VOLCANIC ROCKS.
Trap rocks--Name, whence derived--Their igneous origin at first
doubted--Their general appearance and character--Volcanic cones and
craters, how formed--Mineral composition and texture of volcanic
rocks--Varieties of felspar--Hornblende and augite--Isomorphism--Rocks,
how to be studied--Basalt, greenstone, trachyte, porphyry, scoria,
amygdaloid, lava, tuff--Alphabetical list, and explanation of names
and synonyms, of volcanic rocks--Table of the analyses of minerals
most abundant in the volcanic and hypogene rocks.
The aqueous or fossiliferous rocks having now been described, we have next
to examine those which may be called volcanic, in the most extended sense
of that term. Suppose _a a_ in the annexed diagram, 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_. They also are seen, in some instances, to pass insensibly
into the unstratified division of _a_, or the Plutonic rocks.
[Illustration: Fig. 434. Cross section.
_a._ Hypogene formations, stratified and unstratified.
_b._ Aqueous formations.
_c._ Volcanic 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 volcanos. They also 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, and
amygdaloid. All these, which were recognized 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 on the sides of hills. This configuration appears to be
derived from two causes. First, the abrupt original terminations of sheets
of melted matter, which have spread, whether on the land or bottom of the
sea, over a level surface. For we know, in the case of lava flowing from a
volcano, that a stream, when it has ceased to flow, and grown solid, very
commonly ends in a steep slope, as at _a_, fig. 435. But, secondly, the
step-like appearance arises more frequently from the mode in which
horizontal masses of igneous rock, such as _b c_, intercalated between
aqueous strata, have, subsequently to their origin, been exposed, at
different heights, by denudation. Such an outline, it is true, is not
peculiar to trap rocks; great beds of limestone, and other hard kinds of
stone, often presenting similar terraces and precipices: but these are
usually on a smaller scale, or less numerous, than the volcanic _steps_, or
form less decided features in the landscape, as being less distinct in
structure and composition from the associated rocks.
[Illustration: Fig. 435. Step-like appearance of trap.]
Although the characters of trap rocks are greatly diversified, the
beginner will easily learn to distinguish them as a class from the
aqueous formations. Sometimes they present themselves, as already
stated, in tabular masses, which are not divided into strata: sometimes
in shapeless lumps and irregular cones, forming chains of small hills.
Often they are seen in dikes and wall-like masses, intersecting
fossiliferous beds. The rock is occasionally found divided into columns,
often decomposing into balls of various sizes, from a few inches to
several feet in diameter. The decomposing surface very commonly assumes
a coating of a rusty iron colour, from the oxidation of ferruginous
matter, so abundant in the traps in which augite or hornblende occur;
or, in the felspathic varieties of trap, it acquires a white opaque
coating, from the bleaching of the mineral called felspar. On examining
any of these volcanic rocks, where they have not suffered
disintegration, we rarely fail to detect a crystalline arrangement in
one or more of the component minerals. Sometimes the texture of the mass
is cellular or porous, or we perceive that it has once been full of
pores and cells, which have afterwards become filled with carbonate of
lime, or other infiltrated mineral.
Most of the volcanic rocks produce a fertile soil by their disintegration.
It seems that their component ingredients, silica, alumina, lime, potash,
iron, and the rest, are in proportions well fitted for vegetation. As they
do not effervesce with acids, a deficiency of calcareous matter might at
first be suspected; but although _the carbonate_ of lime is rare, except in
the nodules of amygdaloids, yet it will be seen that lime sometimes enters
largely into the composition of augite and hornblende. (See Table, p. 377.)
