The trilobites are of large size; _Paradoxides Davidis_ (see Fig. 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 (Fig. 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.
Fig. 573: Histioderma Hibernica.
Harlech Grits.—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 Fig. 460) brought
to light by Mr. Salter in Shropshire, and Dr. Kinahan in Wicklow, and
an obscure crustacean form, _Palæopyge Ramsayi,_ they were supposed to
be barren of organic remains. Now, however, through the labours of Mr.
Hicks,[5] 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.
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_ (Fig. 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 Fig. 444 and 445.
Lanberis Slates.—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.
Fig. 574: Oldhamia radiata.
Fig. 575: Oldhamia antiqua.
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 Étage C;
his two first stages A and B consisting of crystalline and metamorphic
rocks and unfossiliferous schists. This Étage 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.
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.
_Fossils of the lowest Fossiliferous Beds in Bohemia, or
“Primordial Zone” of Barrande._
Fig. 576: Paradoxides Bohemicus. Fig. 577: Conocoryphe striata. Fig.
578: Agnostus integer. Fig. 579: Agnostus Rex. Fig. 580: Sao hirsuta in
its various stages of growth.
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 “Palæontologica
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)._—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_ (Fig. 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._
Fig. 581: Dikelocephalus Minnesotensis.
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.
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 Figs. 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.
Fig. 582 and 583: Eozoon Canadense.
Fig. 582. _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. (Fig. 583 shows these
bodies in a decalcified state.) _f._ Stoloniferous passages.
Fig. 583. 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.
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 Fig. 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.
[1] Quart. Geol. Journ., vol. iii, p. 156.
[2] This genus has been substituted for Barrande’s _ Conocephalus,_ as
the latter term had been preoccupied by the entomologists.
[3] Geol. Mag., vol iv.
[4] British Association Report 1865, 1866, 1868 and Quart. Geol.
Journ., vols. xxi, xxv.
[5] Brit. Assoc. Report, 1868.
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.
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 (Fig. 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._
Fig. 584: a. Hypogene formations, stratified and unstratified. b.
Aqueous formations. c. Volcanic rocks.
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 subÆrial 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.
Fig. 585: Part of the chain of extinct volcanoes called the Monts Dome,
Aurvergne.
_Cones and Craters._—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 _scoriæ,_ 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 scoriæ 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 Fig. 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° to 20°
or even more than 30°, 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 quâquâversal 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.[1]
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
subaërial 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 1½ foot to 26 feet. And their planes of
stratification are parallel to those of the overlying and underlying
scoriæ which form part of the same currents.[2]
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.—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 (p.504).
_Analysis of Minerals most abundant in the Volcanic and Hypogene
Rocks._
THE QUARTZ GROUP QUARTZ 100·0
2·6 Silica
Specific gravity TRIDYMITE 100·0
2·3 Silica
Specific gravity THE FELDSPAR GROUP ORTHOCLASE.
—— Carisbad, in granite (bulk) 65·23
16·26
0·27
nil
trace
nil
14·66
1·45
nil
2·55 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Sanadine, Drachenfels in trachyte
(Rammelsberg) 65·87
18·53
nil
nil
0·95
0·30
10·32
3·49
W. 0·44
2·55 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity ALBITE.
—— Arendal, in granite (G. Rose) 68·46
19·30
nil
0·28
0·68
nil
nil
11·27
nil
2·61 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity OLIGOCLASE.
—— Ytterby, in granite (Berzelius) 61·55
23·80
nil
nil
3·18
0·80
0·38
9·67
nil
2·65 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Teneriffe, in trachyte (Deville) 61·55
22·03
nil
nil
2·81
0·47
3·44
7·74
nil
2·59 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity LABRADORITE.
—— Hitteroe, in Labrador-rock (Waage) 51·39
29·42
2·90
nil
9·44
0·37
1·10
5·03
W. 0·71
2·72 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Iceland, in volcanic (Damour) 52·17
29·22
1·90
nil
13·11
nil
nil
3·40
nil
2·71 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity ANORTHITE.
—— Harzburg, in diorite (Streng) 45·37
34·81
0·59
nil
16·52
0·83
0·40
1·45
W. 0·87
2·74 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Hecla, in volcanic (Waltershausen) 45·14
32·10
2·03
0·78
18·32
nil
0·22
1·06
nil
2·74 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity LEUCITE.
—— Vesuvius, 1811, in lava (Rammelsberg) 56·10
23·22
nil
nil
nil
nil
20·59
0·57
nil
2·48 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity NEPHELINE.
