in all but the most transparent and perfect crystals, may partly
explain the discordant results at which experienced chemists have arrived in
their analysis of the same mineral. For the reader will often find that crystals
of a mineral determined to be the same by physical characters, crystalline form,
and optical properties, have been declared by skilful analysers to be composed
of distinct elements. This disagreement seemed at first subversive of the atomic
theory, or the doctrine that there is a fixed and constant relation between the
crystalline form and structure of a mineral and its chemical composition. The
apparent anomaly, however, which threatened to throw the whole science of
mineralogy into confusion, was reconciled to fixed principles by the discoveries
of Professor Mitscherlich at Berlin, who ascertained that the composition of the
minerals which had appeared so variable was governed by a general law, to which
he gave the name of ISOMORPHISM (from isos, equal, and morphe, form). According
to this law, the ingredients of a given species of mineral are not absolutely
fixed as to their kind and quality; but one ingredient may be replaced by an
equivalent portion of some analogous ingredient. Thus, in augite, the lime may
be in part replaced by portions of protoxide of iron, or of manganese, while the
form of the crystal, and the angle of its cleavage planes, remain the same.
These vicarious substitutions, however, of particular elements can not exceed
certain defined limits.
BASALTIC ROCKS.
The two principal families of trappean or volcanic rocks are the basalts and the
trachytes, which differ chiefly from each other in the quantity of silica which
they contain. The basaltic rocks are comparatively poor in silica, containing
less than 50 per cent of that mineral, and none in a pure state or as free
quartz, apart from the rest of the matrix. They contain a larger proportion of
lime and magnesia than the trachytes, so that they are heavier, independently of
the frequent presence of the oxides of iron which in some cases forms more than
a fourth part of the whole mass. Abich has, therefore, proposed that we should
weigh these rocks, in order to appreciate their composition in cases where it is
impossible to separate their component minerals. Thus, basalt from Staffa,
containing 47.80 per cent of silica, has a specific gravity of 2.95; whereas
trachyte, which has 66 per cent of silica, has a specific gravity of only 2.68;
trachytic porphyry, containing 69 per cent of silica, a specific gravity of only
2.58. If we then take a rock of intermediate composition, such as that
prevailing in the Peak of Teneriffe, which Abich calls Trachyte-dolerite, its
proportion of silica being intermediate, or 58 per cent, it weighs 2.78, or more
than trachyte, and less than basalt. (Dr. Daubeny on Volcanoes second edition
pages 14, 15.)
BASALT.
The different varieties of this rock are distinguished by the names of basalts,
anamezites, and dolerites, names which, however, only denote differences in
texture without implying any difference in mineral or chemical composition: the
term BASALT being used only when the rock is compact, amorphous, and often semi-
vitreous in texture, and when it breaks with a perfect conchoidal fracture;
when, however, it is uniformly crystalline in appearance, yet very close-
grained, the name ANAMESITE (from anamesos, intermediate) is employed, but if
the rock be so coarsely crystallised that its different mineral constituents can
be easily recognised by the eye, it is called DOLERITE (from doleros,
deceitful), in allusion to the difficulty of distinguishing it from some of the
rocks known as Plutonic.
MELAPHYRE is often quite undistinguishable in external appearance from basalt,
for although rarely so heavy, dark-coloured, or compact, it may present at times
all these varieties of texture. Both these rocks are composed of triclinic
feldspar and augite with more or less olivine, magnetic or titaniferous oxide of
iron, and usually a little nepheline, leucite, and apatite; basalt usually
contains considerably more olivine than melaphyre, but chemically they are
closely allied, although the melaphyres usually contain more silica and alumina,
with less oxides of iron, lime, and magnesia, than the basalts. The Rowley Hills
in Staffordshire, commonly known as Rowley Ragstone, are melaphyre.
GREENSTONE.
This name has usually been extended to all granular mixtures, whether of
hornblende and feldspar, or of augite and feldspar. The term DIORITE has been
applied exclusively to compounds of hornblende and triclinic feldspar. LABRADOR-
ROCK is a term used for a compound of labradorite or labrador-feldspar and
hypersthene; when the hypersthene predominates it is sometimes known under the
name of HYPERSTHENE-ROCK. GABBRO and DIABASE are rocks mainly composed of
triclinic feldspars and diallage. All these rocks become sometimes very
crystalline, and help to connect the volcanic with the Plutonic formations,
which will be treated of in Chapter 31.