_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 and other gases
are evolved. The explosions are so violent as to hurl up into the air
fragments of broken stone, parts of which are shivered into minute
atoms. At the same time melted stone or _lava_ usually ascends through
the chimney or vent by which the gases make their escape. Although
extremely heavy, this lava is forced up by the expansive power of
entangled gaseous fluids, chiefly steam or aqueous vapour, exactly in
the same manner as water is made to boil over the edge of a vessel when
steam has been generated at the bottom by heat. Large quantities of the
lava are also shot up into the air, where it separates into fragments,
and acquires a spongy texture by the sudden enlargement of the included
gases, and thus forms _scoriae_, other portions being reduced to an
impalpable powder or dust. The showering down of the various ejected
materials round the orifice of eruption gives rise to a conical mound,
in which the successive envelopes of sand and scoriae form layers,
dipping on all sides from a central axis. In the mean time a hollow,
called a _crater_, has been kept open in the middle of the mound by the
continued passage upwards 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 it down on
one side, and often it flows out from a fissure at the base of the
hill (see fig. 436.).[368-A]
[Illustration: Fig. 436. Part of the chain of extinct volcanos called the
Monts Dome, Auvergne. (Scrope.)]
_Composition and nomenclature._--Before speaking of the connection between
the products of modern volcanos and the rocks usually styled trappean, and
before describing the external forms of both, and the manner and position
in which they occur in the earth's crust, it will be desirable to treat of
their mineral composition and names. The varieties most frequently spoken
of are basalt, greenstone, syenitic greenstone, clinkstone, claystone, and
trachyte; while those founded chiefly on peculiarities of texture, are
porphyry, amygdaloid, lava, tuff, scoriae, and pumice. It may be stated
generally, that all these are mainly composed of two minerals, or families
of simple minerals, _felspar_ and _hornblende_; some almost entirely of
hornblende, others of felspar.
These two minerals may be regarded as two groups, rather than species.
Felspar, for example, may be, first, common felspar, that is to say,
potash-felspar, in which the alkali is potash (see table, p. 377.); or,
secondly, albite, that is to say, soda-felspar, where the alkali is soda
instead of potash; or, thirdly, Labrador-felspar (Labradorite), which
differs not only in its iridescent hues, but also in its angle of
fracture or cleavage, and its composition. We also read much of two
other kinds, called glassy felspar and compact felspar, which, however,
cannot rank as varieties of equal importance, for both the albitic and
common felspar appear sometimes in transparent or _glassy_ crystals; and
as to compact felspar, it is a compound of a less definite nature,
sometimes containing both soda and potash; and which might be called a
felspathic paste, being the residuary matter after portions of the
original matrix have crystallized.
The other group, or _hornblende_, consists principally of two varieties;
first, hornblende, and, secondly, augite, which were once regarded as very
distinct, although now some eminent mineralogists are in doubt whether they
are not one and the same mineral, differing only as one crystalline form of
native sulphur differs from another.
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 Hauey,
Mohs, and other celebrated mineralogists. It was agreed that the form of
the crystals of the two species were different, and 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; and that when this happened, as in some lavas
of modern date, the hornblende occurs in the mass of the rock, where
crystallization may have taken place more slowly, while the augite
merely lines cavities where the crystals may have been produced rapidly.
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 in rocks from
Siberia which presented a hornblende _cleavage_, 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
recognized 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 recrystallizing, 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 recrystallizes, 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 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 find that a
mineral determined to be the same by its physical characters, crystalline
form, and optical properties, has often been declared by skilful analyzers
to be composed of distinct elements. (See the table at p. 377.) 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 in a great degree 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+, _isos_, equal, and +morphe+, _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.
Having been led into this digression on the recent progress of mineralogy,
I may here observe that the geological student must endeavour as soon as
possible to familiarize himself with the characters of five at least of
the most abundant simple minerals of which rocks are composed. These are,
felspar, quartz, mica, hornblende, and carbonate of lime. This knowledge
cannot be acquired from books, but requires personal inspection, and the
aid of a teacher. It is well to accustom the eye to know the appearance of
rocks under the lens. To learn to distinguish felspar from quartz is the
most important step to be first aimed at. In general we may know the
felspar because it can be scratched with the point of a knife, whereas the
quartz, from its extreme hardness, receives no impression. But when these
two minerals occur in a granular and uncrystallized state, the young
geologist must not be discouraged if, after considerable practice, he often
fails to distinguish them by the eye alone. If the felspar is in crystals,
it is easily recognized by its cleavage: but when in grains the blow-pipe
must be used, for the edges of the grains can be rounded in the flame,
whereas those of _quartz_ are infusible. If the geologist is desirous of
distinguishing the three varieties of felspar above enumerated, or
hornblende from augite, it will often be necessary to use the reflecting
goniometer as a test of the angle of cleavage, and shape of the crystal.
