. References. Beudant, Voyage en Hongrie, vol. ii. p. 363. v. Pettko, in Haidinger's Abhandlungen, 1847, vol. i. p. 298; and as to Schemnitz, in the Abhandlung. d. geol. Reichstanst. 1853, vol. ii. No. 1. He names the variety with felsite balls 'Spherolite rock.' Erdmann, Journ. f. tech. Chemie, 1832, vol. xv. p. 38. Delesse, Bullet. de la Soc. géol. 1854, [2] vol. xi. p. 109; v. L. u. Br. Jahrb. 1856, p. 195. C. OBSIDIAN and PUMICE-STONE. OBSIDIAN und BIMSTEIN. (Germ.) Obsidian is a volcanic glass, sometimes porphyritic by Spec. grav. 2.3-2.5 This glassy or frothy texture belongs only to the rocks of the trachyte group, and more especially to the trachyte-porphyries or rhyolites. Their colour is (in the case of obsidian) usually dark-black, brown, or greenish ; in the case of pumice-stone, on the other hand, white or yellowish-grey. According to differences of texture, we may distinguish : (a) COMMON OBSIDIAN. GEMEINER OBSIDIAN. (Germ.) (b) OBSIDIAN-PORPHYRY OBSIDIANPORPHYR. (Germ.) (c) SPHÆRULITIC OBSIDIAN. SPHÄROLITISCHER OBSIDIAN. (Germ.) A mere glass. With sanidine crystals, or sometimes also mica plates. With felsite balls, passing over into perlite. (d) VESICULAR OBSIDIAN, PUMICE-STONE. BLASIGER OBSIDIAN (BIMSTEIN). (Germ.) OBSIDIENNE VACUOLAIRE. (Fr.) This rock is often of such long fibre and so porous that it will even float on water. These species of volcanic glass are only found in trachytic volcanic regions. They are very characteristically developed at the Peak of Teneriffe, the Lipari Islands, in Iceland, in Mexico, &c. References. Beudant, Voyage en Hongrie, vol. iii. p. 389. Erdmann, Journ. f. techn. Chem. 1832, vol. xv. p. 36. Mundoch, Phil. Mag. and Journ. 1844, [2] vol. xxv. p. 495. v. d. Boon-Meesch, Pogg. Ann. 1828, vol. xii. p. 616. Herter, Perlstein. Zeitschr. d. deutschen geol. Gesellsch. vol. xv. p. 459. PHONOLITE GROUP. 15. PHONOLITE, CLINKSTONE. PHONOLITH, KLINGSTEIN, PORPHYRSCHIEFER. (Germ.) A compact base or matrix, in its fresh state dark greenish-grey, showing here and there single cleavage surfaces of a vitreous felspar. The mass is as a rule somewhat slaty or schistose in texture, or of thinly tabular jointed structure-gives a clear sound when struck by the hammer; on weathering a sharply defined white crust is formed. Spec. grav. Contains silica. 2.4-2.6 Klaproth proposed the name of phonolite for this rock, as having a more scientific air than that of klingstein, previously in use, of which it is the translation, and the new name has been very generally accepted. The peculiar properties of the rock had long been recognised, its difference from basalt, trachyte, felsite rock, &c., but its exact ingredients had not been investigated. Gmelin first drew attention to its analysis by muriatic acid, in which it is partly soluble and partly insoluble. The soluble part was considered to be a zeolitic substance, the latter a felspar, and the whole was considered to be an intimately blended compound of zeolite and felspar (sanidine). By the more exact microscopic and chemical investigations of later times, however, it has appeared that the composition of the phonolite mass is not so simple, and is in some part wholly different from what was supposed. It is even questionable whether in its fresh state it contains any zeolitic substance at all; certain is it that the nepheline crystals which both Breithaupt and Rose early recognised in phonolite, as well as the mineral forming part of the matrix which Rammelsberg also judged to be nepheline, have frequently been mistaken for zeolite. G. Jenzsch ventures to give the following as the mineralogical composition of this rock, after investigating microscopically and chemically several very characteristic phonolites of Bohemia : These proportional values must, of course, vary greatly with locality. As accessories, the following minerals occur, and are sometimes distinctly to be recognised in the rock; viz. oligoclase, augite, magnetic iron-ore, olivine, haüyne, brown mica, leucite, and nosean; the last two minerals are the least frequent. It is possible that the zeolite (natrolite) which sometimes fills the crevices of the rock may also occur in the principal mass, but if so, it is probably the result of decomposition. In respect of the proportion of silica contained in phonolite, we might equally well group it with the basic as the acidic igneous rocks; it forms one of the intermediate links between the two. As it never contains quartz distinctly and separately developed, it might