—— Miask, in Miascite (Scheerer) 44·30
33·25
0·82
nil
0·32
0·07
5·82
16·02
nil
2·59 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Vesuvius, in volcanic (Arfvedson) 44·11
33·73
nil
nil
nil
nil
nil
20·46
W. 0·62
2·60 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity THE MICA GROUP MUSCOVITE.
—— Finland, in grante (Rose) 46·36
36·80
4·53
nil
nil
nil
9·22
nil
F. 0·67
W. 1·84
2·90 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity LEPIDOLITE.
—— Cornwall, in granite (Regnault) 52·40
26·80
nil
1·50
nil
nil
9·14
nil
F. 4·18
Li. 4·85
2·90 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity BIOTITE.
—— Bodennais (V. Kobel> 40·86
15·13
13·00
nil
nil
22·00
8·83
nil
W. 0·44
2·70 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Vesuvius, in volcanic (Chodnef) 40·91
17·71
11·02
nil
0·30
19·04
9·96
nil
nil
2·75 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity PHLOGOPITE.
—— New York, in metamorphic limestone (Rammelsberg) 41·96
13·47
nil
2·67
0·34
27·12
9·37
nil
F. 2·93
W. 0·60
2·81 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity MARGARITE.
—— Nexos (Smith) 30·02
49·52
1·65
nil
10·82
0·48
1·25
W. 5·55
2·99 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
=Potash
=Soda
Other constituents
Specific gravity RAPIDOLITE.
—— Pyrenees (Delesse) 32·10
18·50
nil
0·06
nil
36·70
nil
nil
W. 12·10
2·61 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity TALC.
—— Zillerthal (Delesse) 63·00
nil
nil
trace
nil
33·60
nil
nil
W. 3·10
2·78 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity THE AMPHIBOLE AND PYROXENE GROUP TREMOLITE.
—— St. Gothard (Rammelsbeg) 58·55
nil
nil
nil
13·90
26·63
nil
nil
F.W. 0·34
2·93 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity ACTINOLITE.
—— Arendal, in granite (Rammelsberg) 56·77
0·97
nil
5·88
13·56
21·48
nil
nil
W. 2·20
3·02 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity HORNBLENDE.
—— Faymont, in diorite (Deville) 41·99
11·66
nil
22·22
9·55
12·59
nil
1·02
W. 1·47
3·20 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Etna, in volcanic (Waltershausen) 40·91
13·68
nil
17·49
13·44
13·19
nil
nil
W. 0·85
3·01 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity URALITE.
—— Ural, (Rammelsberg) 50·75
5·65
nil
17·27
11·59
12·28
nil
nil
W. 1·80
3·14 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity AUGITE.
—— Bohemia, in dolerite (Rammelsberg) 51·12
3·38
0·95
8·08
23·54
12·82
nil
nil
nil
3·35 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Vesuvius, in lava of 1858 (Rammelsberg) 49·61
4·42
nil
9·08
22·83
14·22
nil
nil
nil
3·25 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity DIALLAGE.
—— Harz, in Gabbro (Rammelsberg) 52·00
3·10
nil
9·36
16·29
18·51
nil
nil
W. 1·10
3·23 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity HYPERSTHENE.
—— Labrador, in Labrador-Rock (Damour) 51·36
0·37
nil
22·59
3·09
21·31
nil
nil
nil
3·39 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity THE OLIVINE GROUP BRONZITE.
—— Greenland (V. Kobell) 58·00
1·33
11·14
nil
nil
29·66
nil
nil
nil
3·20 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity OLIVINE.
—— Carlsbad, in basalt (Rammelsberg) 39·34
nil
nil
14·85
nil
45·81
nil
nil
nil
3·40 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity —— Mount Somma, in volcanic (Walmstedt) 10·08
0·18
nil
15·74
nil
44·22
nil
nil
nil
3·33 Silica
Alumina
Sesquioxide of Iron
Protoxides of Iron and Manganese
Lime
Magnesia
Potash
Soda
Other constituents
Specific gravity
In the “Other constituents” the following signs are used: F=Fluorine,
Li=Lithia, W=Loss on igniting the mineral, in most instances only
Water.
From the table above 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.[3] 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, p. 499), 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, p. 499) 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, p. 499) 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 the table 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 (see page 499),
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 Haüy, 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, p. 499) 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
cannot 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.[4]
_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 XXXI.
Trachytic Rocks.—The name trachyte (from [**Greek]_ 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.
Fig. 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 Fig. 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 cannot
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.
Fig. 587: Scoriaceous lava in part converted into an amygdaloid.