The use of this instrument will not be found difficult.
The external characters and composition of the felspars are extremely
different from those of augite or hornblende; so that the volcanic rocks in
which either of these minerals decidedly predominates, are easily
recognized. But there are mixtures of the two elements in every possible
proportion, the mass being sometimes exclusively composed of felspar, at
other times solely of augite, or, again, of both in equal quantities.
Occasionally, the two extremes, and all the intermediate gradations, may be
detected in one continuous mass. Nevertheless there are certain varieties
or compounds which prevail so largely in nature, and preserve so much
uniformity of aspect and composition, that it is useful in geology to
regard them as distinct rocks, and to assign names to them, such as basalt,
greenstone, trachyte, and others, already mentioned.
_Basalt._--As an example of rocks in which augite greatly prevails, basalt
may first be mentioned. Although we are more familiar with this term than
with that of any other kind of trap, it is difficult to define it, the name
having been used so vaguely. It has been very generally applied to any trap
rock of a black, bluish, or leaden-grey colour, having a uniform and
compact texture. Most strictly, it consists of an intimate mixture of
augite, felspar, and iron, to which a mineral of an olive green colour,
called olivine, is often superadded, in distinct grains or nodular masses.
The iron is usually magnetic, and is often accompanied by another metal,
titanium. Augite is the predominant mineral, the felspar being in much
smaller proportions. There is no doubt that many of the fine-grained and
dark-coloured trap rocks, called basalt, contained hornblende in the place
of augite; but this will be deemed of small importance after the remarks
above made. Other minerals are occasionally found in basalt; and this rock
may pass insensibly into almost every variety of trap, especially into
greenstone, clinkstone, and wacke, which will be presently described.
_Greenstone_, or _Dolerite_, is usually defined as a granular rock, the
constituent parts of which are hornblende and imperfectly crystallized
felspar; the felspar being more abundant than in basalt; and the grains or
crystals of the two minerals more distinct from each other. This name may
also be extended to those rocks in which augite is substituted for
hornblende (the dolorite of some authors), or to those in which albite
replaces common felspar, forming the rock sometimes called Andesite.
_Syenitic greenstone._--The highly crystalline compounds of the same two
minerals, felspar and hornblende, having a granitiform texture, and
with occasionally some quartz accompanying, may be called Syenitic
greenstone, a rock which frequently passes into ordinary trap, and
as frequently into granite.
_Trachyte._--A porphyritic rock of a whitish or greyish colour, composed
principally of glassy felspar, with crystals of the same, generally with
some hornblende and some titaniferous iron. In composition it is extremely
different from basalt, this being a felspathic, as the other is an augitic,
rock. It has a peculiar rough feel, whence the name +trachys+, _trachus_,
rough. Some varieties of trachyte contain crystals of quartz.
[Illustration: Fig. 437. Porphyry.
White crystals of felspar in a dark base of hornblende and felspar.]
_Porphyry_ is merely a certain form of rock, 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. 437.). Thus trachyte is porphyritic; for in it, as in many modern
lavas, there are crystals of felspar; 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.
_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, calcedony, 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
_amygdala_, 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.
[Illustration: Fig. 438. Scoriaceous lava in part converted into
an amygdaloid.
Montagne de la Veille, Department of Puy de Dome, France.]
The annexed figure 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.