The name trachyte (from trachus, rough) was originally given to a coarse
granular feldspathic rock which was rough and gritty to the touch. The term was
subsequently made to include other rocks, such as clinkstone and obsidian, which
have the same mineral composition, but to which, owing to their different
texture, the word in its original meaning would not apply. The feldspars which
occur in Trachytic rocks are invariably those which contain the largest
proportion of silica, or from 60 to 70 per cent of that mineral. Through the
base are usually disseminated crystals of glassy feldspar, mica, and sometimes
hornblende. Although quartz is not a necessary ingredient in the composition of
this rock, it is very frequently present, and the quartz trachytes are very
largely developed in many volcanic districts. In this respect the trachytes
differ entirely from the members of the Basaltic family, and are more nearly
allied to the granites.
OBSIDIAN.
Obsidian, Pitchstone, and Pearlstone are only different forms of a volcanic
glass produced by the fusion of trachytic rocks. The distinction between them is
caused by different rates of cooling from the melted state, as has been proved
by experiment. Obsidian is of a black or ash-grey colour, and though opaque in
mass is transparent in thin edges.
CLINKSTONE OR PHONOLITE.
Among the rocks of the trachytic family, or those in which the feldspars are
rich in silica, that termed Clinkstone or Phonolite is conspicuous by its
fissile structure, and its tendency to lamination, which is such as sometimes to
render it useful as roofing-slate. It rings when struck with the hammer, whence
its name; is compact, and usually of a greyish blue or brownish colour; is
variable in composition, but almost entirely composed of feldspar. When it
contains disseminated crystals of feldspar, it is called CLINKSTONE PORPHYRY.
VOLCANIC ROCKS DISTINGUISHED BY SPECIAL FORMS OF STRUCTURE.
Many volcanic rocks are commonly spoken of under names denoting structure alone,
which must not be taken to imply that they are distinct rocks, i.e., that they
differ from one another either in mineral or chemical composition. Thus the
terms Trachytic porphyry, Trachytic tuff, etc., merely refer to the same rock
under different conditions of mechanical aggregation or crystalline development
which would be more correctly expressed by the use of the adjective, as
porphyritic trachyte, etc., but as these terms are so commonly employed it is
considered advisable to direct the student's attention to them.
PORPHYRY.
(FIGURE 586. Porphyry. White crystals of feldspar in a dark base of hornblende
and feldspar.)
PORPHYRY is one of this class, and very characteristic of the volcanic
formations. When distinct crystals of one or more minerals are scattered through
an earthy or compact base, the rock is termed a porphyry (see Figure 586). Thus
trachyte is usually porphyritic; for in it, as in many modern lavas, there are
crystals of feldspar; but in some porphyries the crystals are of augite,
olivine, or other minerals. If the base be greenstone, basalt, or pitchstone,
the rock may be denominated greenstone-porphyry, pitchstone-porphyry, and so
forth. The old classical type of this form of rock is the red porphyry of Egypt,
or the well-known "Rosso antico." It consists, according to Delesse, of a red
feldspathic base in which are disseminated rose-coloured crystals of the
feldspar called oligoclase, with some plates of blackish hornblende and grains
of oxide of iron (iron-glance). RED QUARTZIFEROUS PORPHYRY is a much more
siliceous rock, containing about 70 or 80 per cent of silex, while that of Egypt
has only 62 per cent.
AMYGDALOID.
This is also another form of igneous rock, admitting of every variety of
composition. It comprehends any rock in which round or almond-shaped nodules of
some mineral, such as agate, chalcedony, calcareous spar, or zeolite, are
scattered through a base of wacke, basalt, greenstone, or other kind of trap. It
derives its name from the Greek word amygdalon, an almond. The origin of this
structure can not be doubted, for we may trace the process of its formation in
modern lavas. Small pores or cells are caused by bubbles of steam and gas
confined in the melted matter. After or during consolidation, these empty spaces
are gradually filled up by matter separating from the mass, or infiltered by
water permeating the rock. As these bubbles have been sometimes lengthened by
the flow of the lava before it finally cooled, the contents of such cavities
have the form of almonds. In some of the amygdaloidal traps of Scotland, where
the nodules have decomposed, the empty cells are seen to have a glazed or
vitreous coating, and in this respect exactly resemble scoriaceous lavas, or the
slags of furnaces.
