John Pinkerton.

Petralogy. A treatise on rocks (Volume 1) online

. (page 8 of 35)
Online LibraryJohn PinkertonPetralogy. A treatise on rocks (Volume 1) → online text (page 8 of 35)
Font size
QR-code for this ebook

have pre-existed the fusion, and withstood it.
The form of these crystals, their laminar struc-
ture perfectly preserved, their transparency,
their facility of melting, their manner of being
in these vast masses, and in short their analogy
of composition with the paste which surrounds
them, leads one to believe that they were formecj
during their igneous fluidity, by an approxi-
mation of their integrant parts, which were able
to obey the laws of their affinity. These por-
phyroids are the most ancient of all the volcanic
productions of Auvergne : they are covered with
basalt, and contain veins of that substance.

" However different these productions may
be, however distant the various periods of their
formation, they do not seem the less united in a
certain degree, and form, in some sort, an identic
system. Cantal, Mont Dor, Puy de Dome, &c.
the most ancient of the volcanic masses, are in.


a direct line, running nearly from south to north.
Almost all the basalts of these regions, that may
be, in some manner, retraced to their origin,
seem to have taken their direction in this same
line. It is also in this direction, and among the
ancient products, that the greater part of the
craters have opened, whose vestiges are still
visible. When at two leagues to the westward
of Clermont, we see near sixty volcanic moun-
tains, ranged in a straight line, one can scarcely
believe it to be the effect of chance. A cause
has certainly existed which has produced this
effect : perhaps there was under-ground, and in
this direction, as it were a vein of matter which
contained the germ of volcanic fire, or which
was of a nature to maintain it ; the cause always
existing, its effect might have been renewed at
different periods."

These observations are no doubt cogent, and
worthy of the acknowledged ability of the au-
thor. But in the spirit of perfect candour when
treating a subject where it is difficult, after every
allowance for the weakness of the human mind,
even to suppose that prejudices should exist,
Brochant's able abstract of the arguments for
and against the volcanic origin of basaltin, shall
here be subjoined, as the author thinks it a me-
ritorious service to the science, to lay before the


English reader interesting extracts from such
works in the foreign languages, as, from their
very nature, can scarcely be expected ever to he
known to him by translation.

?t r atement s . * Jt has already been said in the Introduction,
p. 68, that mountains of secondary trap, and
chiefly the basalts, were looked upon by some
mineralogists as produced by volcanic fires, while
Mr. Werner, and almost all the learned men of
Germany, are of opinion, that they have been
formed, like other rocks, by the waters which
inundated the surface of the globe.

" The former ground their opinion on the fol?
lowing reasons.

" 1. In the masses produced by volcanoes
which have burnt in our time, are found pris-r
matic basalts, and other rocks which resemble
trap, and which nevertheless bear no character
of fusion, and which only the locality and po-
sition indicate as volcanic.

" 2. It is an error to believe that all the
masses ejected by volcanoes must be vitrified
substances ; such are on the contrary very rare.

" 3. Neither is the black colour essential to
volcanic products : there are some grey, others
brown, and even white.

" 4. Many observations have proved, that the
fire of volcanoes is very inferior to that of our


furnaces ; therefore it is not surprising that ba*
salts may be changed by an artificial fusion, and
it is no reason for believing that they have not
before undergone the action of volcanic fires.

" 5. Even supposing that volcanic fire pos-
sesses a great heat, it is known, by the beautiful
experiments, made in England by sir James Hall,
comparatively on the whinstone*, and on the
lavas of Vesuvius, that a contexture and aspect
may be given to a rocky mass, melted and cooled,
which shall have the characters of glass or stone,
according as the cooling is quicker or slower.
These experiments having been repeated on
several kinds of whinstone, there has. always been
obtained by a gentle cooling, a stony mass, com-
pact, dull, exactly similar to the whinstone em-
ployed; and on the contrary, a vitreous mass
was obtained by a rapid cooling. The same
essays made on the substance of bottle glass,
gave the same results.

" We see then that the absence of scoriae and *
vitrifications is not a reason for denying the vol-
canic origin of basalt; besides, it is a known
fact that burning volcanoes have produced it.

" * The whinstone of the English is generally a secondary trap ;
but among several specimens that I have seen given under that
name, some resembled basalt, others grunstein ; others in fine had
ihe structure of amygdalite."