_Scoriae_ and _Pumice_ may next be mentioned as porous rocks, produced by
the action of gases on materials melted by volcanic heat. _Scoriae_ are
usually of a reddish-brown and black colour, and are the cinders and slags
of basaltic or augitic lavas. _Pumice_ is a light, spongy, fibrous
substance, produced by the action of gases on trachytic and other lavas;
the relation, however, of its origin to the composition of lava is not yet
well understood. Von Buch says that it never occurs where only
Labrador-felspar is present.
_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, or in consequence of the
contact with water in or upon the damp soil.
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 rents
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-felspar.[374-A]
_Trap tuff, volcanic tuff._--Small angular fragments of the scoriae and
pumice, above mentioned, and the dust of the same, produced by volcanic
explosions, form the tuffs which abound in all regions of active
volcanos, where showers of these materials, together with small pieces
of other rocks ejected from the crater, 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.
Besides the peculiarity of their composition, some tuffs, or _volcanic
grits_, as they have been termed, differ from ordinary sandstones by
the angularity of their grains. When the fragments are coarse, the
rock is styled a volcanic _breccia_. _Tufaceous conglomerates_ result
from the intermixture of rolled fragments or pebbles of volcanic and
other rocks with tuff.
According to Mr. Scrope, the Italian geologists confine the term
_tuff_, or tufa, to felspathose mixtures, and those composed
principally of pumice, using the term _peperino_ for the basaltic
tuffs.[374-B] The peperinos thus distinguished are usually brown,
and the tuffs grey or white.
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 to cause them to resemble
many of those kinds of trap which are found in ordinary dikes. The
chocolate-coloured mud, which was poured for weeks out of the crater of
Graham's Island, in the Mediterranean, in 1831, must, when unmixed with
other materials, have constituted a stone heavier than granite. Each cubic
inch of the impalpable powder which has fallen for days through the
atmosphere, during some modern eruptions, has been found to weigh, without
being compressed, as much as ordinary trap rocks, and to be often identical
with these in mineral composition.
The fusibility of the igneous rocks generally exceeds that of other rocks,
for there is much alkaline matter and lime in their composition, which
serves as a flux to the large quantity of silica, which would be otherwise
so refractory an ingredient.
It is remarkable that, notwithstanding the abundance of this silica,
quartz, that is, crystalline silica, is usually wanting in the volcanic
rocks, or is present only as an occasional mineral, like mica. The elements
of mica, as of quartz, occur in lava and trap; but the circumstances under
which these rocks are formed are evidently unfavourable to the development
of mica and quartz, minerals so characteristic of the hypogene formations.
It would be tedious to enumerate all the varieties of trap and lava which
have been regarded by different observers as sufficiently abundant to
deserve distinct names, especially as each investigator is too apt to
exaggerate the importance of local varieties which happen to prevail in
districts best known to him. It will be useful, however, to subjoin here,
in the form of a glossary, an alphabetical list of the names and synonyms
most commonly in use, with brief explanations, to which I have added a
table of the analysis of the simple minerals most abundant in the volcanic
and hypogene rocks.
_Explanation of the names, synonyms, and mineral composition of the more
abundant volcanic rocks._
AMPHIBOLITE. _See_ Hornblende rock, amphibole being Hauey's name
for hornblende.
AMYGDALOID. A particular form of volcanic rock; _see_ p. 372.
AUGITE ROCK. A kind of basalt or greenstone, composed wholly or principally
of granular augite. (_Leonhard's Mineralreich_, 2d edition, p. 85.)
AUGITIC-PORPHYRY. Crystals of Labrador-felspar and of augite, in a green or
dark grey base. (_Rose_, _Ann. des Mines_, tom. 8. p. 22. 1835.)
BASALT. Chiefly augite--an intimate mixture of augite and felspar with
magnetic iron, olivine, &c. _See_ p. 371. The yellowish green mineral
called olivine, can easily be distinguished from yellowish felspar by
its infusibility, and having no cleavage. The edges turn brown in the
flame of the blow-pipe.
BASANITE. Name given by Alex. Brongniart to a rock, having a base of
basalt, with more or less distinct crystals of augite disseminated
through it.