(FIGURE 587. Scoriaceous lava in part converted into an amygdaloid. Montagne de
la Veille, Department of Puy de Dome, France.)
Figure 587 represents a fragment of stone taken from the upper part of a sheet
of basaltic lava in Auvergne. One-half is scoriaceous, the pores being perfectly
empty; the other part is amygdaloidal, the pores or cells being mostly filled up
with carbonate of lime, forming white kernels.
LAVA.
This term has a somewhat vague signification, having been applied to all melted
matter observed to flow in streams from volcanic vents. When this matter
consolidates in the open air, the upper part is usually scoriaceous, and the
mass becomes more and more stony as we descend, or in proportion as it has
consolidated more slowly and under greater pressure. At the bottom, however, of
a stream of lava, a small portion of scoriaceous rock very frequently occurs,
formed by the first thin sheet of liquid matter, which often precedes the main
current, and solidifies under slight pressure.
The more compact lavas are often porphyritic, but even the scoriaceous part
sometimes contains imperfect crystals, which have been derived from some older
rocks, in which the crystals pre-existed, but were not melted, as being more
infusible in their nature. Although melted matter rising in a crater, and even
that which enters a rent on the side of a crater, is called lava, yet this term
belongs more properly to that which has flowed either in the open air or on the
bed of a lake or sea. If the same fluid has not reached the surface, but has
been merely injected into fissures below ground, it is called trap. There is
every variety of composition in lavas; some are trachytic, as in the Peak of
Teneriffe; a great number are basaltic, as in Vesuvius and Auvergne; others are
andesitic, as those of Chili; some of the most modern in Vesuvius consist of
green augite, and many of those of Etna of augite and labrador-feldspar. (G.
Hose, Ann. des Mines tome 8 page 32.)
SCORIAE and PUMICE may next be mentioned, as porous rocks produced by the action
of gases on materials melted by volcanic heat. SCORIAE are usually of a reddish-
brown and black colour, and are the cinders and slags of basaltic or augitic
lavas. PUMICE is a light, spongy, fibrous substance, produced by the action of
gases on trachytic and other lavas; the relation, however, of its origin to the
composition of lava is not yet well understood. Von Buch says that it never
occurs where only labrador-feldspar is present.
VOLCANIC ASH OR TUFF, TRAP TUFF.
Small angular fragments of the scoriae and pumice, above-mentioned, and the dust
of the same, produced by volcanic explosions, form the tuffs which abound in all
regions of active volcanoes, where showers of these materials, together with
small pieces of other rocks ejected from the crater, and more or less burnt,
fall down upon the land or into the sea. Here they often become mingled with
shells, and are stratified. Such tuffs are sometimes bound together by a
calcareous cement, and form a stone susceptible of a beautiful polish. But even
when little or no lime is present, there is a great tendency in the materials of
ordinary tuffs to cohere together. The term VOLCANIC ASH has been much used for
rocks of all ages supposed to have been derived from matter ejected in a melted
state from volcanic orifices. We meet occasionally with extremely compact beds
of volcanic materials, interstratified with fossiliferous rocks. These may
sometimes be tuffs, although their density or compactness is such as the cause
them to resemble many of those kinds of trap which are found in ordinary dikes.
WACKE is a name given to a decomposed state of various trap rocks of the
basaltic family, or those which are poor in silica. It resembles clay of a
yellowish or brown colour, and passes gradually from the soft state to the hard
dolerite, greenstone, or other trap rock from which it has been derived.
AGGLOMERATE.
