Letter to Major Denham, on the Rock Specimens brought from Africa. By Charles Konig, Esq. F.R.S.
British Museum, Feb. 25th, 1826.
My dear Sir,
I have great pleasure in transmitting to you, for whatever use you may think proper to make of it in the Appendix to your forthcoming work, the little I have to advance on the geological and a few other objects, that were collected by you, the late Dr. Oudney, and Captain Clapperton, on your journey through the great African desert, and are now deposited in the British Museum. It chiefly consists of the descriptive catalogue of a small series of rock specimens, originally drawn up by me without particular regard to their geognostic occurrence; to which I now prefix a few desultory remarks that occurred to me, when, with a view to its publication, I subjected that list to a second perusal. My materials are, indeed, very scanty; but their description may, nevertheless, (in conjunction with the observations dispersed in the body of the work, of which I have not been able to avail myself) lead some of your geological readers to more or less important results relative to the structure of the tract of country in which they were collected.
There are among the specimens I have examined none that might be referred to the primitive formations, except those gathered south and west of Kouka. The principal specimen of granite (No. 1.) brought by you from the Mandara mountains strongly resembles some of the fresh large-grained varieties of the same from the Fetish rocks in Congo. Those from Soudan, with feldspar, in its progress to kaolin, (No. 5.) betray the principal cause of the striking appearance of the granite mountains in that part of central Africa. The effects of atmospheric influence on that component, spreading chiefly in the direction of the natural rifts of the rock, are, in the Soudan mountains as well as in those of the Hartz, the Riesengebirge, and other European granitic districts, manifested partly by the immense and numberless blocks, wholly or partially detached, and confusedly piled up on the sides of the mountains, or strewed over the plains in fantastic groups; and partly by the almost total disintegration of the masses into gravel at the foot of the ridges. The former of these effects is illustrated by the sketch of a granite mountain in Soudan, which you were so good as to show me. There are a few other specimens of granite from Soudan; but they present no characters from which any useful information is likely to be derived; the less so, as they appear to be casual fragments, not found in situ. The mica slate (No. 9.), as Captain Clapperton informed me, occurs at the upper part of the ridges between Quarra and Zurma; and a ticket, accompanying a micaceous rock specimen, (No. 11.) from the same locality, as I suppose, states it to be “used for glazing earthen ware.”
These are all the specimens of primitive rocks. As to those of secondary and tertiary formations, which have been brought home, it will appear from the subjoined catalogue, (as, indeed, with regard to part of the kingdoms of Tripoli and Fezzan, has already been pointed out by Dr. Buckland), that they may be referred to three formations, viz. 1. to the fletz-trap or basaltic formation; 2. to a formation analogous to the Paris limestone, (calcaire grossier, grob-kalk); and 3. to some members of the secondary formation from the chalk to the alpine limestone inclusively.
The few specimens of the first mentioned of these formations are specified under Nos. 14 to 18. To the second I would refer the sandstone No. 26. with fragments of small univalve shells, not unlike a species of Paludina, and the plastic clay of Cano, of which, I suppose, are made the light pipe bowls, brought from thence; together with some other specimens of clay, and also some fragments of shells, apparently from the tertiary limestone, but the localities of which are not mentioned.
But by far the greater proportion of the specimens brought home by you belongs to that series of the secondary formations of which the variegated sandstone is the principal member. Subordinate to this is the ferruginous sandstone, (Nos. 47, &c.) with its beds of brown hydrous oxide, into which, as also into ochrey ironstone, (Nos. 84—90.) it appears to pass by gradual transition. The white sandstone, called quader-sandstein by most German geologists, and sandstone of Konigstein by Baron Humboldt, is not specified as such in the list; I am, however, inclined to think it is not entirely wanting in several parts of the tract you have traversed, especially to the southward of the boundary of Fezzan. I conclude it, from what you have mentioned to me of the picturesque and ruin-like appearance of many of the mountain ranges in those parts, which is peculiar to that sandstone; from the external character of some of the specimens, which, however, have no localities affixed to them; as also from the impressions of small bivalves on one or two of them. No stress is perhaps to be laid on the occurrence of fibrous limestone, which substance is known to be among the mineral contents of quader-sandstone at the foot of the Hartz, where this link of the fletz sandstone formation rests on the red marle sandstone, without the intervention of shell limestone, of which latter I have seen no specimens among those from North Africa.
More characteristic are the specimens of the variegated sandstone, properly speaking. The varieties, which, by their patches, stripes, and flamed delineations, are more particularly entitled to that denomination, are Nos. 31—36. The friable sandstone of Traghen (No. 23.) probably owes its green colour to oxide of nickel: the colouring matter seems chiefly to reside in the clay by which the grains are held together.
Particularly remarkable are those varieties of sandstone in which the cement is quartz, both with and without ferruginous admixture (Nos. 37—46). They are all (with the exception of a few of more loose texture, which belong to a tertiary formation) referable to the variegated sandstone series. In some of these the cement of various colours, yellowish, red, brown, bluish, is so completely conferruminated with the grains, that, upon breaking a specimen, the fracturing plane invariably passes through them, producing a uniformly smooth and frequently conchoidal surface. Sometimes both cement and grains are united into one homogeneous quartzy mass, in which, especially when thoroughly impregnated with oxide of iron, scarcely the slightest vestiges of former granulation remain perceptible. Such an extraordinary transformation of the cementing mass, observable not only in this but likewise in the quader-sandstone, the iron sand, and the variety of the newest sandstone called molasse, presupposes a state of liquefaction, and is but imperfectly accounted for by those who ascribe it to infiltration.