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. 1 1 .) 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 grassier, 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 appeal's 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 lis t; I am, however, inclined to think it is not entirely wanting iu several parts of the tract you have traversed, especially to the southward of the boun-. dary of Fezzan. I ymnclqde .it, from what you have mentioned to me of the picturesque and ruin-j, ike. appearance of many of the mountain ranges ip.thop 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 impvpssiqps of.small bivalves.on one or two of then]. No stress ¡S perhaps to be.laid on ¡the occurrence, of fibrous limestone, which substançe is known, to be,, among the mineral .contents of quader-sandstqne at the, foot of the Htptz, vvhere'tliis link qf the fletz. sandsto.ne formation rests. on .the fed marie smidstone.wdthout.the.interyention ofshell limestcp^ of w^iich 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, sjripes, and flajned delineations, are,¡moçe particularly entitled to that cjeudmination, are Nos. 31 —36. , The irjabffi sandstone of Traghen (No, 23.), probably, p>ves its green colour, tp. oxide pf .nickeJU 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 admixturei(Nos. 87—r4fi). They are all (with the exception of a few of more loose texture, which belqng to a tertiary formation) referable to the variegated sandstone series. In spme of these the cement ,of various colours, yellowish,.,red, brown, bluish, is,so completely cqnferrummated with the grains, that, upon breaking a specimen, the fracturing plane invariably passes through them, prodiping a uniformly smooth and frequently conehoidal surface. Sometimes both cepent 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 iu tins 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. The result of another, equally enigmatical, liquefaction of siliceous matter, (which is, however/generally, though gratuitously, attributed to thp agency of lightning) we see in the tubular concretions that have been found near Drigg in k!> !
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