In the neighbourhood of volcanic vents, we frequently observe accumulations of
angular fragments of rocks formed during eruptions by the explosive action of
steam, which shatters the subjacent stony formations, and hurls them up into the
air. They then fall in showers around the cone or crater, or may be spread for
some distance over the surrounding country. The fragments consist usually of
different varieties of scoriaceous and compact lavas; but other kinds of rock,
such as granite or even fossiliferous limestones, may be intermixed; in short,
any substance through which the expansive gases have forced their way. The
dispersion of such materials may be aided by the wind, as it varies in direction
or intensity, and by the slope of the cone down which they roll, or by floods of
rain, which often accompany eruptions. But if the power of running water, or of
the waves and currents of the sea, be sufficient to carry the fragments to a
distance, it can scarcely fail to wear off their angles, and the formation then
becomes a CONGLOMERATE. If occasionally globular pieces of scoriae abound in an
agglomerate, they may not owe their round form to attrition. When all the
angular fragments are of volcanic rocks the mass is usually termed a volcanic
breccia.
Laterite is a red or brick-like rock composed of silicate of alumina and oxide
of iron. The red layers called "ochre beds," dividing the lavas of the Giant's
Causeway, are laterites. These were found by Delesse to be trap impregnated with
the red oxide of iron, and in part reduced to kaolin. When still more
decomposed, they were found to be clay coloured by red ochre. As two of the
lavas of the Giant's Causeway are parted by a bed of lignite, it is not
improbable that the layers of laterite seen in the Antrim cliffs resulted from
atmospheric decomposition. In Madeira and the Canary Islands streams of lava of
subaerial origin are often divided by red bands of laterite, probably ancient
soils formed by the decomposition of the surfaces of lava-currents, many of
these soils having been coloured red in the atmosphere by oxide of iron, others
burnt into a red brick by the overflowing of heated lavas. These red bands are
sometimes prismatic, the small prisms being at right angles to the sheets of
lava. Red clay or red marl, formed as above stated by the disintegration of
lava, scoriae, or tuff, has often accumulated to a great thickness in the
valleys of Madeira, being washed into them by alluvial action; and some of the
thick beds of laterite in India may have had a similar origin. In India,
however, especially in the Deccan, the term "laterite" seems to have been used
too vaguely to answer the above definition. The vegetable soil in the gardens of
the suburbs of Catania which was overflowed by the lava of 1669 was turned or
burnt into a layer of red brick-coloured stone, or in other words, into
laterite, which may now be seen supporting the old lava-current.
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 nine feet, and those of
Morven an inch or less. (MacCulloch System of Geology volume 2 page 137.) 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.
(FIGURE 588. Lava of La Coupe d'Ayzac, near Antraigue, in the Department 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 has descended and occupied 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. Figure 588 represents
the remnant of the lava at one of these points. 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
degrees, at g, and are nearly horizontal at f, their position having been
everywhere determined, according to the law before mentioned, by the form of the
original valley.
(FIGURE 589. Columnar basalt in the Vincentin. (Fortis.)
In Figure 589, 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. (Fortis Mem. sur l'Hist.
Nat. de l'Italie tome 1 page 233 plate 7.) 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.
(FIGURE 590. Basaltic pillars of the Kasegrotte, Bertrich-Baden, half-way
between Treves and Coblentz. Height of grotto, from 7 to 8 feet.)
The columnar structure is by no means peculiar to the trap rocks in which augite
abounds; it is also observed in trachyte, and other feldspathic rocks of the
igneous class, although in these it is rarely exhibited in such regular
polygonal forms. 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 (Figure 590). 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.
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. According to M. Delesse, the centre of each spheroid has
been a centre of crystallisation, around which the different minerals of the
rock arranged themselves symmetrically during the process of cooling. But it was
also, he says, a centre of contraction, produced by the same cooling, the
globular form, therefore, of such spheroids being the combined result of
crystallisation and contraction. (Delesse sur les Roches Globuleuses Mem. de la
Soc. Geol. de France 2 ser. tome 4.)
(FIGURE 591. Globiform pitchstone. Chiaja di Luna, Isle of Ponza. (Scrope.))
Mr. Scrope gives as an illustration of this structure 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 Figure
591). 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." (Scrope Geological Transactions second series volume 2 page 205.)
VOLCANIC OR TRAP DIKES.
