action of granite, in the Pyrenees at the 'Montagnes de Rancie', see
Dufrenoy, in the 'Memoires Geologiques', t. ii., p. 440; and on similar
changes in the 'Montagnes de l'Oisans', see Elie de Beaumont, in the 'Mem.
Geolog.', t. ii., p. 379-415; on a similar effect produced by the action of
dioritic and pyroxenic porphyry (the 'ophite' described by Elie de Beaumont,
in the 'Geologie de la France', t. i., p. 72), between Tolosa and St.
Sebastian, see Dufrenoy, in the 'Mem. Geolog.', t. ii., p. 130; and by
syenite in the Isle of Skye, where the fossils in the altered limestone may
still be distinguished, see Von Dechen, in his 'Geognosie', p. 573. In the
transformation of chalk by contact with basalt, the transposition of the
most minute particles in the processes of crystallization and granulation is
the more remarkable, because the excellent microscopic investigations of
Ehrenberg have shown that the particles of chalk previously existed in the
form of closed rings. See Poggend., 'Annalen der Physic', bd. xxxix., s.
105; and on the rings of aragonite deposited from solution, see Gustav Rose
in vol. xlii., p. 354, of the same journal.
I would here wish to make special mention of Parian and Carrara marbles,
which have acquired such celebrity from the noble works of art into which
they have been converted, and which have too long been considered in our
geognostic collections as the main types of primitive limestone. The action
of granite has been manifested sometimes by immediate contact, as in the
Pyrenees,* and sometimes, as in the main land of Greece, and in the insular
groups in the gean Sea, through the intermediate layers of gneiss or mica
slate.
[footnote] *Beds of granular limestone in the granite at Port d'Oo and in
the Mont de Labourd. See Charpentier, 'Constitution Geologique des
Pyrenes', p. 144, 146.
Both cases presuppose a simultaneous but heterogeneous process of
transformation.
p 263
In Attica, in the island of Euboea, and in the Peloponnesus, it has been
remarked, "that the limestone, when superposed on mica slate, is beautiful
and crystalline in proportion to the purity of the latter substance and to
the smallness of its argillaceous contents; and, as is well known, this
rock, together with beds of gneiss, appears at many points, at a
considerable depth below the surface, in the islands of Paros and
Antiparos."*
[footnote] *Leop. von Buch, 'Descr. des Canaries', p. 394; Fiedler, 'Reise
durch das Konigreich Griechenland', th. ii., s., 181, 190, und 516.
We may here infer the existence of an imperfectly metamorphosed flotz
formation, if faith can be yielded to the testimony of Origen, according to
whom, the ancient Eleatic, Xenophanes of Colophon* (who supposed the whole
earth's crust to have been once covered by the sea), declared that marine
fossils had been found in the quarries of Syracuse, and the impression of a
fish (a sardine) in the deepest rocks of Paros.
[footnote] *I have previously alluded to the remarkable passage in Origen's
'Philosophumena', cap. 14 ('Opera', ed. Delarue, t. i., p. 893). From the
whole context, it seems very improbable that Xenophanes meant an impression
of a laurel ([Greek words]) instead of an impression of a fish ([Greek
words]). Delarue is wrong in blaming the correction of Jacob Gronovius in
changing the laurel into a sardel. The petrifaction of a fish is also much
more probable than the natural picture of Silenus, which, according to Pliny
(lib. xxxvi., 5), the quarry-men are stated to have met with in Parian
marble from Mount Marpessos. 'Servius ad Virg., AEn.', vi., 471.
The Carrara or Luna marble quarries, which constituted the principal source
from which statuary marble was derived even prior to the time of Augustus,
and which will probably continue to do so until the quarries of Paros shall
be reopened, are beds of calcareous sandstone -- macigno -- altered by
Plutonic action, and occurring in the insulated mountain of Apuana, between
gneiss-like mica and talcose schist.*
[footnote] *On the geognostic relations of Carrara ('The City of the Moon',
Strabo, lib. v., p. 222), see Savi 'Osservazioni sui terreni antichi
Toscani', in the 'Nuova Giornale de' Letterati di Pisa', and Hoffmann, in
Karsten's 'Archiv fur Mineralogie', bd. vi., s. 258-263, as well as in his
'Geogn. Reise durch Italien', s. 244-265.
