Cumberland, on the Senner heath in Westphalia, at Pillau in the vicinity of Königsberg, 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 end 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, Astraphya/ite, Cerauruan Sinter, JilitzrShren, names under which these tubular concretions have been introduced into our systems of mineralogy. In most of the specimens of the variegated aeries 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. S3, and some others, are inscribed “ Aluminous s latebut these specimens belonu to the micaceous variety into which the red marie sandstone so frequently passes, and which sometimes forms distinct beds in it, overlaid by variegated and white marie 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 marie, 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 salz- thon 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 rlone, 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 k k 2
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