which have been comprised under the old
p 254
systematic, but not very correct designation of 'transition, flot' or
'secondary', and 'tertiary formations'. If the erupted rocks had not
exercised an elevating, and, owing to the simultaneous shock of the earth, a
disturbing influence on these sedimentary formations, the surface of our
planet would have consisted of strata arranged in a uniformly horizontal
direction above one another. Deprived of mountain chains, on whose
declivities the gradations of vegetable forms and the scale of the
diminishing heat of the atmosphere appear to be picturesquely reflected --
furrowed ony here and there by valleys of erosion, formed by the force of
fresh water moving on in gentle undulations, or by the accumulation of
detritus, resulting from the action of currents of water -- continents would
have presented no other appearance from pole to pole than the dreary
uniformity of the llanos of South America or the steppes of Northern Asia.
The vault of heaven would everywhere have appeared to rest on vast plains,
and the stars to rise as if they emerged from the depths of ocean. Such a
condition of things could not, however, have generally prevailed for any
length of time in the earlier periods of the world, since subterranean
forces must have striven in all epochs to exert a counteracting influence.
Sedimentary strta have been either precipitated or deposited from liquids,
according as the materials entering into their composition are supposed,
whether as limestone or argillaceous slate, to be either chemically
dissolved or suspended and commingled. But earth, when dissolved in fluids
impregnated with carbonic acid, must be regarded as undergoing a mechanical
process while they are being precipitated, deposited, and accumulated into
strata. This view is of some importance with respect to the envelopment of
organic bodies in petrifying calcareous beds. The most ancient sediments of
the transition and secondary formations have probably been formed from water
at a more or less high temperature, and at a time when the heat of the upper
surface of the earth was still very considerable. Considered in this point
of view, a Plutonic action seems to a certain extent also to have taken
place in the sedimentary strata, especially the more ancient; but these
strata appear to have been hardened into a schistose structure, and under
great pressure, and not to have been solidified by cooling, like the rocks
that have issued from the interior, as, for instance, granite, porphyry, and
basalt. By degrees, as the waters lost their temperature, and were able to
absorb a copious supply of the carbonic acid gas with which
p 255
the atmosphere was overcharged, they became fitted to hold in solution a
larger quantity of lime.
'The sedimentary strata', setting aside all other exogenous, purely
mechanical deposits of sand or detritus, are as follows:
'Schist', of the lower and upper transition rock, compositing the silurian
and devonian formations; from the lower silurian strata, which were once
termed cambrian, to the upper strata of the old red sandstone or devonian
formation, immediately in contact with the mountain limestone.
'Carboniferous deposits':
'Limestones' imbedded in the transition and carboniferous formations;
zechstein, muschelkalk, Jura formation and chalk, also that portion of the
tertiary formation which is not included in sandstone and conflomerate.
'Travertine', fresh-water limestone, and silicious concretions of hot
springs, formations which have not been produced under the pressure of a
large body of sea water, but almost in immediate contact with the
atmosphere, as in shallow marshes and streams.
'Infusorial deposits': geognostical phenomena, whose great importance in
proving the influence of organic activity in the formation of the solid part
of the earth's crust was first discovered at a recent period by my
highly-gifted friend and fellow-traveler, Ehrenberg.
If, in this short and superficial view of the mineral constituents of the
earth's crust, I do not place immediately after the simple sedimentary rocks
the conglomerates and sandstone formations which have also been deposited as
sedimentary strata from liquids, and which have been imbedded alternately
with schist and limestone, it is only because they contain, together with
the detritus of eruptive and sedimentary rocks, also the detritus of gneiss,
mica slate, and other metamorphic masses. The obscure process of this
metamorphism, and the action if produces, must therefore compose the third
class of the fundamental forms of rock.
Endogenous or erupted rocks (granite, porphyry, and melaphyre) produce, as I
have already frequently remarked, not only cynamical, shaking, upheaving
actions, either vertically or laterally displacing the strata, but they also
occasion changes in their chemical composition as well as in the nature of
their internal structure; new rocks being thus formed, as gneiss, mica
slate, and granular limestone (Carrara and Parian marble). The old silurian
or devonian transition schists, the belemnitic limestone of Tarantaise, and
the dull gray calcareous
p 256
sandstone ('Macigno'), which contains alggae found in the northern
Apennines, often assume a new and more brilliant appearance after their
metamorphosis, which renders it difficult to recognize them. The theory of
metamorphism was not established until the individual phases of the change
were followed step by step, and direct chemical experiments on the
difference in the fusion point, in the pressure and time of cooling, were
brought in aid of mere inductive conclusions. Where the study of chemical
combinations is regulated by leading ideas,* it may be the means of throwing
a clear light on the wide field of geognosy, and over the vast laboratory of
nature in which rocks are continually being formed and modified by the
agency of subterranean forces.