" 6. But without mentioning basalts, which
have so many volcanic characters, a great ana-
logy is found between secondary mandelstein
and porous lavas; between the clay of trap
mountains and the products of muddy eruptions;
between basaltic tufa and volcanic tufa; and
almost all minerals which are found scattered
in volcanic masses, are found in trap moun-
tains, &c.

" 7. The position of secondary traps, which
lie over all secondary rocks, while the hardness,
compactness, and other characters of many
among them, such as grunstein, basalt, and
others, are so different from those of the se-
condary rocks on which they lie. That kind
of dryness to the feel which they present, and
which is characteristic of volcanic productions
in general, all these approximations will not
permit us to acknowledge that basalts have had
the same origin as all secondary rocks.

" 8. It has been objected that in basaltic
countries basalt is found on almost all the sum-
mits, and that such would not be the case if
basalts were lavas: this might be true if these
lavas proceeded from a recent deposition ; but
on the contrary this deposit seems very ancient,
and has undergone many changes. M. Reuss
himself observed in Bohemia, that the basaltic


summits he there met with, seem to be the
remains of a vast bed of decayed basalt.

" 9. In short, the conical form of trap moun-
tains, and above all those of basalt, has the most
perfect resemblance to that of volcanic moun-
tains ; and it is this resemblance which gave the
first idea of attributing a volcanic origin to

" The advocates for the formation by the hu-
mid way, or the Neptunists, on their part sup-
port their opinions by many observations, of
which these are the principal :

"1. It is true basalts are found among pro-
ducts of burning volcanoes, but they are ex-
tremely rare, and modern eruptions have not
produced any.

"2. Whatever origin may be attributed to
the division in prisms, tables, &c. it is not pecu-
liar to trap rocks: there are gypsums, marls,
sand-stones, which frequently offer this struc-
ture. Thus then this division in prisms, very
rare among real volcanic products, on the con-
trary exists in many stratified rocks.

" 3. Basalts often repose immediately on coal,
as at Meissner, near Cassel : now, if this basalt
was volcanic, it must necessarily have produced
$he combustion of these beds of coal,

" 4. The remains of vegetables and animals,


which are found in some trap rocks, could not
in like manner have resisted the volcanic heat
without being destroyed.

" It is the same with many minerals which
are very fusible, and which are there met with :
indeed, some are also found in volcanic rocks,
but these instances are rare, and cannot serve as
a basis to a general rule.

" 5. Cavities filled with water, such as en-
hydritic agates found near Vicenza, in Italy, in
secondary trap mountains, entirely destroy all
supposition of a volcanic origin*.

" 6. There are not observed in trap rocks either
that black colour, or those indications of vitri-
faction, that are apparent, at least in certain por-
tions, of the products of burning volcanoes : real
craters have never been observed. All those
which have been cited were hollows, chasms
filled with water, so common in some moun-

" 7. Mandelstein has certainly some resem-
blance to porous lava; but there are mandel-
steins evidently not volcanic. Besides, the
cavities of the mandelstein of trap mountains
contain very different minerals, and which could

" * The Vulcanists answer that these agates have a latter origin
rom infiltration."


not have undergone the action of fire without
being changed*.

" 8. It is true, that according to the experi-
ments of sir James Hall, and some late ob-
servations made upon burning volcanoes, it is
known that rocky substances may, after fusion,
reassume their stony character; but when this
takes place in burning volcanoes, there are al-
ways found in the vicinity substances which
have not experienced this effect, and which on
the contrary are scorified or vitrified; which
denotes the action of fire.

" 9. In different countries, and especially in
Bohemia and the Vicentine, beds of basalt have
been observed, which alternate with grit, or stra-
tiform limestone : does not this reunion of these
two rocks prove that they have had the same
origin ? The Vulcanists, to make this agree with
their theory, are obliged to have recourse to
quite a forced supposition, according to which
there have been alternately volcanic eruptions
and submarine deposits ; whereas this alternation
of beds of different rocks, of nearly contemporary
formation, has more than one example in moun-

" 10. There are many basaltic regions where

" * The same observation, as in note upon article 5."


basalt is only found on summits, and it is evi-
dently perceived by the correspondence of the
beds ; that all these summits were parts of one
and the same bed, which spread over all the
country : now, that is not the nature of volcanic
deposits ; they form currents, which take a cer-
tain direction, and no similar examples of such
vast deposits are known, but among rocks pro-
duced by water, and particularly among strati-
form rocks.