seem to be more allied mineralogically to the basic rocks; but geologically its character is nearer that of the trachytes than the basalts. Where it occurs together with the latter, as is very frequently the case, it seems to play the same part as the trachytes under similar circumstances. Its small content of water (0.6-0.8 per cent.) appears to be (at least in part) a secondary product, the result of a commencing decomposition; and in the same manner the occurrence of many accessory minerals in the mass, more especially those appearing in the clefts and vesicular cavities. Phonolite often acquires a porphyritic texture from the prominence of distinct crystals of sanidine and acicular hornblende. The most marked porphyritic varieties are as a rule little slaty and somewhat decomposed. As decomposition progresses, the crystals become more prominent, and even the titanite then is frequently to be easily recognised. Many phonolites are dark-spotted, or they contain round grains of peculiar composition and colour; these, however, as in the case of basalt, appear chiefly to arise from commencing decomposition. Many are entirely decomposed (kaolinised), and show an earthy frac ACIDIC IGNEOUS ROCKS. (1) VOLCANIC. ture, with a light colour. Whole mountains of ph have, apparently at least, decayed in this manne scarcely a trace of slaty texture remaining. Nauma this variety Trachytic phonolite; it is almost the only in which vesicular and amygdaloidal texture is fou never occurs in the fresh, dark, and slaty kinds. J is even of opinion that the apparent vesicular and daloidal cavities of the phonolite are not genuine b of the original rock, but have arisen subsequentl kind of corrosive process of decay. This view ce agrees with the absence of cavities in the perfectly rock. Yet in some few phonolites are found very d vesicular cavities. These cavities, as also the cle fissures, most usually contain zeolites; especially phyllite, chabasite, comptonite, desmine, natrolite cime, or calespar and hyalite. Dark-coloured, compact, so PHONOLITHE COMMUNE. (Fr.)) with ringing sound when st or imperfect slaty cleavag the hammer. Mileschauer in Bohemia; Milzburg on the The same mass wi tinct crystals of horn augite, or sanidine. Not slaty, not cl TRACHYTÄHNLICHER PHONOLITH. (Germ.) rough, of a rather grey colour; frequently porphyritic, geodic, or amygdaloidal. in Bohemia. Luschwitz, near Aussig, hemia. Marienberg, nea The slaty or schistose phonolites are those whic most usually of tabular or columnar jointed stru Those which are not slaty are usually only irreg massive. This rock forms isolated conical hills, even more fectly than basalt, especially so in the Bohemian M gebirge and in the Oberlausitz. Much more rarely it form great connected mountain ranges, and it is rarely found in the form of dykes than basalt. On the continent of Europe, phonolite is only known as of tertiary or of still more recent origin, and never as a genuine plutonic rock. It is, on the other hand, also unknown as actual lava at active volcanoes, and from this it would appear that its state must be more or less the result of cooling under pressure or of transmutation. In favour of the latter supposition (of transmutation) is the presence of zeolite, which is, however, not a constant ingredient. Lyell, in his Geology, has instanced the occurrence of a phonolite of the Devonian period in Forfarshire. If this be a genuine phonolite, it is the only recorded instance of such being found of earlier than tertiary origin, but as the notice is quite incidental, and has reference to a different subject, and is moreover very brief, we cannot, without further explanation, accept it as authority in contravention of a law which otherwise appears universal. References. Gmelin, in Poggend. Ann. 1828, vol. xiv. p. 259. Schill, in G. Leonhard's Beitr. z. miner. Kenntn. von Baden, 1854, vol. iii. p. 59. Schmid, in Poggend. Ann. 1853, vol. lxxxix. Br. Jahrb. 1856, p. 845. Jenzsch, in the Zeitschr. d. d. geol. Ges. 1856, p. 167; and v. Rath, in the Zeitschr. d. d. geol. Ges. 1856, p. 291, and 1860, Engelbach, in the Erl. z. geogn. Karte v. Hessen, Sect. Schotten, 1859, p. 45. Fischer, Die Trachyte u. Phonolithe des Höhganes, v. L. Jahrb. Rammelsburg, Analysen von Phonolithen, Zeitschr. der d. geol. v. Fritsch has lately set up a distinction between nepheline- 2. Plutonic. Granite is the principal rock of the plutonic division of the acidic igneous rocks, as trachyte is of the volcanic division of the same rocks. |