CLAYSTONE and CLAYSTONE-PORPHYRY. An earthy and compact stone, usually of a
purplish colour, like an indurated clay; passes into hornstone; generally
contains scattered crystals of felspar and sometimes of quartz.
CLINKSTONE. _Syn._ Phonolite, fissile Petrosilex; a greenish or greyish
rock, having a tendency to divide into slabs and columns; hard, with clean
fracture, ringing under the hammer; principally composed of compact
felspar, and, according to Gmelin, of felspar and mesotype. (_Leonhard_,
_Mineralreich_, p. 102.) A rock much resembling clinkstone, and called by
some Petrosilex, contains a considerable percentage of quartz and felspar.
As both trachyte and basalt pass into clinkstone, the rock so called must
be very various in composition.
COMPACT FELSPAR, which has also been called Petrosilex; the rock so called
includes the hornstone of some mineralogists, is allied to clinkstone, but
is harder, more compact, and translucent. It is a varying rock, of which
the chemical composition is not well defined, and is perhaps the same as
that of clay. (_MacCulloch's Classification of Rocks_, p. 481.) Dr.
MacCulloch says, that it contains both potash and soda.
CORNEAN. A variety of claystone allied to hornstone. A fine homogeneous
paste, supposed to consist of an aggregate of felspar, quartz, and
hornblende, with occasionally epidote, and perhaps chlorite; it passes into
compact felspar and hornstone. (_De la Beche_, _Geol. Trans._ second
series, vol. 2. p. 3.)
DIALLAGE ROCK. _Syn_. Euphotide, Gabbro, and some Ophiolites. Compounded of
felspar and diallage, sometimes with the addition of serpentine, or mica,
or quartz. (_MacCulloch. ibid_. p. 648.)
DIORITE. A kind of greenstone, which see. Components, felspar and
hornblende in grains. According to _Rose_, _Ann. des Mines_, tom. 8. p. 4.,
_diorite_ consists of albite and hornblende.
DIORITIC-PORPHYRY. A porphyritic greenstone, composed of crystals of albite
and hornblende, in a greenish or blackish base. (_Rose_, _ibid._ p. 10.)
DOLERITE. Formerly defined as a synonym of greenstone, which see. But,
according to Rose (_ibid._ p. 32.), its composition is black augite and
Labrador-felspar; according to Leonhard (_Mineralreich_, &c. p. 77.),
augite, Labrador-felspar, and magnetic iron.
DOMITE. An earthy _trachyte_, found in the Puy de Dome, in Auvergne.
EUPHOTIDE. A mixture of grains of Labrador-felspar and diallage. (_Rose_,
_ibid._ p. 19.) According to some, this rock is defined to be a mixture of
augite or hornblende, and saussurite, a mineral allied to jade. (_Allan's
Mineralogy_, p. 158.) _See_ Diallage rock.
FELSPAR-PORPHYRY. _Syn._ Hornstone-porphyry; a base of felspar,
with crystals of felspar, and crystals and grains of quartz. _See_
also Hornstone.
GABBRO, _see_ Diallage rock.
GREENSTONE. _Syn._ Dolerite and diorite; components, hornblende and
felspar, or augite and felspar in grains. See above, p. 372.
GREYSTONE. (Graustein of Werner.) Lead grey and greenish rock, composed of
felspar and augite, the felspar being more than seventy-five per cent.
(_Scrope_, _Journ. of Sci._ No. 42. p. 221.) Greystone lavas are
intermediate in composition between basaltic and trachytic lavas.
HORNBLENDE ROCK. A greenstone, composed principally of granular hornblende,
or augite. (_Leonhard_, _Mineralreich_, &c., p. 85.)
HORNSTONE, HORNSTONE-PORPHYRY. A kind of felspar porphyry (_Leonhard_,
_ibid._), with a base of hornstone, a mineral approaching near to flint,
differing from compact felspar in being infusible.