"11. Basalt has no appearance of fusion ;
heated in a furnace it melts to glass. It is true,
that from Hall's experiments, a stony substance
has been obtained ; and that may very well hap-
pen, since nature produces it in burning vol-
canoes. But these cases are very rare, and Hall
has justly observed, that in his experiments this
appearance depended on the management of the
cooling : but it must then be supposed, that this
circumstance is always met with in the volcanic
eruptions, which are supposed to have produced
the mountains of trap.

" 12. The prismatic division of basalt has
been attributed to the water of the sea, which
they say then covered all the region upon which
these lavas have run: that is possible; but this
accelerated refrigeration should, according to
Hall's experiments, give the lavas a vitreous


appearance ; which is not the case even in por-
tions of the mass.

" 13. The conical form of basaltic mountains
proves nothing ; it is true, that such is the form
of volcanic mountains, but in general it is that
of all mountains whose sides are covered with
earthy substances. Melted substances, ashes,
give this form to volcanic mountains; and if
basaltic mountains assume also more particularly
this appearance, it is because their fragments
are quickly reduced to this earthy state, so that
they naturally form slopes on the sides of moun-

" Moreover, the conical form of basaltic moun-
tains is not that of burning volcanoes: the former
are cones, isolated one from another, nearly
equal in height; whereas volcanic mountains
are grand coniform elevations, whose slopes and
sides are loaded with little conical summits.

<e One might extend much farther this chain
of motives on which both theories are founded,
but longer details would be here superfluous*;

" * It may be observed, that the points of division are often in
matters of fact; as the existence of scoriae, vitrifactions, that of
traters, &c. I do not pretend to discuss their legitimacy.

*' Perhaps both parties may think that I have not done justice to
their arguments, and that I have overlooked some important ones.
I believe not : I endeavoured to reconcile them, at least the princi-
pal ; but I confess if any have escaped me, I should easily console


time may perhaps some day afford the definitive
solution of this great geological problem. Do-
Jomieu occupied himself much upon it; and he
doubtless would have succeeded in uniting both
parties, if death had not overtaken him in the
midst of his labours. He adopted neither of the
two opinions : he was persuaded that both were
admissible, according to localities; because hav-
ing often seen in the products of the burning
volcanoes of Italy, rock entirely resembling ba-
salt, and even other primitive rocks, he had
found by long experience, that only the charac-
ters of locality would decide on the origin of
either. He had, according to this principle,
observed some basaltic countries, among others
Auvergne and the Vicentine, and he had re-
garded them as volcanic. I chiefly cite these
two examples, because I know that many cele-
brated German mineralogists are of a contrary

A yet later French mineralogist has thus ex-
pressed his sentiments upon this curious and
long- agitated subject.

myself, if I thought that would induce the advocates of the two
opinions to publish fresh memqjrs, to undertake their own defence.
This great quarrel has been long hushed, and probably both parties
have collected new observations."
* Brochant, ii. 6l2.


cc We shall give a third opinion upon the ori-
gin of basalt, in a medium between the two pre-
ceding ones, and which appears to us the most
probable. The naturalists who profess it, as
Fortis, Dolomieu, Delrio, Spallanzani, think that
the discussion on basalt is often a dispute of
mere words : that if this name is given to those
stones whose characters we explained at the be-
ginning of this article; some are truly volcanic;
while others have entirely an aqueous origin;
that the basalts of Saxony, and those of Ethiopia,
certainly belong to this second division, and that
it is probable that those of Scotland and Ireland
also belong to it; while those of Italy, and Au-
vergne, should be arranged in the first class to-
tally, or at least in part.