Whether at some points granular limestone may not have been formed in the
interior of the earth, and been raised by gneiss and syenite to the surface,
where it forms vein-like fissures,* is a question on which I can not hazard
an opinion, owing to my own want of personal knowledge of the subject.
[footnote] *According to the assumption of an excellent and very
experienced observer, Karl von Leonhard. See his 'Jahrbuch fur
Mineralogie', 1834 s. 329, and Bernhard Cotta, 'Geognosie', s. 310.
p 264
According to the admirable observations of Leopold von Buch, the masses of
dolomite found in Southern Tyrol, and on the Italian side of the Alps,
present the most remarkable instance of metamorphism produced by massive
eruptive rocks on compact calcareous beds. The formation of the limestone
seems to have proceeded from the fissures which traverse it in all
directions. The cavities are every where covered with rhomboidal crystals
of magnesian bitter spar, and the whole formation, without any trace of
strtification, or of the fossil remains which it once contained, consists
only of a granular aggregation of crystals of dolomite. Talc laminae lie
scattered here and there in the newly-formed rock, traversed by masses of
serpentine. In the valley of the Fassa, dolomite rises perpendicularly in
smooth walls of dazzling whiteness to a height of many thousand feet. It
forms sharply-pointed conical mountains, clustered together in large
numbers, but yet not in contact with each other. The contour of their forms
recalls to mind the beautiful landscape with which the rich imagination of
Leonardi da Vinci has embellished the back-ground of the portrait of Mona
Lisa.
The geognostic phenomena which we are now describing, and which excite the
imagination as well as the powers of the intellect, are the result of the
action of augite porphyry manifested in its elevating, destroying, and
transforming force.*
[footnote] *Leop. von Buch, 'Geognostische Briefe an Alex. von Humboldt',
1824, s. 86 and 82; also in the 'Annalen de Chemie', t. xxiii., p. 276, and
in the 'Abhandl. der Berliner Akad. aus der Jahren 1822 'und' 1823, s.
83-136; Von Dechen, 'Geognosie.' s. 574-576.
The process by which limestone is converted into dolomite is not regarded by
the illustrious investigator who first drew attention to the phenomenon as
the consequence of the tale being derived from the black porphyry, but
rather as a transformatiion simultaneous with the appearance of this erupted
stone through wide fissures filled with vapors. It remains for future
inquirers to determine how transformation can have been effected without
contact with the endogenous stone, where strata of dolomite are found to be
interspersed in imestone. Where, in this case, are we to seek the concealed
channels by which the Plutonic action is conveyed? Even here it may not,
however, be necessary, in conformity with the old Roman adage, to believe
"that much that is alike in nature may have been formed in wholly different
ways." When we find, over widely extended parts of the earth, that two
phenomena are always associated together, as, for instance, the occurrence
of melaphyre
p 265
and the transformation of compact limestone into a crystaline mass differing
in its chemical character, we are, to a certain degree, justified in
believing, where the second phenomenon is manifested unattended by the
appearance of the first, that this apparent contradiction is owing to the
absence, in certain cases, of some of the conditions attendant upon the
exciting causes. Who would call in question the volcanic nature and igneous
fluidity of basalt merely because there are some rare instances in which
basaltic veins, traversing beds of coal or strata of sandstone and chalk,
have not materially deprived the coal of its carbon, nor broken and slacked
the sandstone, not converted the chalk into granular marble? Wherever we
have obtained even a faint light to guide us in the obscure domain of
mineral formation, we ought not ungratefully to disregard it, because there
may be much that is still unexplained in the history of the relations of the
transitions, or in the isolated interposition of beds of unaltered strata.
After having spoken of the alteration of compact carbonate of lime into
granular limestone and dolomite, it still remains for us to mention a third
mode of transformation of the same mineral, which is ascribed to the
emission, in the ancient periods of the world, of the vapors of sulphuric
acid. This transformation of limestone into gypsum is analogous to the
penetration of rock salt and sulphur, the latter being deposited from
sulphureted aqueous vapor. In the lofty Cordilleras of Quindin, far from
all volcanoes, I have observed deposits of sulphur in fissures in gneiss,
while in Sicily (at Cattolica, near Girgenti), sulphur, gypsum, and rock
salt belong to the most recent secondary strata, the chalk formations.*
[footnote] *Horrman, 'Geogn. Reise', edited by Von Dechen, s. 113-119, and
380-386; Poggend., 'Annalen der Physik', bd. xxvi., s. 41.