[footnote] *See the admirable researches of Mitscherlich, in the 'Abhandl.
der Berl. Akad.' for the years 1822 and 1823, s. 25-41; and in Poggend.,
'Annalen', bd. x., s. 137-152; bd. xi., s. 323-332; bd. sli., s. 213-216
(Gustav Rose, 'Ueber Gildung des Kalkspaths und Aragonits', in Poggend.,
'Annalen', bd. xli., s, 353-366; Haidinger, in the 'Transactions of the
Royal Society of Edinburgh', 1827, p. 148.)
The philosopohical inquirer will escape the deception of apparent analogies,
and the danger of being led astray by a narrow view of natural phenomena, if
he constantly bear in view the complicated conditions which may, by the
intensity of their force, have modified the counteracting effect of those
individual substances whose nature is better known to us. Simple bodies
have, no doubt, at all periods, obeyed the same laws of attraction, and,
wherever apparent contradictions present themselves, I am confident that
chemistry will in most cases be able to trace the cause to some
corresponding error in the experiment.
Observations made with extreme accuracy over large tracts of land, show that
erupted rocks have not been produced in an irregular and unsystematic
manner. In parts of the globe most remote from one another, we often find
that granite, basalt, and diorite have exercised a regular and uniform
metamorphic action, even in the minutest details, on the strata of
argillaceous slate, dense limestone, and the grains of quartz in sandstones.
As the same endogenous rock manifests almost every where the same degree of
activity, so on the contrary, different rocks belonging to the same class,
whether to the endogenous or the erupted, exhibit great differences in
their character. Intense heat has undoubtedly influenced all these
phenomena, but the degree of fluidity (the more or less perfect mobility of
the particles -- their more viscous composition) has varied very
considerably from the granite to the basalt, while at different geological
p 257
periods (or metamorphic phases of the earth's crust) other substances
dissolved in vapors have issued from the interior of the earth
simultaneously with the eruption of granite, basalt, greenstone porphyry,
and serpentine. This seems a fitting place again to draw attention to the
fact that, according to the admirable views of modern geognosy, the
metamorphism of rocks is not a mere phenomenon of contact, limited to the
effect produced by the apposition of two rocks, since it comprehends all the
generic phenomena that have accompanied the appearance of a particular
erupted mass. Even where there is no immediate contact, the proximity of
such a mass gives rise to modifications of solidification, cohesion,
granulation, and crystallization.
All eruptive rocks penetrate, as ramifying veins either into the sedimentary
strata, or into other equally endogenous masses; but there is a special
importance to be attached to the difference manifested between 'Plutonic'
rocks* (granite, porphyry, and serpentine) and those termed 'volcanic' in
the strict sense of the word (as trachyte, basalt, and lava).
[footnote] ([Lyell, 'Principales of Geology', vol. i.i., p. 353 and 359.]
-- Tr.
The rocks produced by the activity of our present volcanoes appear as
band-like streams, but by the confluence of several of them they may form an
extended basin. Wherever it has been possible to trace basaltic eruptions,
they have generally been found to terminate in slender threads. Examples of
these narrow openings may be found in three places in Germany: in the
'Pflaster-kaute', at Marksuhl, eight miles from Eisenach; in the blue
'Kuppe', near Eschwege, on the banks of the Werra; and in the Druidical
stone on the Hollert road (Siegen), where the basalt has broken through the
variegated sandstone and graywacke slate, and has spread itself into
cup-like fungoid enlargements, which are either grouped together like rows
of columns, or are sometimes stratified in thin laminae. The case is
otherwise with granite, syenite, quartzose porphyry, serpentine, and the
whole series of unstratified compact rocks, to which, from a predilection
for a mythological nomenclature, the term Plutonic has been applied. These,
with the exception of occasional veins, were probably not erupted in a state
of fusion, but merely in a softened condition; not from narrow fissures, but
from long and widely-extending gorges. They have been protruded, but have
not flowed forth, and are found not in streams like lava, but in extended
masses.*
[footnote] *The description here given of the relation of position under
which granite occurs, expresses the general or leading character of the
whole formation. But its aspect at some places leads to the belief that it
was occasionally more fluid at the period of its eruption. The description
given by Rose, in his 'Reise nach dem Ural', bd. i., s. 599, of part of the
Narym chain, near the frontiers of the Chinese territories, as well as the
evidence afforded by trachyte, as described by Dufrenoy and Elie de
Beaumont, in their 'Description Geologique de la France', t. i., p. 70.