"Other naturalists, and particularly M.Patrin,
imagine that basalts are the productions of the
muddy eruptions of submarine volcanoes ; and
that the nature of the eruption, and the influence
of the water, have given to this lava those par-
ticular characters for which it is remarkable.
They believe that the latter influence prevented
the basaltic matter from calcining or burning
those substances on which it flowed. This hy-
pothesis, which seems one of the most probable,
if not applied without exception to all basalts,
explains well enough the alternation of beds of
VOL. i, F


prismatic basalt with beds of basalt, or stony
and earthy matter without order j that of these
same beds of basalt with sand-stone, with car-
bonate of lime, or with coal, which are not
altered by it; in short, the presence of fossil
shells in some basaltic beds. The causes which,
in this hypothesis, concurred in the formation of
prismatic basalt, no longer existing, we see why
basalt is no longer formed in those vast currents
of lava which in our days have issued from vol-
canoes. It seems that it is with basalt, as with
veins, crystallised beds, fossils properly so called,
&c. Nature in her present quiescent state no
longer forms any*."

The extent of these observations will be par-
doned, as there is not, in this science, a topic
more difficult or interesting : but we must now
return to a more immediate view of this cele-
brated substance.


This rock, as already mentioned, is the trap of
the Swedes, who first recommended it to modern
notice ; while the basaltic columns of Saxony had

* Brongniart, i. 473. He had observed, p. 4?0, that lava enter-
ing the sea becomes fixed on the surface, and does not assume a
columnar form, which rather proceeds from slow cooling.


been observed by Agricola, the restorer of mine-
ralogy in the sixteenth century. Whatever be
their origins, these two substances are identically
the same ; as the same results may be produced
either by the humid or the dry processes of che-

Aspect 1. Uniform. The columnar basaltin
had, as already mentioned, attracted great atten-
tion by the beauty and regularity of its forms, as
early as the sixteenth century ; but trap, or stra-
tiformed basalt, may be called a discovery of the
Swedes. The hill of Kinnekulla, in Westrogothia,
was one of the first observed; and also that of
Hunneberg, in the same province.

Black basaltin, from Kinnekulla and other parts
of Sweden.

Grey basaltin, from the same.

Greenish, from Norberg.

Reddish, from Sweden.

Black basaltin, with small needles or scales of
siderite, from Sweden.

Stratified basaltin, from Faroe, Staffa, the
Giants' Causey, &c. where it sometimes underlies
the columnar.

The same, from the castle hill of Edinburgh,
Dunbarton, and other parts of the south of Scot-

F 2


The same, from Andernach on the Rhine.

The same, from the Sierra Morena, or Black
Mountains, in Spain.

The same, from Toplitz in Bohemia.

Black basaltin, from Egypt.

Green, from the same.

Red basaltin, from Channelkirk in Scotland.

Brown, from the same.

Stratiformed basaltin, from Saxony.

The same, from Etna, Vesuvius, the isle of
Bourbon, New Spain, and other volcanic re*

Aspect 2. Mingled. Basaltin, with nodules
of steatite, from the isle of Skey in Scotland,
Westrogothia, &c.

Black basaltin, with red zeolite, from Sweden.

The same, with white zeolite, from Staffa, Giants'
Causey, c.

The same, with many beautiful varieties of zeo-
lite, from the Faroe isles.

The same, with zeolite, from Etna, &e.

The same, with grains of pyrites, from Hunne-
berg in Sweden, Dauphiny, &c.

* Saussure mentions, 14Q7, a kind of basalt which may be
called laminar: and, 548, a singular roche de corne, (basaltin ?)
in thin leaves, with mica, quartz, and felspar. If compact, he
says, it would have formed a genuine porphyry. How ?


The same, with nodules of calcareous spar,

from various parts.

The same, with red jasper, from Derbyshire,
Basaltin, passing in veins through granite, from

Norway. With inherent pitchstone, from the


Aspect 3. Basaltic Tufa. This substance has
been observed at Staffa, and in some other basaltic
countries. A considerable portion of Arthur's
seat, near Edinburgh, is composed of it.

Aspect 4. Basaltic Bricia. Bricia, with frag-
ments of granite, on a base of basaltin, either
black, grey, or green, from Westrogothia in Swe-
den, or from Dauphiny in France.

The same base, with fragments of quartz, from
the same.

The same, with fragments of limestone, from
the Alps of Dauphiny.

The same, with fragments of slate, from the
same, and from the mountain of Tarare, near

The same, with fragments of granite, slate, and
limestone, all mingled, from Tarare, and Dau-

The same, with fragments of porphyry, from
the hill of Lesterelle in Provence.


Bricia of fragments of basaltin, joined by a
cement of quartz, intermixed with particles of
basaltin. Uncertain.