I have also seen on the edge of the crater of Vesuvius, fissures filled with
rock salt, which occurred in such considerable masses as occasionally to
lead to its being disposed of by contraband trade. On both declivities of
the Pyrenees, the connection of diorite and pyroxene, and colomite, gypsum,
and rock salt, can not be questioned;* and here, as in the other phenomena
which we have been considering, every thing bears evidence of the action of
subterranean forces on the sedimentary strata of the ancient sea.
[footnote] *Dufrenoy, in the 'Memoires Geologiques', t. ii., p. 145 and 179.
There is much difficulty in explaining the origin of the beds of pure
quartz, which occur in such large quantities in South America, and impart so
peculiar a character to the chain of
p 266
the Andes.*
[footnote] *Humboldt, 'Essai Geogn. sur le Gisement des Roches', p. 93;
'Asie Centrale', t. iii., p. 532.
In descending toward the South Sea, from Caxamarca toward Guangamarca, I
have observed vast masses of quartz, from 7000 to 8000 feet in height,
superposed sometimes on porphyry devoid of quartz, and sometimes on diorite.
Can these beds have been transformed from sandstone, as Elie de Beaumont
conjectures in the case of the quartz strata on the Col de la Poissonniere,
east of Briançon?*
[footnote] *Elie de Beaumont, in the 'Annales des Sciences Naturelles', t.
xv., p. 362; Murchison, 'Silurian System', p. 286.
In the Brazils, in the diamond district of Minas Geraes and St. Paul, which
has recently been so accurately investigated by Clausen, Plutonic action has
developed in dioritic veins sometimes ordinary mica, and sometimes specular
iron in quartzose itacolumite. The diamonds of Grammagoa are imbedded in
strata of solid silica, and are occasionally enveloped in laminae of mica,
like the garnets found in mica slate. The diamonds that occur furthest to
the north, as those discovered in 1829 at 58 degrees lat., on the European
slope of the Uralian Mountains, bear a geognostic relation to the black
carboniferous dolomite of Adolffskoi* and to augitic porphyry, although more
accurate observations are required in order fully to elucidate this subject.
[footnote] *Rose, 'Reise nach dem Ural', bd. i., s. 364 und 367.
Among the most remarkable phenomena of contact, we must, finally, enumerate
the formation of garnets in argillaceous schist in contact with basalt and
dolerite (as in Northumberland and the island of Anglesea), and the
occurrence of a vast number of beautiful and most various crystals, as
garnets, vesuvian, augite, and ceylanite, on the surfaces of contact between
the erupted and sedimentary rock, as, for instance, on the junction of the
syenite of Monzon with dolomite and compact limestone.
[footnote] *Leop. von Buch, 'Briefe', s. 109-129. See also, Elie de
Beaumont 'On the Contact of Granite with the Beds of the Jura', in the 'Mem.
Geol.' t. ii., p. 408.
In the island of Elba, masses of serpentine, which perhaps nowhere more
clearly indicate the character of erupted rocks, have occasioned the
sublimation of iron glance and red oxyd of iron in fissures of calcareous
sandstone.
[footnote] *Hoffman, 'Reise', s. 30 und 37.
We still daily find the same iron glance formed by sublimation from the
vapors and the walls of the fissures of open veins on the margin of the
crater, and in the fresh lava currents of the volcanoes of Stromboli,
Vesuvius, and AEtna.*
[footnote] *On the chemical process in the formation of specular iron, see
Gay Lussac, in the 'Annales de Chimie', t. xxii., p. 415, and Mitscherlich,
in Poggend., 'Annalen', bd. xv., s. 630. Moreover, crystals of olivine have
been formed (probaby by sublimation) in the cavities of the obsidian of
Cerro del Jacal, which I brought from Mexico (Gustav Rose, in Poggend.,
'Annalen', bd. x., s. 323). Hence olivine occurs in basalt, lava, obsidian,
artificial scoriae in meteoric stones, in the syenite of Elfdale, and (as
hyalosiderite) in the wacke of the Kaiserstuhl.