HYPERSTHENE ROCK, a mixture of grains of Labrador-felspar and hypersthene
(_Rose_, _Ann. des Mines_, tom. 8. p. 13.), having the structure of syenite
or granite; abundant among the traps of Skye. Some geologists consider it a
greenstone, in which hypersthene replaces hornblende.
LATERITE. A red jaspery rock, composed of silicate of alumina and oxide of
iron. Abundant in the Deccan, in India; and referred to the trap formation;
from Later, a brick or tile.
MELAPHYRE. A variety of black porphyry, the base being black augite with
crystals of felspar; from +melas+, _melas_, black.
OBSIDIAN. Vitreous lava like melted glass, nearly allied to pitchstone.
OPHIOLITE, sometimes same as Diallage rocks (_Leonhard_, p. 77.); sometimes
a kind of serpentine.
OPHITE. A green porphyritic rock composed chiefly of hornblende, with
crystals of that mineral in a base of the same, mixed with some felspar.
It passes into serpentine by a mixture of talc. (_Burat's d'Aubuisson_,
tom. ii. p. 63.)
PEARLSTONE. A volcanic rock, having the lustre of mother of pearl;
usually having a nodular structure; intimately related to obsidian,
but less glassy.
PEPERINO. A form of volcanic tuff, composed of basaltic scoriae.
_See_ p. 374.
PETROSILEX. _See_ Clinkstone and Compact Felspar.
PHONOLITE. _Syn._ of Clinkstone, which see.
PITCHSTONE. Vitreous lava, less glassy than obsidian; a blackish green rock
resembling glass, having a resinous lustre and appearance of pitch;
composition various, usually felspar and augite; passes into basalt; occurs
in veins, and in Arran forms a dike thirty feet wide, cutting through
sandstone; forms the outer walls of some basaltic dikes.
PORPHYRY. Any rock in which detached crystals of felspar, or of one or more
minerals, are diffused through a base. _See_ p. 372.
POZZOLANA. A kind of tuff. _See_ p. 36.
PUMICE. A light, spongy, fibrous form of trachyte. _See_ p. 373.
PYROXENIC-PORPHYRY, same as augitic-porphyry, pyroxene being Hauey's
name for augite.
SCORIAE. _Syn._ volcanic cinders; reddish brown or black porous form of
lava. _See_ p. 373.
SERPENTINE. A greenish rock, in which there is much magnesia; usually
contains diallage, which is nearly allied to the simple mineral called
serpentine. Occurs sometimes, though rarely, in dikes, altering the
contiguous strata; is indifferently a member of the trappean or
hypogene series.
SYENITIC-GREENSTONE; composition, crystals or grains of felspar and
hornblende. _See_ p. 372.
TEPHRINE, synonymous with lava. Name proposed by Alex. Brongniart.
TOADSTONE. A local name in Derbyshire for a kind of wacke, which see.
TRACHYTE. Chiefly composed of glassy felspar, with crystals of glassy
felspar. _See_ p. 372.
TRAP TUFF. _See_ p. 374.
TRASS. A kind of tuff or mud poured out by lake craters during eruptions;
common in the Eifel, in Germany.
TUFACEOUS CONGLOMERATE. _See_ p. 374.
TUFF. _Syn._ Trap-tuff, volcanic tuff. _See_ p. 374.
VITREOUS LAVA. _See_ Pitchstone and Obsidian.
VOLCANIC TUFF. _See_ p. 374.
WACKE. A soft and earthy variety of trap, having an argillaceous aspect. It
resembles indurated clay, and when scratched exhibits a shining streak.
WHINSTONE. A Scotch provincial term for greenstone and other hard
trap rocks.
ANALYSIS OF MINERALS MOST ABUNDANT IN THE VOLCANIC AND HYPOGENE ROCKS.
Silica. Alu- Mag- Lime. Pot- Soda. Iron. Manga- Remain-
mina. nesia. ash. Oxide. nese. der.
Actinolite 64. -- 22. -- -- -- 3. a 43.05 C.