(FIGURE 592. Dike in valley, near Brazen Head, Madeira. (From a drawing of
Captain Basil Hall, R.N.))
The leading varieties of the trappean rocks-- basalt, greenstone, trachyte, 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. 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, scoriae,
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 Figure 592).
(FIGURE 593. Ground-plan of greenstone dikes traversing sandstone. Arran.)
In the islands of Arran and Skye, and in 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. 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. In Figure 593 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 five to
seven feet in width, which will afford a scale of measurement for the whole.
(FIGURE 594. Trap dividing and covering sandstone near Suishnish, in Skye.
(MacCulloch.))
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 downward in veins or dikes, which
probably unite with other masses of igneous rock at a greater depth. The largest
of the dikes represented in Figure 594, and which are seen in part of the coast
of Skye, is no less than 100 feet in width.
Every variety of trap-rock is sometimes found in dikes, as basalt, greenstone,
feldspar-porphyry, and trachyte. The amygdaloidal traps also occur, though more
rarely, 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 eruption 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.
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 heat of melted matter and of the
entangled steam and gases might be expected to cause.
PLAS-NEWYDD: DIKE CUTTING THROUGH SHALE.
A striking example, near Plas-Newydd, in Anglesea, has been described by
Professor Henslow. (Cambridge Transactions volume 1 page 402.) The dike is 134
feet wide, and consists of a rock which is a compound of feldspar 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, of 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 porcelanous 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. (Ibid.
volume 1 page 410.) 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. (Ibid. volume 2 page 175.)
ANTRIM: DIKE CUTTING THROUGH CHALK.
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 eight or ten
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 porcelanous 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." (Dr. Berger Geological Transactions 1st series volume 3 page
172.) All traces of organic remains are effaced in that part of the limestone
which is most crystalline.
(FIGURE 595. Basaltic dikes in chalk in Island of Rathlin, Antrim. Ground-plan
as seen on the beach. (Conybeare and Buckland. (Geological Transactions 1st
series volume 3 page 210 and plate 10.
From left to right: chalk: dike 35 ft.: dike 1 ft.: dike 20 ft.: chalk.)
Figure 595 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. By another, the
shale of the coal-measures has been indurated, assuming the character of flinty
slate; and in another place the slate-clay of the lias has been changed into
flinty slate, which still retains numerous impressions of ammonites. (Ibid.
volume 3 page 213; and Playfair Illustration of Huttonian Theory s. 253.)
It might have been anticipated that beds of coal would, from their combustible
nature, be affected 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 nine feet on each side. At
Cockfield Fell, in the north of England, a similar change is observed. Specimens
taken at the distance of about thirty 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. (Sedgwick Cambridge Transactions volume 2 page 37.)
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 to 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 being
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 heat supplied by the dike and its
gases will act more powerfully.
INTRUSION OF TRAP BETWEEN STRATA.
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 were caused by gases propelling the
lava upward.
RELATION OF TRAPPEAN ROCKS TO THE PRODUCTS OF ACTIVE VOLCANOES.
When we reflect on the changes above described in the strata near their contact
with trap dikes, and consider how complete is the analogy or often identity in
composition and structure of the rocks called trappean and the lavas of active
volcanoes, 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. A large portion of the trappean rocks first studied in the north of
Germany, and in Norway, France, Scotland, and other countries, were such as had
been formed entirely under 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 the 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 downward 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.
(FIGURE 596. Strata intercepted 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 Chapter 6); and this fact prepares us to
expect a similar destruction of whatever may once have formed the uppermost part
of ancient submarine or subaerial volcanoes, 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
Figure 596), 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 volcanoes.
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 above); 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. (MacCulloch Western Islands volume 2 page 487.)
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 volcanoes; for they are
such in every essential point, although they no longer eject fire and smoke."
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. (System of Geology
volume 2 page 114.)
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 volcanoes, 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.
CHAPTER XXIX.
ON THE AGES OF VOLCANIC ROCKS.
Tests of relative Age of Volcanic Rocks.
Why ancient and modern Rocks can not be identical.
Tests by Superposition and intrusion.
Test by Alteration of Rocks in Contact.