The veins that
p 267
are thus formed beneath our eyes by volcanic forces, where the contiguous
rock has already attained a certain degree of solidification, show us how,
in a similar manner, mineral and metallic veins may have been every where
formed in the more ancient periods of the world, where the solid but thinner
crust of our planet, shaken by earthquakes, and rent and fissured by the
change of volume to which it was subjected in cooling, may have presented
many communications with the interior, and many passages for the escape of
vapors impregnated with earthy and metallic substances. The arrangement of
the particles in layers parallel with the margins of the beins, the regular
recurrence of analogous layers on the opposite sides of the veins (on their
different walls), and, finally, the elongated cellular cavities in the
middle, frequently afford direct evidence of the Plutonic process of
sublimation in metalliferous veins. As the traversing rocks must be of more
recent origin than the traversed, we learn from the relations of
stratification existing between the porphyry and the argentiferous ores in
the Saxon mines (the richest and most important in Germany), that these
formations are at any rate more recent than the vegetable remains found in
carboniferous strata and in the red sandstone.*
[footnote] *Constantin von Veust, 'Ueber die Porphyrgebilde', 1835, s.
89-96; also his 'Belenchtung der Werner'schen Gangtheorie', 1840, s. 6; and
C. von Wissenbach, 'Abbildungen merkwurdiger Gangverhaltnisse', 1836, fig.
12. The ribbon-like structure of the veins is, however, no more to be
regarded of general occurrence than the periodic order of the different
members of these masses.
All the facts connected with our geological hypotheses on the formation of
the earth's crust and the metamorphism of rocks have been unexpectedly
elucidated by the ingenious idea which led to a comparison of the slags or
scoriae of our smelting furnaces with natural minerals, and to the attempt
of reproducing the latter from their elements.*
[footnote] *Mitscherlich, 'Ueber die kunstliche Darstellung der
Mineralien', in the 'Abhandl. der Akademie der Wiss. zu Berlin', 1822-3, s.
25-41.
In all these operations, the same affinities manifest themselves which
determine chemical combinations both in our laboratories and in the interior
of the earth. The most considerable part of
p 268
the simple minerals which characterize the more generally diffused Plutonic
and erupted rocks, as well as those on which they have exercised a
metamorphic action, have been produced in a crystalline state, and with
perfect identify, in artificial mineral products. We must, however,
distinguish here between the scoriae accidentally formed, and those which
have been designedly produced by chemists. To the former belong feldspar,
mica, augite, olivine, hornblende, crystallized oxyd of iron, magnetic iron
in octahedral crystals, and metallis titanium;* to the latter, garnets,
idocrase, rubies (equal in hardness to those found in the East), olivine,
and augite.**
[footnote] *In scoriae crystals of feldspar have been discovered by Heine
in the refuse of a furnace for copper fusing, near Sangerhausen, and
analyzed by Kersten (Poggend., 'Annalen', bd. xxxiii., s. 337); crystals of
augite in scoriae at Sahle (Mitscherlich, in the 'Abhandl. der Akad. zu
Berlin', 1822-23, s. 40); of oliving by Seifstrom (Leonhard,
'Basalt-Gebilde', bd. ii., s. 495); of mica in old scoriae of Schloss
Garpenberg (Mitscherlich, in Leonhard, op. cit., s. 506); of magnetic iron
in the scoriae of Chatillon sur Seine (Leonhard, s. 441); and of micaceous
iron in potter's clay (Mitscherlich, in Leohnard, op. cit., s. 234).
[See Ebelmer's papers in 'Ann. de Chimie et de Physique', 1847; also 'Report
on the Crystalline Slags', by John Percy, M.D., F.R.S., and William Hallows
Miller, M.A., 1847. Dr. Percy, in a communication with which he has kindly
favored me, says that the minerals which he has found artificially produced
and proved by analysis are Humboldtilite, gehlenite, olivine, and magnetic
oxyd of iron, in octahedral crystals. He suggests that the circumstance of
the production of gehlenite at a high temperature in an iron furnace may
possibly be made available by geologists in explaining the formation of the
rocks in which the natural mineral occurs, as in Fassathal in the Tyrol.] --
Tr.
[footnote] **Of minerals purposely produced, we may mention idocrase and
garnet (Mitscherlich, in Poggend., 'Annalen der Physik', bd. xxxii., s.
