ART. X.-On the Geognosy of the Cape of Good Hope. By Professor JAMESON. THE peninsula of the Cape of Good Hope is a mountainous ridge, stretching nearly north and south for thirty or forty miles, and connected on the east side, and near its northern extremity, with the main body of Africa, by a flat sandy isthmus, about ten miles broad, having Table Bay on the north of it, and False Bay on the south. The southern extremity of this peninsula, extending into the sea, with False Bay on the east, and the ocean on the south and west, is properly the Cape of Good Hope, and is the most southern point of Africa. At this point, the chain of mountains which forms the peninsula, though rugged, is lower than at the north end, where it is terminated by Table Mountain and two others, which form an amphitheatre overlooking Table Bay, and opening to the north. The mountains of the ridge extending from the Cape to the termination of the peninsula on the north, vary in shape; but the most frequent forms incline more or less to sharp conical. The three mountains which terminate the peninsula on the north, are, the Table Mountain in the middle; the Lion's Head, sometimes called the Sugar Loaf, on the west side; and the Devil's Peak on the east. The Lion's Head, which is about 2100 feet above the level of the sea, is separated from the Table Mountain by a valley, that descends to the depth of 1500 or 2000 feet below the summit of the Table Mountain, which is itself 3582 above the level of the sea. On the west of the Lion's Head, there is a lower eminence, named the Lion's Rump, from which the ground declines gradually to the sea. The amphitheatre, formed by these three mountains, is about five or six miles in diameter, in the centre of which is placed Cape Town *. The rocks of which the peninsula is composed are few in number, and of simple structure. They are granite, gneiss, clay-slate, sandstone, and greenstone. Of these the most abun • Transactions of the Royal Society of Edinburgh, vcl. vii. p. 271. ខ dant are sandstone and granite; the next, in frequency, are clayslate and gneiss; and the rarest is greenstone. The strata in general have a direction from E. to W. that is, across the peninsula. The southern and middle parts of the peninsula have been very imperfectly examined. Captain Basil Hall, in an interesting account of some mineralogical appearances he observed near Cape Town, published in the Edinburgh Philosophical Transactions, remarks, that the same general structure and relations seem to occur all over the peninsula as in the mountains around Cape Town. More lately Captain Wauchope, an active and enterprising officer, pointed out to Mr Clarke Abel a fine display of stratification in a mountain that faces the sea, in the neighbourhood of Simon's Bay. The following is the description, as given by Mr Abel: "The sandstone, forming the upper part of the mountain, is of a reddish colour, very crystalline in its structure, and approaching, in some specimens, to quartz rock. Immediately beneath the sandstone is a bed of compact dark red argillaceous sandstone, passing, in many places, into slate of the same colour. This bed rests upon another of very coarse loosely combined sandstone, resembling gravel. Under this is another layer of dark red sandstone, terminating in a conglomerate, consisting of decomposed crystals of felspar, and of rounded and angular fragments of quartz, from the size of a millet seed to that of a plover's egg, imbedded in a red sandstone base. Beneath the conglomerate commences a bed, which I at first took for granite, and which is composed of the constituents of granite in a decomposed state, intermixed with green steatite, and a sufficient quantity of the dark red sandstone to give it a reddish hue. The felspar of the bed is decomposed, and exactly resemble that of the conglomerate above it. The mica seems, in a good measure, to have passed into steatite. The quartz is in small crystals, frequently having their angles rounded. This bed is several feet in thickness, and gradually terminates in the granite; but the precise line of junction I was unable to trace. The appearances, then, were in the following order: 1. Horizontally stratified sandstone. 2. Bed of compact dark red sandstone, passing into slate. 3. A bed of coarser sandstone, resembling gravel. 4. A second layer of compact dark red sandstone, passing 5. Into a conglomerate, consisting of decomposed crystals of felspar, and fragments of quartz in a sandstone basis. 6. A bed composed of the decomposed constituents of granite and red sandstone, passing 7. Into granite." The above is the only spot to the southward of the range of mountains near Cape Town, which has been particularly described. To the northward of Cape Town, it is reported that the mountains are principally composed of the same rocks as those which occur throughout the peninsula, and whose characters and position have been examined with considerable attention in the Lion's Rump, Lion's Head, Table Mountain, and Devil's Peak. As these mountains give a good general idea of the composition and structure of the whole peninsulà, and also of much of southern Africa, we shall now present our readers with a concise description of them, drawn up from information communicated to us by Dr Adam of Calcutta, and from the published accounts of Captain Hall and Mr Clarke Abel. Lion's Rump. The Lion's Rump rises by an easy ascent, and, excepting at one or two points, is covered to the summit with a thin soil, bearing a scanty vegetation +. It is composed of clay-slate, and sandstone. The sandstone rests upon the slate. The clay-slate is distinctly stratified; • Clarke Abel's Travels, p. 295. and 297. + Dr Adam remarks, that vegetables appeared to be most luxuriant over the sandstone, less so on the soil formed by the decomposition of the granite, and least of all over clay-slate, as on the Lion's Rump, where clay-slate is the predominating rock. Although this latter hill has been cultivated in some places, yet it presents a stunted vegetation, while the upper parts of Lion's Head and Table Mountain, though much more elevated, display rich and more vigorous shrubs. Constantia, so much celebrated for its wine, is situated at the bottom of the range leading from Cape Town to Simmon's Bay, where sandstone is the predominating rock, and the soil of the farm of the neighbouring ground appears to be composed of it, in a state of decomposition and of vegetable mould. That it is the sandstone which essentially contributes to the excellence of the soil, Dr Adam is inclined to believe, from having observed several spots at the foot of the same range nearer Cape Town, with a soil richer in vegetable mould, but whose produce was held much inferior. The principal rock there was granite, and its superincumbent sandstone has suffered less decomposition than that adjoining to Constantią. VOL. I. NO. 2. OCTOBER 1819. |