Test by Organic Remains.
Test of Age by Mineral Character.
Test by Included Fragments.
Recent and Post-pliocene volcanic Rocks.
Vesuvius, Auvergne, Puy de Come, and Puy de Pariou.
Newer Pliocene volcanic Rocks.
Cyclopean Isles, Etna, Dikes of Palagonia, Madeira.
Older Pliocene volcanic Rocks.
Italy.
Pliocene Volcanoes of the Eifel.
Trass.
Having in the former part of this work referred the sedimentary strata to a long
succession of geological periods, we have now 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: first, superposition and
intrusion, with or without alteration of the rocks in contact; second, organic
remains; third, mineral characters; fourth, included fragments of older rocks.
Besides these four tests it may be said, in a general way, that volcanic rocks
of Primary or Palaeozoic antiquity differ from those of the Secondary or
Mesozoic age, and these again from the Tertiary and Recent. Not, perhaps, that
they differed originally in a greater degree than the modern volcanic rocks of
one region, such as that of the Andes, differ from those of another, such as
Iceland, but because all rocks permeated by water, especially if its temperature
be high, are liable to undergo a slow transmutation, even when they do not
assume a new crystalline form like that of the hypogene rocks.
Although subaerial and submarine denudation, as before stated, remove, in the
course of ages, large portions of the upper or more superficial products of
volcanoes, yet these are sometimes preserved by subsidence, becoming covered by
the sea or by superimposed marine deposits. In this way they may be protected
for ages from the waves of the sea, or the destroying action of rivers, while,
at the same time, they may not sink so deep as to be exposed to that Plutonic
action (to be spoken of in Chapter 31) which would convert them into crystalline
rocks. But even in this case they will not remain unaltered, because they will
be percolated by water often of high temperature, and charged with carbonate of
lime, silex, iron, and other mineral ingredients, whereby gradual changes in the
constitution of the rocks may be superinduced. Every geologist is aware how
often silicified trees occur in volcanic tuffs, the perfect preservation of
their internal structure showing that they have not decayed before the
petrifying material was supplied.
The porous and vesicular nature of a large part, both of the basaltic and
trachytic lavas, affords cavities in which silex and carbonate of lime are
readily deposited. Minerals of the zeolite family, the composition of which has
already been alluded to in Chapter 28, occur in amygdaloids and other trap-rocks
in great abundance, and Daubree's observations have proved that they are not
always simple deposits of substances held in solution by the percolating waters,
being occasionally products of the chemical action of that water on the rock
through which they are filtered, and portions of which are decomposed. From
these considerations it follows that the perfect identity of very ancient and
very modern volcanic formations is scarcely possible.
TESTS BY SUPERPOSITION.
(FIGURE 597. Section through sedimentary mass with melted matter.)
If a volcanic rock rest upon an aqueous deposit, the volcanic 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 below D in Figure 597, 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 (Figure 597), 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, on the other hand, we must conclude the trap
to be intrusive, if the stratum a be altered by b at the point of contact, 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.
(FIGURE 598. Section through sedimentary mass with melted matter.)
We may, however, be easily deceived in supposing the 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, can not 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 (Figure
598) 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 overlying strata, d, e, to have been altered at their junction, as if
by heat.
The test of age by superposition is strictly applicable to all stratified
volcanic tuffs, according to the rules already explained in the case of
sedimentary deposits (see Chapter 8).
TEST OF AGE BY ORGANIC REMAINS.
We have seen how, in the vicinity of active volcanoes, 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 volcanoes now 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 evidence of this kind we can
establish a coincidence in age between volcanic rocks and the different primary,
secondary, and tertiary fossiliferous strata.
The tuffs alluded to may not always be marine, but may include, in some places,
fresh-water 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 ten
feet, for a distance of eight leagues from the crater, in a southerly direction.
Birds, cattle, and wild animals were scorched to death in great numbers, and
buried in ashes. Some volcanic dust fell at Chiapa, upward of 1200 miles, not to
leeward of the volcano, as might have been anticipated, but to windward, 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. (Caldcleugh Philosophical Transactions 1836 page 27.)