(Bergman) trace
Albite (Rose) 68.84 20.53 -- a trace -- 9.12 -- -- --
--(mean of 4 69.45 19.44 0.13 0.22 -- 9.95 a -- --
analyses) trace
Augite (Rose) 53.36 -- 4.99 22.19 -- -- 17.38 0.09 --
--(mean of 4 53.57 1. 11.26 20.9 -- -- 10.75 0.67 --
analyses)
Carbonate of -- -- -- 56.33 -- -- -- -- --
Lime (Biot)
Chiastolite 68.49 30.17 4.12 -- -- -- 2.7 -- 0.27 W.
(Landgrabe)
Chlorite 26. 18.5 8. -- -- 2. 43. -- --
(Vauquelin)
--(mean of 3 27.43 17.9 14.56 0.50 1.56 -- 30.63 -- 6.92 W.
analyses)
Diallage 60. -- 27.5 -- -- -- 10.5 -- --
(Klaproth)
--(mean of 3 43.33 2.2 26.41 5.58 -- -- 11.53 -- 8.54 W.
analyses)
Epidote 37. 21. -- 15. -- -- 24. 1.5 --
(Vauquelin)
Felspar, 62.83 17.02 -- 3. - 13. -- 1. -- --
common (Vauq.)
--(Rose) 66.75 17.5 -- 1.25 12. -- 0.75 -- --
--(mean of 7 64.04 18.94 -- 0.76 13.66 -- 0.74 -- --
analyses)
Garnet 35.75 27.25 -- -- -- -- 36. 0.25 --
(Klaproth)
--(Phillips) 43. 16. -- 20. -- -- 16. -- --
Hornblende 42. 12. 2.25 11. a -- 30. 0.25 --
(Klap.) trace
--(Bonsdorff.) 45.69 12.18 18.79 13.85 -- -- 7.32 0.22 1.5 F.
Hypersthene 54.25 2.25 14. 1.5 -- -- 24.5 a 1. W.
(Klaproth) trace
Labrador- 55.75 26.5 -- 11. -- 4. 1.25 -- 0.5 W.
felspar (Klap.)
Leucite 53.75 24.62 -- -- 21.35 -- -- -- --
(Klap.)
Mesotype 54.64 19.70 -- 1.61 -- 15.09 -- -- 9.83 W.
(Gehlen)
Mica 42.5 11.5 9. -- 10. -- 22. 2. --
(Klaproth)
--(Vauquelin) 50. 35. -- 1.33 -- -- 7. -- --
--(mean of 3 45.83 22.58 -- -- 11.08 -- 14. 1.45 --
analyses)
Olivine 50. -- 38.5 -- -- -- 12. -- --
(Klaproth)
Schorl or 35.48 34.75 4.68 -- 0.48 1.75 17.44 1.89 4.02 B.
Tourmaline (Gmelin)
--(mean of 6 36.03 35.82 4.44 0.28 0.71 1.96 13.71 1.62 --
analyses)
Serpentine 43.07 0.25 40.37 0.5 -- -- 1.17 -- 12.45 W.
(Hisinger)
--(mean of 5 37.29 4.97 36.8 2.89 -- -- 3.14 -- 12.77 W.
analyses)
Steatite 64. -- 22. -- -- -- 3. -- 5. W.
(Vauquelin)
--(mean of 3 48.3 6.18 26.65 -- -- -- 2. -- 9.5 W.
anal. by Klap.)
Talc. 61.75 -- 30.5 -- 2.75 -- 2.5 -- --
(Klaproth)
In the last column of the above Table, the letters B. C. F. W. represent
Boracic acid, Carbonic acid, Fluoric acid, and Water.
FOOTNOTES:
[368-A] For a description and theory of active volcanos, see Principles of
Geology, chaps. xxiv. to xxvii.
[374-A] G. Rose, Ann. des Mines, tom. viii. p. 32.
[374-B] Geol. Trans. vol. ii. p. 211. 2d series.