Basaltin of Aspect 1. Uniform. Basaltin, from Stolpen,
in Saxony. Remarkable as having attracted the
attention of Agricola, and other naturalists since
the sixteenth century. The little town of Stolpen
is built upon the side of a basaltic hill, a few
miles to the east of Dresden. The lower part of
the hill consists of a granite, of white felspar,
grey quartz, and black mica, upon which the ba-
salt reposes, presenting the most beautiful and
regular columns observable in Germany*. They
have commonly six sides; but some have four,
five, seven, or eight; yet their length does not
seem to exceed fifteen or sixteen feet. The co-
lumns are vertical; .but on the south-east there is
a rock of stratified basalt, of that kind which ap-
pears in thin plates or tables. The basaltin of
Stolpen is black with a bluish cast, the grain being
impalpable, the fracture conchoidal, and the frag-
ments sharp. Its hardness, like that of siderite
and basalt, equals that of iron the hardest metal,
being more than 800 of Quist's gradation. This

* Daub, sur les basaltcs, 42.


basaltin often presents little cavities, lined with
chalcedony, and quartz crystals ; sometimes filled
with green steatite, calcareous spar, zeolite, or a
lithomarga, resembling semiopal. Small grains of
olivine also occur, and dots of siderite, or perhaps
augite. The pillars are used for many useful and
ornamental purposes of architecture ; an example
which might be followed in other basaltic coun-
tries, with a sacred regard however to the more
regular, grand, and conspicuous parts.

Columnar basaltin, from Italy, Sicily, Auvergne,
Hungary, Bohemia, Saxony, Lusatia, Thuringia,
Hessia, Goetingen, Nassau, in Germany; from
the isle of Bourbon, New Zealand, and other isles
in the South Sea, &c. &c. The columns are often
$o small as to be chosen as specimens.

Aspect 2. Mingled. Columnar basaltin, min-
gled with zeolite, from many countries.

With nodules of steatite, calcareous spar, chal-
cedony, lithomarga, olivine, &c. from Stolpen, and
other places.






Texture coarse, and of a large grain, mixed
with quartz or felspar, but lax, and incapable of
the fine polish of basalt pr basaltin.

Hardness marmoric. Fracture commonly
even. Fragments blunt and amorphous.

Weight sometimes siderose, generally gran>

Lustre glimmering. Opake.

Colour grey or greenish.

As the Italian termination ino designates di-
minutives and substances of a finer nature, so
that in one is employed to discriminate those of
a coarse appearance or large grain. Hence the
name basalton is adopted for another branch of
the basaltic family, that called gnmsteins by the
Germans, an appellation alike vague and baiv
barous, as are most of those terms derived from
the vulgar miners. The most important ancj
beautiful of the grunsteins, a mixture of crystal
lised siderite with felspar, has been already de-
scribed after siderite. . By basalton are under-
stood the other kinds of grunstein, except the
porphyries ; being a mixture of coarse basalt,
without the splendour or cohesion of that sub-
stance, with either felspar or quartz. Even that


with a finer grain must still be regarded as a
coarse rock, as it does not admit the polish of
basalt or basaltin. The common whin-stones
of the north of England and of Scotland belong
to this class. It is unnecessary to indicate many
examples of so common a substance, which is
chiefly interesting from its intimate connexion
with basalt and basaltin, often passing either
into the one or the other of these substances.

Werner has considered grunstein as either
primitive or stratiform. The former has been
here described under the venerable name of
Wallerite; the latter, which commonly covers
the beds of basalt, is that about to be mentioned.
It would appear that he has since added a trans-
itive grimstein, distinguished by veins or grains
of quartz, in Voigtland cabled kberfetts, or li-
ver rock, being coloured with a reddish brown
oxyd of iron. This transitive grunstein occurs
in the Hartz, in Bohemia; and, according to
Mr. Jameson, in the upper part of Dumfries-
shire. The Wernerians regard grunstein as a
more chemical solution than basalt, though it
commonly rest upon the latter; while in general
the more chemical dissolutions are the lowest :
a circumstance which they endeavour to ex-
plain by supposing the superincumbent waters
more agitated at one period than at another.



Compact basalton, from some of the interior
pillars of Stonehenge.

Online LibraryJohn PinkertonPetralogy. A treatise on rocks (Volume 1) → online text (page 8 of 35)