Having already spoken in the text of the narrow apertures through which the
basalts have sometimes been effused, I will here notice the large fissures,
which have acted as conducting passages for melaphyres, which must not be
confounded with basalts. See Murchison's interesting account ('The Silurian
System', p. 126) of a fissure 480 feet wide, through which melaphyre has
been ejected, at the coal-mine at Cornbrook, Hoar Edge.
Some groups of dolerite and trachyte indicate
p 258
a certain degree of basaltic fluidity; others, which have been expanded into
vast craterless domes, appear to have been only in a softened condition at
the time of their elevation. Other trachytes, like those of the Andes, in
which I have frequently perceived a striking analogy with the greenstones
and syenitic porphyries (which are argentiferous, and without quartz), are
deposited in the same manner as granite and quartzose porphyry.
Experiments on the changes which the texture and chemical constitution of
rocks experience from the action of heat, have shown that volcanic masses*
(diorite, augitic porphyry, basalt, and the lava of AEtna) yield different
products, according to the difference of the pressure under which they have
been fused, and the length of time occupied during their cooling; thus,
where the cooling was rapid, they form a black glass, having a homogeneous
fracture, and where the cooling was slow, a stony mass of granular
crystalline structure.
[footnote] *Sir James Hall, in the 'Edin. Trans.', vol. v., p. 43, and vol.
vi., p. 71; Gregory Watt, in the 'Phil. Trans. of the Roy. Soc. of London
for' 1804, Part ii., p. 279; Dartigues and Fleurieu de Bellevue, in the
'Journal de Physique', t. lx., p. 456; Bischof, 'Warmelchre', s. 313 und 443.
In the latter case, the crystals are formed partly in cavities and partly
inclosed in the matrix. The same materials yield the most dissimilar
products, a fact that is of the greatest importance in reference to the
study of the nature of erupted rocks, and of the metamorphic action which
they occasion. Carbonate of lime, when fused under great pressure, does not
lose its carbonic acid, but becomes, when cooled, granular limestone; when
the crystallization has been effected by the dry method, saccharoidal
marble; while by the humid method, calcareous spar and aragonite and
produced, the former under a lesser degree of temperature than the latter.*
[footnote] *Gustav Rose, in Poggend., 'Annalen.' bd. xliii., s 364.
Differences of temperature
p 259
likewise modify the direction in which the different particles arrange
themselves in the act of crystallization, and also affect the form of the
crystal.*
[footnote] *On the dimorphism of sulphur, see Mitscherlich, 'Lehrbuch der
Chemie', 55-63.
Even when a body is not in a fluid condition, the smallest particles may
undergo certain relations in their various modes of arrangement, which are
manifested by the different action on light.*
[footnote] *On gypsum as a uniaxal crystal, and on the sulphate of
magnesia, and the oxyds of zinc and nickel, see Mitscherlich, in Poggend.,
'Annalen.' bd. xi., s. 328.
The phenomena presented by devitrification, and by the formation of steel by
cementation and casting -- the transition of the fibrous in the granular
tissue of the iron, from the action of heat* and probably, also, by regular
and long-continued concussions -- likewise throw a considerable degree of
light on the geological process of metamorphism.
[footnote] *Coste, 'Versuche am Creusot uber das bruchig werden des
Stabeisens.' Elie de Beaumont, 'Mem. Geol.', t. ii., p. 411.
Heat may even simultaneously induce opposite actions in crystalline bodies;
for the admirable experiments of Mitscherlich have established the fact*
that calcareous spar, without altering its condition of aggregation, expands
in the direction of one of its axes and contracts in the other.