340); ruby (Gaudin, in the 'Comptes Rendus de l'Academie de Science', t.
iv., Part i., p. 999); olivine and augite (Mitscherlich and Berthier, in the
'Annales de Chimie et de Physique', t. xxiv., p. 376). Notwithstanding the
greatest possible similarity in crystalline form, and perfect identity in
chemical composition, existing, according to Gustav Rose, between augite and
hornblende, hornblende has never been found accompanying augite in scoriae,
nor have chemists ever succeeded in artificially producing either hornblende
or feldspar (Mitscherlich in Poggend., 'Annalen', bd. xxxiii., s. 340, and
Rose, 'Reise nach dem Ural', bd. ii., s. 358 und 363). See also, Beaudant,
in the 'Mem. de l'Acad. des Sciences', t. viii., p. 221, and Becquerel's
ingenious experiments in his 'Trait de l'Electricite,' t. i., p. 334; t.
iii., p. 218; and t. v., p. 148 and 185.
These minerals constitute the main constituents of granite, gneiss, and mica
schist, of basalt, dolerite, and many porphyries. The artificial production
of feldspar and mica is of most especial geognostic importance with
reference to the theory of the formation of gneiss by the metamorphic agency
of argillaceous schist, which contains all the constituents of granite,
p 269
potash not excepted.*
[footnote] *D'Aubuisson, in the 'Journal de Physique', t. lxviii., p. 128.
It would not be very surprising, therefore, as is well observed by the
distinguished geognosist, Von Dechen, if we were to meet with a fragment of
gneiss formed on the walls of a smelting furnace which was built of
argillaceous slate and graywacke.
After having taken this general view of the three classes of erupted,
sedimentary, and metamorphic rocks of the earth's crust, it still remains
for us to consider the fourth class, comprising 'conglomerates', or 'rocks
of detrius'. The very term recalls the destruction which the earth's crust
has suffered, and likewise, perhaps reminds us of the process of
cementation, which has connected together, by means of oxyd of iron, or of
some argillaceous and calcareous substances, the sometimes rounded and
sometimes angular portions of fragments. Conglomerates and rocks of
detritus, when considered in the widest sense of the term, manifest
characters of a double origin. The substances which enter into their
mechanical composition have not been alone accumulated by the action of the
waves of the sea or currents of fresh water, for there are some of these
rocks the formation of which can not be attributed to the action of water.
"When basaltic islands and trachytic rocks rise on fissures, friction of the
elevated rock against the walls of the fissures causes the elevated rock to
be inclosed by conglomerates composed of its own matter. The granules
composing the sandstones of many formations have been separated rather by
friction against the erupted volcanic or Plutonic rock than destroyed by the
erosive force of a neighboring sea. The existence of these friction
'conglomerates', which are met with in enormous masses in both hemispheres,
testifies the intensity of the force with which the erupted rocks have been
propelled from the interior through the earth's crust. This detritus has
subsequently been taken up by the waters, which have then deposited it in
the strata which it still covers."*
[footnote] *Leop. von Buck, 'Geognost. Briefe', s. 75-82, where it is also
shown why the new red sandstone (the 'Todtliegende' of the Thuringian flotz
formation) and the coal measures must be regarded as produced by erupted
porphyry.
Sandstone formations are found imbedded in all strata, from the lower
silurian transition stone to the beds of the tertiary formations, superposed
on the chalk. They are found on the margin of the boundless plains of the
New Continent, both within and without the tropics, extending like
breast-works along the ancient shore, against which the sea once broke its
foaming waves.
p 270
If we cast a glance on the geographical distribution of rocks, and their
relations in space, in that portion of the earth's crust which is accessible
to us, we shall find that the most universally distributed chemical
substance is 'silicic acid', generally in a variously-colored and opaque
form. Next to solid silicic acid we must reckon carbonate of lime, and then
the combinations of silicic acid with alumina, potash, and soda, with lime,
magnesia, and oxyd of iron.
The substances which we designate as 'rocks' are determinate associations of
a small number of minerals, in which some combine parasitically, as it were,
with others, but only under definite relations; thus, for instance, although
quartz (silica), feldspar, and mica are the principal constituents of
granite, these minerals also occur, either individually or collectively, in
many other formations. By way of illustrating how the quantitative
relations of one feldspathic rock differ from another, richer in mica than
the former, I would mention that, according to Mitscherlich, three times
more alumina and one third more silica than that ossessed by feldspar, give
the constituents that enter into the composition of mica. Potash is
contained in both -- a substance whose existence in many kinds of rocks is
probably antecedent to the dawn of vegetation on the earth's surface.