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; thus in Iceland, in 1783, 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. (See
Principles Index "Skaptar Jokul.") 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 much in mineral composition and texture.
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; and in Madeira trachytic rocks overlie an older basaltic
series; but the trachyte occupies more generally an inferior position, and is
cut through and overflowed by basalt. It can by no means be inferred that
trachyte predominated 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 lavas
containing feldspar more rich in silica 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
feldspathic lavas; thus, for example, hornblende, augite, and olivine are each
more than three times the weight of water; whereas common feldspar and albite
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 allied
feldspathic lavas. 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 upward to the surface by the expansive power of gases. Those
materials, therefore, which occupy 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 a
conglomerate almost exclusively composed of rolled pebbles of trap, associated
with some fossiliferous stratified formation in the neighbourhood of massive
trap. If the pebbles agree generally in mineral character with the latter, we
are then enabled to determine its relative age 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 volcanoes, as at the base of Etna.
RECENT AND POST-PLIOCENE VOLCANIC ROCKS.
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. They have been perpetually shifting the places where
they have broken out at the earth's surface, and we can sometimes prove that
those areas which are now the great theatres of volcanic action were in a state
of perfect tranquillity at remote geological epochs, and that, on the other
hand, in places where at former periods the most violent eruptions took place at
the surface and continued for a great length of time, there has been an entire
suspension of igneous action in historical times, and even, as in the British
Isles, throughout a large part of the antecedent Tertiary Period.
In the absence of British examples of volcanic rocks newer than the Upper
Miocene, I may state that in other parts of the world, especially in those where
volcanic eruptions are now taking place from time to time, there are tuffs and
lavas belonging to that part of the Tertiary era the antiquity of which is
proved by the presence of the bones of extinct quadrupeds which co-existed with
terrestrial, fresh-water, and marine mollusca of species still living. 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 testacea, but when
certain species of elephant, rhinoceros, and other quadrupeds now extinct,
inhabited Europe.
VESUVIUS.
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 twenty 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 has also been stated (Chapter 13), 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. Some of these strata contain marine shells
which are exclusively of living species, others contain a slight mixture, one or
two per cent of species not known as living.
The ancient part of Vesuvius is called Somma, and consists of the remains of an
older cone which appears to have been partly destroyed by explosion. In the
great escarpment which this remnant of the ancient mountain presents towards the
modern cone of Vesuvius, there are many dikes which are for the most part
vertical, and traverse the inclined beds of lava and scoriae which were
successively superimposed during those eruptions by which the old cone was
formed. 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 one to twelve 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 halfway. 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.
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 fact:
"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.
"These channels (says the same observer), I have found, upon examination after
an eruption, 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." I was able to verify
this phenomenon in 1858, when a stream of lava issued from a lateral cone.
(Principles of Geology volume 1 page 626.) 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 upward, can not 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 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.
(See Chapter 28.)
AUVERGNE.
Although the latest eruptions in central France seem to have long preceded the
historical era, they are so modern as to have a very intimate connection with
the present superficial outline of the country and with the existing valleys and
river-courses. Among a great number of cones with perfect craters, one called
the Puy de Tartaret sent forth a lava-current which can be traced up to its
crater, and which flowed for a distance of thirteen miles along the bottom of
the present valley to the village of Nechers, covering the alluvium of the old
valley in which were preserved the bones of an extinct species of horse, and of
a lagomys and other quadrupeds all closely allied to recent animals, while the
associated land-shells were of species now living, such as Cyclostoma elegans,
Helix hortensis, H. nemoralis, H. lapicida, and Clausilia rugosa. That the
current which has issued from the Puy de Tartaret may, nevertheless, be very
ancient in reference to the events of human history, we may conclude, not only
from the divergence of the mammiferous fauna from that of our day, but from the
fact that a Roman bridge of such form and construction as continued in use only
down to the fifth century, but which may be older, is now seen at a place about
a mile and a half from St. Nectaire. This ancient bridge spans the river Couze
with two arches, each about fourteen feet wide. These arches spring from the
lava of Tartaret, on both banks, showing that a ravine precisely like that now
existing had already been excavated by the river through that lava thirteen or
fourteen centuries ago.
