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Geological Survey 7th Annual Report
Obsidian Cliff, Yellowstone National Park |
HISTORICAL REVIEW
Having attempted an explanation of the nature and origin of lithophysæ which seems to accord most closely with the phenomena of their occurrence at Obsidian Cliff, it will be of interest to review the opinions of those who have studied the question in other fields and upon different material.
Ferdinand von Richthofen.—The first notice of these curious rock structures is the very full and graphic description of those found in the rhyolites of Hungary, given by von Richthofen1 more than twenty-five years ago, at which time he proposed the name of Lithophysen, because the rock appeared to have been inflated by the expanding of gas. He considered them quite distinct from spherulites and said that only through the most careful chemical analysis could positive conclusions as to the process of their formation be reached. He thought that a gas had been evolved, for the oft-repeated, lamellar banding of the surrounding rock showed that it had been forced apart, not violently but gradually, since the separate layers of the rock bent themselves exactly to the curvature of the hollows. Moreover the gas had been evolved from the inclosed substance for the hollow spaces are found only in connection with this and do not occur by themselves in the groundmass. Further, they have come into existence after the laminated arrangement of the rock; consequently the gas must have been connected with the inclosed substance in either a solid or fluid condition. From the form of the repeated shells within the lithophysæ it would appear that their substance was exceedingly viscous at the time the gas was set free. The question then suggests itself how and for what reason was the substance of the inclusion separated out of the rock and what sort of gas was evolved. He thought that the experiments of Daubree threw some light on the process. They showed, he said, that water at a temperature of 400° C. under high pressure dissolves all the constituents of glass and upon cooling deposits them as different crystals, in part anhydrous. He referred to the effect of hydration in increasing the fluidity of lavas and added that, if in consequence of this any hydrate had been developed in particular parts of the rock, then the process already suggested would easily take place, for a part of the water could no longer remain in combination, because of the lessening of pressure upon the eruption of the rock, and would disengage itself as gas. It would force the viscous rock mass apart, but could not escape, and thus the forms in question must have been produced.
1Studien aus den ungarisch-siebenbürgischen Trachytgebirgen: Jahrbuch k. k. geol. Reichsanstalt, vol. 11, 1860, p. 180.
With this view, expressed by von Richthofen in the early days of the science of microscopical petrography, that of the present paper is in accord at many essential points; but the earlier view represents bubbles of gas expanding from their point of liberation within the viscous lava and forcing back the surrounding matrix; they carry up successive films of the plastic glass and thus produce concentric shells. The objections to this are the physical difficulty of expansion under the circumstances and the radially fibrous structure observed in connection with the often incomplete shells, which correlate them with the concentric layers of spherulites.
Dr. Joseph Szabó,1 in a paper on the trachytes and rhyolites in the vicinity of Tokay, Hungary, expresses the opinion that lithophysæ are only a stage in the mechanical and chemical alteration of spherulites, which through chemical alteration first lose their luster and then their coherence. He thinks the bases are removed chemically, the insoluble particles mechanically, and that the silica is concentrated in the cavity.
1Die Trachyte und Rhyolite der Umgebung von Tokaj: Jahrbuch k. k. geol. Reichsanstalt, vol. 16, 1866, p. 89.
Karl Ritter von Hauer1 published in the same year the results of the chemical analyses of four rhyolites which carry lithophysæ, and also of the substance of the lithophysæ. They are given further on, in the table of chemical analyses (p. 43). They show that the lithophysæ do not differ chemically from the groundmass which contains them and give some idea, he said, of their mode of origin. He thought that the lithophysæ were separated out from the groundmass only in a mechanical way through the development of gas, and not as the product of metamorphosing influences on the groundmass; that the gases (in this case water vapor), upon being evolved in the still viscous mass, were able to find exit only slowly and occasionally not at all; in this way pores, larger hollows, and bladder-like inflations were produced.
1Die Gesteine mit Lithophysenbildungen von Telki-Banya in Ungarn: Verhandl. k. k. geol. Reichsanstalt, 1866, p. 98.
Justus Roth2 in 1869 adopted Szabo's views that lithophysæ are only mechanical and chemical alterations of spherulites and repeats them in his Allgemeine und chemische Geologic, 1883, vol. 2, p. 216.
2Beiträge zur Petrographie der plutonischen Gesteine, 1869, p. 168.
Ferdinand Zirkel,3 in describing the rhyolites in the collection of the Geological Exploration of the Fortieth Parallel, calls attention to spherulites in the rock from Shoshone Mesa, which "develop, by decomposition, a concentric layer-structure." These he considers to be the same as the lithophysæ of von Richthofen and that they in like manner were only the result of chemical alteration.
3U. S. Geol. Ex. Fortieth Par., vol. 6, Microscopical Petrography, 1876, p. 212.
Ch. E. Weiss4 considers the cavity of a hollow spherulite as playing the same role as a solid body around which the spherulite forms. He thinks hollow spherulites and hollow spheres are only larger and smaller spherulites which have formed around gas bubbles; where several bubbles were near together and touched one another there arose the chambered, hollow spherulites, or lithophysæ. The coarser grained spherulites, he says, show they are composed of two minerals, quartz and feldspar.
4Quartzporphyren aus Thüringen: Zeitschr. Deutsch. geol. Gesell., Berlin, vol. 29, 1877, p. 418.
Grenville A. J. Cole5reviews the previous opinions on the subject and concludes that the hollows are due to the decomposition of solid spherulites by chemical agents, the material having been carried out through cracks in the rock.
5On hollow spherulites and their occurrence in ancient British lavas: Quart. Jour. Geol. Soc. London, May, 1885.
C. A. Tenne6 describes the lithophysæ in the obsidian from Cerro de las Navajas, Mexico, collected by von Humboldt, to which von Richthofen calls attention in the paper already cited and from which Rose obtained the olivine crystals. He says that the substance of the lithophysæ must be devitrified obsidian, which a microscopical investigation makes probable. He gives chemical analyses (see analyses VII and VIII, next page) which show that the obsidian and lithophysæ have the same chemical composition.
6Uebr Gesteine des Cerro de las Navajas (Messerberg) in Mexico: Zeitschr. Deutsch. geol. Gesell., Berlin, 1885, p. 610.
Whitman Cross,1 in a paper "On the occurrence of topaz and garnet in lithophyses of rhyolite," points out the fact that these minerals are not of secondary formation in the cavities, but primary, "produced by sublimation or crystallization from presumably heated solutions, contemporaneous or nearly so with the final consolidation of the rock. The lithophysal cavities seem plainly caused by the expansive tendency of confined gases or vapors, while the shrinkage cracks in the walls and white masses of the Nathrop rock suggest the former presence of moisture. Certainly the history of the lithophyses themselves embraces that of both topaz and garnet."
1Am. Jour. Sci., 3d series, vol. 31, 1886, p. 432.
From this it is seen that two distinct views of the origin of cavities within the lithophysæ have been taken: One, that the hollows were of primary origin, formed while the lava was still plastic, and were due to inclosed gases or vapors. Among those who held this opinion some considered the lithophysæ as wholly distinct from spherulites, while others thought them simply hollow varieties of spherulites. The second view was that the hollows had been produced in solid spherulites by chemical decomposition and alteration, and were subsequent to the solidification of the lavas in which they occur.
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