Fig. 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.[5]
_Scoriæ_ and _Pumice_ may next be mentioned, as porous rocks produced
by the action of gases on materials melted by volcanic heat. _Scoriæ_
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
scoriæ 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 scoriæ 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
subaërial 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,
scoriæ, 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.[6] 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.
Fig. 588: Lava of La Coupe d’Ayzac, near Antraigue, in the Department
of Ardêche.
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 scoriæ. 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. Fig. 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°, 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.
Fig. 589: Columnar basalt in the Vicentin.
In Fig. 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.[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.
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 (Fig.
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.
Fig. 590: Basaltic pillars of Käsegrotte, Bertrich-Baden, half-way
between Trèves and Coblenz.
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.[8]
Fig. 591: Globiform pitchstone. Chiaja di Luna, Isle of Ponza.
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 Fig. 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 laminæ of this nucleus have not been so much
loosened by decomposition; but the application of a ruder blow will
produce a still further exfoliation.”[9]
Fig. 592: Dike in valley, near Brazen Head, Madeira. (From a drawing of
Captain Basil Hall, R.N.)
Volcanic or Trap Dikes.—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, scoriæ, 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
Fig. 592).
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 Fig. 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.
Fig. 593: Ground-plan of greenstone dikes traversing sandstone.
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 Fig. 594, and which are seen in part of the coast of Skye, is no
less than 100 feet in width.
Fig. 594: Trap dividing and covering sandstone near Suishnish, in Skye.
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.[10]
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.[11] 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.[12]
_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.”[13] All traces of organic remains are effaced in that part of
the limestone which is most crystalline.
Fig. 595: Basaltic dikes in chalk in Island of Rathlin, Antrim.
Ground-plan as seen on the beach. Fig. 595: Basaltic dikes in chalk in
Island of Rathlin, Antrim. Ground-plan as seen on the beach. (Conybeare
and Buckland[14])
Fig. 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.[15]
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.[16]
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 scoriæ
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 scoriæ, 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
Ætherias, 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.
Fig. 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 VI); and this fact
prepares us to expect a similar destruction of whatever may once have
formed the uppermost part of ancient submarine or subaërial 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 Fig. 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 p. 507); 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.[17]
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.[18]
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 scoriæ 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.
[1] Principles, vol. ii, pp. 56 and 145.
[2] Memoir on Mount Etna, Phil. Trans., 1858.
[3] For analyses of these minerals see the Mineralogies of Dana and
Bristow.
[4] Dr. Daubeny on Volcanoes, 2nd ed., pp. 14, 15.
[5] G. Hose, Ann. des Mines, tome viii, p. 32.
[6] MacCulloch Sys. of Geol., vol. ii, p. 137.
[7] Fortis, Mém. sur l’Hist. Nat. de l’Italie, tome 1., p. 233, plate
7.
[8] Delesse, sur les Roches Globuleuses, Mém. de la Soc. Géol. de
France, 2 sér., tome iv.
[9] Scrope, Geol. Trans., 2nd series, vol. ii, p. 205.
[10] Cambridge Transactions, vol. i, p. 402.
[11] Ibid., vol. i, p. 410.
[12] Ibid., vol. ii, p. 175.
[13] Dr. Berger, Geol. Trans., 1st series, vol. iii, p. 172.
[14] Geol. Trans., 1st series, vol. iii, p. 210 and plate 10.
[15] Ibid., vol. iii, p. 213; and Playfair, Illus. of Hutt. Theory, s.
253.
[16] Sedgwick, Camb. Trans., vol. ii, p. 37.)
[17] MacCulloch, West. Islands, vol. ii, p. 487.
[18] Syst. of Geol., vol. ii, p. 114.
CHAPTER XXIX.
ON THE AGES OF VOLCANIC ROCKS.
Tests of relative Age of Volcanic Rocks. — Why ancient and modern Rocks
cannot 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 Côme, 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 Palæozoic 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 subaërial 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 XXXI) 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, p. 500, occur in
amygdaloids and other trap-rocks in great abundance, and Daubrée’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.
Fig. 597: Showing melted matter forced between two strata.
Tests by Superposition.—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 Fig. 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 (Fig. 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.
Fig. 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, cannot rest everywhere
upon the same stratum, either because these have been denuded, or
because, if newly thrown down, they thin out in certain places, thus
allowing the lava to cross their edges. Besides, the heavy igneous
fluid will often, as it moves along, cut a channel into beds of soft
mud and sand. Suppose the submarine lava F (Fig. 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 p. 124).
Test of Age by Organic Remains.—We have seen how, in the vicinity of
active volcanoes, scoriæ, 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 scoriæ 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.[1]
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.[2] 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 2½ 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.