The result of another, equally enigmatical, liquefaction of siliceous matter, (which is, however, generally, though gratuitously, attributed to the agency of lightning) we see in the tubular concretions that have been found near Drigg in Cumberland, on the Senner heath in Westphalia, at Pillau in the vicinity of Konigsberg, at Halle upon the Saale, and, lastly, on the plains near Dibla in the Tibbou country. The African sand tubes differ from those of Drigg, as the sands themselves differ from each other in the two localities; the texture of the former is more homogeneous and pure, some being translucent and almost colourless; and, when cylindrical, not unlike some tubular varieties of stalactic carbonate of lime. Others are internally of a light-grey colour, here and there marked with white specks from semi-fused grains of sand: their outer surface is either approaching to smooth, or studded with snow-white opaque grains of sand, sunk in the vitrified substance; but they are not coated by the agglutinated sand which, in the English tubes, forms a rough crust, gradually passing into the vitreous substance of the sides. The stem, to judge from the short fragments I have seen, is very irregular in its circumference, and (except in the small cylindrical tubes, whose surface is even) polymorphously jagged, compressed, and contorted. Their interior lustre is superior to that observable in the European Fulgurite, Astraphyalite, Ceraunian Sinter, Blitzröhren,—names under which these tubular concretions have been introduced into our systems of mineralogy.
In most of the specimens of the variegated series described in the catalogue, the presence of common salt is obvious to the eye or the taste, or at least discoverable on the application of chemical re-agents. The tickets placed with No. 35. and some others, are inscribed “Aluminous slate;” but these specimens belong to the micaceous variety into which the red marle sandstone so frequently passes, and which sometimes forms distinct beds in it, overlaid by variegated and white marle slate; neither is the salt with which they are impregnated any other than muriate of soda.
The gypsum, of which some varieties are among the specimens found in the red marle, both at the most northern and the most southern points where Dr. Oudney collected, is of course referable to the newer, and some even to the newest, formation: it is mostly foliated-fibrous, and, in several specimens, intermixed with red clay.
I have little to say on the specimens of limestone enumerated in the list: most of them agree perfectly well with our new magnesian limestone; but external as well as chemical characters in detached fragments, not observed in situ, are but uncertain guides to the determination of the various modifications of Werner’s older fletz limestone, to which, I suppose, those specimens must all be referred.
There are only two specimens of common salt brought home, neither of which exhibit any thing peculiar in their appearance: but were they ever so numerous or remarkable in their exterior, they would not contribute greatly towards illustrating the history of the saline deposites and saliferous formations of the regions from which they come. The excessive abundance of salt in the variegated sand all over the central part of northern Africa, indicates either the existence of an extensive deposition of that substance beneath the prevailing rock formation, or the uninterrupted operation of causes by which those superficial saline masses, crusts, and efflorescences (the last of these observable in most specimens of sandstone brought from thence), are produced independently of briny waters emanating from such deposits of rocksalt. Mr. Keferstein, who has collected a multitude of instances of the occurrence of salt springs in situations which would seem to preclude the possibility of an immediate connexion between them and extensive beds of rock salt, has by ingenious reasoning and a number of interesting facts endeavoured to prove, that, in contact with waters circulating in the earth, the members of the saliferous formation (especially the clay to which, as an inseparable concomitant of salt, the name of salzthon has been given) are endowed with the power of generating salt by means of a chemical process, of which the rationale (as that of many others) remains among the desiderata of the science.
Those who are of opinion that brine springs are, under all circumstances, derived from great salt formations, and that their rise and presence, in any given situation, may be satisfactorily accounted for, by hydrostatic pressure alone, will probably find nothing extraordinary in the peculiar occurrence of that substance in the clay of the extensive salt fields of Mafen, Hamera, &c.; they will consider the various forms under which it presents itself in those tracts merely as the result of aggregation of saline particles conveyed from the great depot to the crevices of the clay, gypsum, and sandstone, and left there by the simple process of evaporation. I shall not attempt examining which of the two theories derives most support from the different phenomena relative to the production of salt observed by you on your journey, or to the occurrence of fresh water springs in the centre of salt hills, dwelt upon by Herodotus, and other circumstances belonging to halurgic geology; but refer you, for materials necessary for this inquiry, to the important facts detailed by the abovementioned author in the second volume of his periodical work (Teutschland, geognostisch-geologisch dargestellt, Weimar, 1823), as also to those opposed to them by no less an authority than Mr. V. Langsdorff in his last work (Anleitung zur Salzwerkskunde, Heidelberg, 1824).
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 of 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 red; 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 often 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 divergingly 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 yellowish-white, 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; 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. “From the Mandara mountains.”