[footnote] * Mitscherlich, 'Ueber die Ausdehnung der Krystallisirten Korper
durch die Warmelehre', in Poggend., 'Annalen', bd. x., s. 151.
If we pass from these general considerations to individual examples, we find
that schist is converted, by the vicinity of Plutonic erupted rocks, into a
bluish-black, glistening roofing slate. Here the planes of stratification
are intersected by another system of divisional stratification, almost at
right angles with the former,* and thus indicating an action subsequent to
the alteration.
[footnote] * On the double system of divisional planes, see Elie de
Beaumont, 'Geologie de la France', p. 41; Credner, 'Geognosie Thuringens und
des Harzes', s. 40; and Romer, 'Das Rheinische Uebergangsgebirge', 1844. s.
5 und 9.
The penetration of silica causes the argillaceous schist to be traversed by
quartz, transforming it, in part, into whetstone and silicious schist; the
latter sometimes containing carbon, and being then capable of producing
galvanic effects on the nerves. The highest degree of silicifaction of
schist is that observed in ribbon jasper, a material highly valuable in the
arts,* and which is produced in the Oural Mountains
p 260
by the contact and eruption of augitic porphyry (at Orsk), of dioritic
porphyry (at Aufschkul), or of a mass of hypersthenic rock conglomerated
into spherical masses (at Bogoslowsk). At Monte Serrato, in the island of
Elba, according to Frederic Hoffman, and in Tuscany, according to Alexander
Brongniart, it is formed by contact with euphotide and serpentine.
[footnote] *The silica is not merely colored by peroxyd of iron, but is
accompanied by clay, lime, and potash. Rose, 'Reise', bd. ii., s. 187. On
the formation of jasper by the action of dioritic porphyry, augite, and by
persthene rock, see Rose, bd. ii., s. 169, 187, und 192. See, also, bd. i.,
s. 427, where there is a drawing of the porphyry spheres between which
jasper occurs, in the calcareous graywacke of Bogoslowsk, being produced by
the Plutonic influence of the augitic rock; bd. ii., s. 545; and likewise
Humboldt, 'Asie Centrale', t. i., p. 486.
The contact and Plutonic action of granite have sometimes made argillaceous
schist granular, as was observed by Gustav Rose and myself in the Altai
Mountains (within the fortress of Buchtarminsk),* and have transformed it
into a mass resembling granite, consisting of a mixture of feldspar and
mica, in which larger laminae of the latter were again imbedded.**
[footnote] *Rose, 'Reise nach dem Ural', bd. i., s. 586-588.
[footnote] **In respect to the volcanic origin of mica, it is important to
notice that crystals of mica are found in the basalt of the Bohemian
Mittelgebirge, in the lava that in 1822 was ejected from Vesuvius
(Monticelli, 'Storia del Vesuvio negli Anni 1821 e 1822', 99), and in
fragments of agrillaceous alte imbedded in scoriaceous basalt at Hohenfels,
not far from Gerolstein, in the Eifel (see Mitscherlich, in Leonhard,
'Basalt-Gebilde', s. 244). On the formation of feldspar in argillaceous
schist, through contact with porphyry, occurring between Urval and Poïet
(Forez), see Dufrenoy, in 'Geol. de la France', t. i., p. 137. It is
probably to a similar contact that certain schists near Paimpol, in
Brittany, with whose appearance I was much struck, while making a geological
pedestrian tour through that interesting country with Professor Kunth, owe
their amygdaloid and cellular character, t. i., p. 234.
Most geognosists adhere, with Leopold von Buch, to the well-known hypothesis
"that all the gneiss in the silurian strata of the transition formation,
between the Icy Sea and the Gulf of Finland, has been produced by the
metamorphic action of granite.*
[footnote] * Leopold von Buch, in the 'Abhandlungen der Akad. der
Wissenschaft zu Berlin, aus dem Jahr' 1842, s. 63, and in the 'Jahrbuchern
fur Wissenschaftliche Kritik Jahrg.' 1840, s. 196.
In the Alps, at St. Gothard, calcareous marl is likewise changed from
granite into mica slate, and then transformed into gneiss." Similar
phenomena of the formation of gneiss and mica slate through granite present
themselves in the oolitic group of the Tarantaise,* in which belemnites are
p 261
found in rocks, which have some claim to be considered as mica slate, and in
the schistose group in the western part of the island of Elba, near the
promontory of Calamita, and the Fichtelgebirge in Baireuth, between Loomitz
and Markleiten.**
[footnote] * Elie de Beaumont, in the 'Annales des Sciences Naturelles', t.
xv., p. 362-372. "In approaching the primitive masses of Mont Rosa, and the
mountains situated to the west of Coni, we perceive that the secondary
strata gradually lose the characters inherent in their mode of deposition.
