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 ma.s.s 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 Po?et (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 ma.s.ses 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 a.s.suming 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 quant.i.ties 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 ma.s.ses, 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 (x.x.xvii., 8 and 9), who rank jasper among the "opaque gems," the name appears to have been given to fragments of "jas.p.a.chat", 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.*
[footnote] *On the transformation of compact into granular limestone by the 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. x.x.xix., 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 n.o.ble 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, "Const.i.tution 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 pa.s.sage 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. x.x.xvi., 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 const.i.tuted the princ.i.p.al 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 a.s.sumption 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 ma.s.ses of dolomite found in Southern Tyrol, and on the Italian side of the Alps, present the most remarkable instance of metamorphism produced by ma.s.sive 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 ma.s.ses of serpentine. In the valley of the Fa.s.sa, dolomite rises perpendicularly in smooth walls of dazzling whiteness to a height of many thousand feet. It forms sharply-pointed conical mountains, cl.u.s.tered 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 ill.u.s.trious 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 a.s.sociated together, as, for instance, the occurrence of melaphyre p 265 and the transformation of compact limestone into a crystaline ma.s.s 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 a.n.a.logous 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 ma.s.ses 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.