Cosmos: A Sketch of the Physical Description of the Universe Part 27

To the second or sedimentary rocks belong the greater part of the formations 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 acc.u.mulation 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 acc.u.mulated 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 quant.i.ty 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 const.i.tuents 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 ma.s.ses. The obscure process of this metamorphism, and the action if produces, must therefore compose the third cla.s.s 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 a.s.sume 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 a.n.a.logies, 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 cla.s.s, 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 ma.s.s. Even where there is no immediate contact, the proximity of such a ma.s.s 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 ma.s.ses; 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, 'Princ.i.p.ales 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 ma.s.ses.*

[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 pa.s.sages 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, h.o.a.r 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 a.n.a.logy 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 const.i.tution of rocks experience from the action of heat, have shown that volcanic ma.s.ses*

(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 gla.s.s, having a h.o.m.ogeneous fracture, and where the cooling was slow, a stony ma.s.s 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 pa.s.s 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 ma.s.s of hypersthenic rock conglomerated into spherical ma.s.ses (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.