Frequently assuming a character apparently arising from a perfectly distinct
cause, but not losing their stratification, they somewhat resemble in their
physical structure a brand of half-consumed wood, in which we can follow the
traces of the ligneous fibers beyond the spots which continue to present the
natural characters of wood." (See, also, the 'Annales des Sciences
Naturelles', t. xiv., p. 118-122, and von Dechen, 'Geognosie', s. 553.)
Among the most striking proofs of the transformation of rocks by Plutonic
action, we must place the belemites in the schists of Nuffenen (in the
Alpine valley of Eginen and in the Gries-glaciers), and the belemnites found
by M. Charpentier in the so-called primitive limestone on the western
descent of the Col de la Seigne, between the Enclove de Monjovet and the
'chalet' of La Lanchette, and which he showed to me at Bex in the autumn of
1822 ('Annales de Chimie', t. xxiii., p. 262).
[footnote] ** Hoffmann, in Poggend., 'Annalen', bd. xvi., s. 552, "Strate
of transition argillaceous schist in the Fichtelgebirge, which can be traced
for a length of 16 miles, are transformed into gneiss only at the two
extremities, where they come in contact with granite. We can there follow
the gradual formation of the gneiss, and the development of the mica and of
the feldspathic amygdaloids, in the interior of the argillaceous schist,
which indeed contains in itself almost all the elements of these substances."
Jasper, which,* as I have already remarked, is a production formed by the
volcanic action of augitic porphyry, could only be obtained in small
quantities by the ancients, while another material, very generally and
efficiently used by them in the arts, was granular or saccharoidal marble,
which is likewise to be regarded solely as a sedimentary stratum altered by
terrestrial heat and by proximity with erupted rocks.
[footnote] * Among the works of art which have come down to us from the
ancient Greeks and Romans, we observe that none of any size -- as columns or
large vases -- are formed from jasper; and even at the present day, this
substance, in large masses, is only obtained from the Ural Mountains. The
material worked as jasper from the Rhubarb Mountain (Raveniaga Sopka), in
Altai, is a beautiful ribboned porphyry. The word 'jasper' is derived from
the Semitic languages; and from the confused description of Theophrastus
('De Lapidibus', 23 and 27) and Pliny (xxxvii., 8 and 9), who rank jasper
among the "opaque gems," the name appears to have been given to fragments of
'jaspachat', and to a substance which the ancients termed 'jasponyx', which
we now know as 'opal-jasper'. Pliny considers a piece of jasper eleven
inches in length so rare as to require his mentioning that he had actually
seen such a specimen: "Magnitudinem jaspidis undecim unciarum vidimus,
formatamque inde effigem Neronis thoracatam." According to Theophrastus,
the stone which he calls emerald, and from which large obelists were cut,
must have been an imperfect jasper.
This opinion is corroborated by the accurate observations on the phenomena
of contact, by the remarkable experiments on fusion
p 262
made by Sir James Hall more than half a century ago, and by the attentive
study of granitic veins, which has contributed so largely to the
establishment of modern geognosy. Sometimes the erupted rock has not
transformed the compact into granular limestone to any great depth from the
point of contact. Thus, for instance, we meet with a slight transformation
-- a penumbra -- as at Belfast, in Ireland, where the basaltic veins
traverse the chalk, and, as in the compact calcareous beds, which have been
partially inflected by the contact of syenitic granite, at the Bridge of
Boscampo and the Cascade of Conzocoli, in the Tyrol (rendered celebrated by
the mention made of it by Count Mazari Peucati).*
[footnote[ *Humboldt, 'Lettre a M. Brochant de Villiers', in the 'Annales
de Chimie et de Physique', t. xxiii., p. 261; Leop. von Buch, 'Geog. Briefe
uber das sudliche Tyrol', s. 101, 105, und 273.
Another mode of transformation occurs where all the strata of the compact
limestone have been changed into granular limestone by the action of
granite, and syenitic or dioritic porphyry.*