as among the many lead salts produced by the decomposition of bournonite and galena at the Mina Beatriz, Sierra Gorda, Chili. Miscellaneous.-Pseudomorphs of talc after quartz, magnesite and calamine are found not uncommonly in a talc layer at Göpfersgrün, near Wunsiedel in the Fichtelgebirge, near a contact of granite and limestone, that has suffered dolomitization as the result of the action of magnesian solutions emanating from the eruptive rock. The genesis and growth of the talc pseudomorph after quartz are carefully described by Weinschenck 15 as taking place in the following manner. The quartz crystals are first traversed by numerous capillary cracks, running parallel to the prismatic faces and rarely parallel to the rhombohedral planes. Along the sides of these, little plates of talc are developed. From certain points within the crystal other fissures then begin to form, and along the sides of these more talc is formed, until finally there remain only a few isolated areas in which the original mineral can be detected. The production of the new mineral along the cracks leads to the supposition that the change is due entirely to the medium of circulating water, a view that is substantiated by experimental researches.--An important contribution to the discussion of optical anomalies has been made by Erb 16 in a study of sodium acetates of copper, magnesium, nickel, and other metals. When allowed to crystallize slowly these salts form at first uniaxial crystals belonging to the hexagonal system. As they increase in size the crystals become twinned according to certain orthorhombic laws. In thin section they show twinning lamellæ, which disappear when the temperature is raised to 65°. The etched figures produced in both the simple and twinned crystals are of the same shape. They lie in the same relative positions, and are not symmetrically developed about the twinning planes of the lamellæ, but they have the symmetry belonging to the hexagonal system. The twinned crystals are pyroelectric, but upon assuming the isotropic condition they lose this property entirely. To account for these anomalies the author thinks that a strain has been superinduced in them during their growth. If they are mimetitic forms it is odd that the crystals should possess a certain grade of symmetry when small, and assume it again when heated. Upon treating freshly precipitated ferric hydroxide at 250° with water, to which a trace of ammonium fluoride has been added, Bruhns " obtained little plates of hematite with hexagonal cross sections. Freshly precipitated alumina 15 Zeits. f. Kryst., XIV., p. 305. 16 Neues Jahrb. f. Min. etc., B. B. VI., when treated in the same way at 300°, yielded little crystals of corundum with pyramidal terminations. Quartz crystals with rhombohedral terminations were produced by heating pulverized glass or amorphous silica to 300° under the same conditions. Microcline gave tridymite plates. A mixture of metallic iron, iron oxide and amorphous silica produced litte black amorphous plates of ilmenite and crystals of magnetite. The syntheses are of importance as indicating the possibility of the formation of contact minerals at a low temperature in the presence of traces of fluorine.Johnston 18 has subjected muscovite and biotite to the action of pure water and to that of dilute carbonic acid for the length of one year. The muscovite undergoes no change in this time except slight hydration, in consequence of which it becomes a hydro-muscovite resembling margarodite in composition. Biotite during this time becomes bleached under the influence of the carbonic acid. It loses some of its magnesium and iron, assumes water, and passes like the muscovite into a hydromuscovite. Anhydrous micas when they undergo hydration increase in bulk, a fact that may help to explain the cause of the rapid weathening of micaceous sandstones.-Bruhns 19 has succeeded in obtaining genuine. glass inclusions in quartz by heating in a bath of molten granite specimens of phrase and pieces of quartz containing inclusions of fibrolite. The resulting glass is entirely surrounded by quartz, and is not merely a portion of the granitic substance filling cracks produced in the mineral by heating. The inclusions are arranged in straight and curved lines, and have all the properties of inclusions found in porphyritic quartzes.A brief comparison of the shapes of the etched figures in diopside and spodumene is made by Greim 20. The depressions found on the ∞ P faces of the first mineral are nearly triangular, with their apices inclined toward the positive hemi-pyramid.—Mr. Lane 21 describes a method of determining the values of the optical angles of minerals in thin sections of rocks without the use of converged light. 18 Quart. Jour. Geol. Soc., May, 1889, p. 363. 19 Neues. Jahrb. f. Min., etc., 1889, I., p. 268. 20 Miner. Magazine, May, 1889, p. 252. 21 Amer. Jour. Sci., Jan., 1890, p. 53. Published July 31, 1890. |