other circumstances belonging to halurgic geology; but refer you, for materials necessary for this inquiry, to the important facts detailed by the above- mentioned author in the second volume of his periodical work (Teutschland, geognostisch-geologisch dargcstellt, Weimar, 1S2S), as also to those opposed to them by no less an authority than Mr. V. Langsdorff in his last work (Anlei- tung zur Salzwerkskunde, Heidelberg, 1S24). The specimens of trona (carbonate of soda), collected on the expedition, exhibit this salt in various degrees of purity. In some, it is mechanically mixed with muriate and sulphate of soda ; in others, it appears, when divested ot the casually adhering substances, to be perfectly pure. Of the latter, we have two very distinct varieties, as far, at least, as external characters are concerned. The one consists of layers, or crusts, of about one-third of an inch in thickness, opaque, and of a white colour, generally with an almost imperceptibly slight tinge of r e d ; and yellowish stains are sometimes observable. These layers, when broken, display distinct concretions between lamellar and granular; the lower surface is rough, and more or less impure, the small interstices being partly filled up with reddish earthy matter; the distinct concretions terminate at the upper surface, in small, slightly cuneiform, apparently four-sided compressed prisms, acuminated by two planes which meet under an angle of about 95°; but both the lateral and terminal edges are constantly rounded off, and the planes uneven and striated. These crystals, not much inferior in hardness to carbonate of lime, are internally splendent, externally dull, and generally covered by a fine efflorescence, and also' oftdh studded with small limpid cubical crystals of muriate of soda. This variety is formed in the lakes of Ghraat, apparently in the same manner as the carbonate of soda (urao) of the Lagunilla lake of Venezuela, or that of the S. Macarius lake in Egypt, but appears to be less contaminated with other salts than either of these. The other variety of carbonate of soda (the locality of which is doubtful, two tickets having been found accompanying the specimen, the one with “ Kanem,” the other with “ hills of Traghen,”) occurs in pieces which appear* to have been part of a vein or layer. They are composed of groups of diverg- ingly radiated acicular crystals, closely grown together, intermixed with indeterminable capillary crystals, confusedly aggregated. Some of these crystals appeared as very compressed four-sided prisms, indistinctly acuminated by two planes set on the acute lateral edges. Its fracture is lamellar-fibrous, passing into splintery; fragments wedge-shaped. Its colour is a dingy greenish, or yellowislr-wlrite, appearing brownish in the more compact parts of the pieces. The internal lustre is splendent; the external surface covered by a yellowish white powder. In the crystalline state, both these varieties of trona appear to be perfectly pure j it is, however, possible that they may differ from each other in the proportion of the water and carbonic acid with which the soda is combined in them. If, as Berthollet thinks, the origin of native carbonate of soda is to be looked for in the decomposition of common salt by carbonate of lime, we may farther conjecture, from the traces of bituminous matter found with the trona, that the rock instrumental to it is fetid limestone, beds of which are frequent concomitants of saliferous formations. The following is the catalogue of the specimens of which I have been able to determine the localities. 1 . Large grained granite; the deep-flesh-coloured feldspar in greater proportion than the greyish quartz, and the black, small-scaly mica. “ Prom the Mandara mountains.** 2. The same ; with feldspar* of dirty-yellowish colour. From the Mandara range, and two similar varieties from the “ hills of Dutchie Zangia, Soudan.” 3. Similarly coloured variety, but of smaller grain. Prom the same places. 4. Large grained variety of the same; the feldspar of a yellowish colour, and in a state of incipient decomposition, with,little black mica. Mandara. 5 . Variety similar to the preceding; in a state of disintegration: the feldspar decomposing into a reddish earth. From Quarra and Zurmee, Soudan. 6. Granite, both fine and coarse-grained, almost entirely composed of flesh-red feldspar, with indeterminable particles of a black substance, apparently mica. “ From the high ridges of Zurma.” 7 . The same, small-grained, rather slaty, approaching to gneiss, composed of greyish-white feldspar and quartz, with predominant black small-scaly mica. From Nansarena, Soudan. 8. Portion of a boulder, chiefly composed of fine-grained dirty-grey semicompact feldspar and some quartz ; with disseminated particles of magnetic iron ore (a syenitic rock). “ Found near Agutefa.” 9. Greenish-grey mica slate, with little admixed quartz and feldspar. “ I t forms the upper part of the ridges between Quarra and Zurma, Soudan.”
27f 24
To see the actual publication please follow the link above