While the river Couze has in most cases, as at the site of this ancient bridge,
been simply able to cut a deep channel through the lava, the lower portion of
which is shown to be columnar, the same torrent has in other places, where the
valley was contracted to a narrow gorge, had power to remove the entire mass of
basaltic rock, causing for a short space a complete breach of continuity in the
volcanic current. The work of erosion has been very slow, as the basalt is tough
and hard, and one column after another must have been undermined and reduced to
pebbles, and then to sand. During the time required for this operation, the
perishable cone of Tartaret, occupying the lowest part of the great valley
descending from Mont Dor (see Chapter 30), and damming up the river so as to
cause the Lake of Chambon, has stood uninjured, proving that no great flood or
deluge can have passed over this region in the interval between the eruption of
Tartaret and our own times.
PUY DE COME.
The Puy de Come and its lava-current, near Clermont, may be mentioned as another
minor volcano of about the same age. This conical hill rises from the granitic
platform, at an angle of between 30 and 40 degrees, to the height of more than
900 feet. Its summit presents two distinct craters, one of them with a vertical
depth of 250 feet. A stream of lava takes its rise at the western base of the
hill instead of issuing from either crater, and descends the granitic slope
towards the present site of the town of Pont Gibaud. Thence it pours in a broad
sheet down a steep declivity into the valley of the Sioule, filling the ancient
river-channel for the distance of more than a mile. The Sioule, thus
dispossessed of its bed, has worked out a fresh one between the lava and the
granite of its western bank; and the excavation has disclosed, in one spot, a
wall of columnar basalt about fifty feet high. (Scrope's Central France page 60
and plate.)
The excavation of the ravine is still in progress, every winter some columns of
basalt being undermined and carried down the channel of the river, and in the
course of a few miles rolled to sand and pebbles. Meanwhile the cone of Come
remains unimpaired, its loose materials being protected by a dense vegetation,
and the hill standing on a ridge not commanded by any higher ground, so that no
floods of rain-water can descend upon it. There is no end to the waste which the
hard basalt may undergo in future, if the physical geography of the country
continue unchanged-- no limit to the number of years during which the heap of
incoherent and transportable materials called the Puy de Come may remain in an
almost stationary condition.
PUY DE PARIOU.
The brim of the crater of the Puy de Pariou, near Clermont, is so sharp, and has
been so little blunted by time, that it scarcely affords room to stand upon.
This and other cones in an equally remarkable state of integrity have stood, I
conceive, uninjured, not IN SPITE of their loose porous nature, as might at
first be naturally supposed, but in consequence of it. No rills can collect
where all the rain is instantly absorbed by the sand and scoriae, as is
remarkably the case on Etna; and nothing but a water-spout breaking directly
upon the Puy de Pariou could carry away a portion of the hill, so long as it is
not rent or ingulfed by earthquakes.
NEWER PLIOCENE VOLCANIC ROCKS.
The more ancient portion of Vesuvius and Etna originated at the close of the
Newer Pliocene period, when less than ten, sometimes only one, in a hundred of
the shells differed from those now living. In the case of Etna, it was before
stated (Chapter 13) that Post-pliocene formations occur in the neighbourhood of
Catania, while the oldest lavas of the great volcano are Pliocene. These last
are seen associated with sedimentary deposits at Trezza and other places on the
southern and eastern flanks of the great cone (see Chapter 13).
CYCLOPEAN ISLANDS.
The Cyclopean 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.
(FIGURE 599. View of the Isle of Cyclops, in the Bay of Trezza. (Drawn by
Captain Basil Hall, R.N.))
The loftiest of the Cyclopean 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 Figure 599) 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 Figure 599), 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.
(FIGURE 600. Contortions of strata in the largest of the Cyclopean Islands.)
(FIGURE 601. Newer Pliocene strata invaded by lava. Isle of Cyclops (horizontal
section).
a. Lava.
b. Laminated clay and sand.
c. The same altered.)
In Figure 600 I have represented a portion of the altered rock, a few feet
square, where the alternating thin laminae of sand and clay are contorted in a
manner often observed in ancient 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 Figure 601). 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
honey-comb 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
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