Missionary Travels and Researches in South Africa, David Livingstone [novels for teenagers .TXT] 📗

4000 feet above the level of the sea, and 1000 feet lower than the top of the western ridge we had already crossed; yet, instead of lofty snow-clad mountains appearing to verify the conjectures of the speculative, we had extensive plains, over which one may travel a month without seeing any thing higher than an ant-hill or a tree. I was not then aware that any one else had discovered the elevated trough form of the centre of Africa.

I had observed that the old schistose rocks on the sides dipped in toward the centre of the country, and their strike nearly corresponded with the major axis of the continent; and also that where the later erupted trap rocks had been spread out in tabular masses over the central plateau, they had borne angular fragments of the older rocks in their substance; but the partial generalization which the observations led to was, that great volcanic action had taken place in ancient times, somewhat in the same way it does now, at distances of not more than three hundred miles from the sea, and that this igneous action, extending along both sides of the continent, had tilted up the lateral rocks in the manner they are now seen to lie. The greater energy and more extended range of igneous action in those very remote periods when Africa was formed, embracing all the flanks, imparted to it its present very simple literal outline. This was the length to which I had come.

The trap rocks, which now constitute the "filling up" of the great valley, were always a puzzle to me till favored with Sir Roderick Murchison's explanation of the original form of the continent, for then I could see clearly why these trap rocks, which still lie in a perfectly horizontal position on extensive areas, held in their substance angular fragments, containing algae of the old schists, which form the bottom of the original lacustrine basin: the traps, in bursting through, had broken them off and preserved them.

There are, besides, ranges of hills in the central parts, composed of clay and sandstone schists, with the ripple mark distinct, in which no fossils appear; but as they are usually tilted away from the masses of horizontal trap, it is probable that they too were a portion of the original bottom, and fossils may yet be found in them.*

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After dwelling upon the geological structure of the Cape Colony as developed by Mr. A. Bain, and the existence in very remote periods of lacustrine conditions in the central part of South Africa, as proved by fresh-water and terrestrial fossils, Sir Roderick Murchison thus writes:

"Such as South Africa is now, such have been her main features during countless past ages anterior to the creation of the human race; for the old rocks which form her outer fringe unquestionably circled round an interior marshy or lacustrine country, in which the Dicynodon flourished, at a time when not a single animal was similar to any living thing which now inhabits the surface of our globe. The present central and meridian zone of waters, whether lakes or marshes, extending from Lake Tchad to Lake 'Ngami, with hippopotami on their banks, are therefore but the great modern residual geographical phenomena of those of a mesozoic age. The differences, however, between the geological past of Africa and her present state are enormous.

Since that primeval time, the lands have been much elevated above the sea-level -- eruptive rocks piercing in parts through them; deep rents and defiles have been suddenly formed in the subtending ridges through which some rivers escape outward.

"Travelers will eventually ascertain whether the basin-shaped structure, which is here announced as having been the great feature of the most ancient, as it is of the actual geography of South Africa (i.e., from primeval times to the present day), does, or does not, extend into Northern Africa. Looking at that much broader portion of the continent, we have some reason to surmise that the higher mountains also form, in a general sense, its flanks only." -- President's Address, Royal Geographical Society, 1852, p. cxxiii.

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The characteristics of the rainy season in this wonderfully humid region may account in some measure for the periodical floods of the Zambesi, and perhaps the Nile. The rains seem to follow the course of the sun, for they fall in October and November, when the sun passes over this zone on his way south. On reaching the tropic of Capricorn in December, it is dry; and December and January are the months in which injurious droughts are most dreaded near that tropic (from Kolobeng to Linyanti).

As he returns again to the north in February, March, and April, we have the great rains of the year; and the plains, which in October and November were well moistened, and imbibed rain like sponges, now become supersaturated, and pour forth those floods of clear water which inundate the banks of the Zambesi. Somewhat the same phenomenon probably causes the periodical inundations of the Nile. The two rivers rise in the same region; but there is a difference in the period of flood, possibly from their being on opposite sides of the equator.

The waters of the Nile are said to become turbid in June; and the flood attains its greatest height in August, or the period when we may suppose the supersaturation to occur. The subject is worthy the investigation of those who may examine the region between the equator and 10 Deg. S.; for the Nile does not show much increase when the sun is at its farthest point north, or tropic of Cancer, but at the time of its returning to the equator, exactly as in the other case when he is on Capricorn, and the Zambesi is affected.*

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The above is from my own observation, together with information derived from the Portuguese in the interior of Angola; and I may add that the result of many years' observation by Messrs. Gabriel and Brand at Loanda, on the west coast, is in accordance therewith.

It rains there between the 1st and 30th of November, but January and December are usually both warm and dry.

The heavier rains commence about the 1st of February, and last until the 15th of May. Then no rain falls between the 20th of May and the 1st of November. The rain averages from 12 to 15 inches per annum. In 1852 it was 12.034 inches; in 1853, 15.473 inches. Although I had no means of measuring the amount of rain which fell in Londa, I feel certain that the annual quantity exceeds very much that which falls on the coast, because for a long time we noticed that every dawn was marked by a deluging shower, which began without warning-drops or thunder.

I observed that the rain ceased suddenly on the 28th of April, and the lesser rains commenced about a fortnight before the beginning of November.

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From information derived from Arabs of Zanzibar, whom I met at Naliele in the middle of the country, the region to the east of the parts of Londa over which we have traveled resembles them in its conformation.

They report swampy steppes, some of which have no trees, where the inhabitants use grass, and stalks of native corn, for fuel.

A large shallow lake is also pointed out in that direction, named Tanganyenka, which requires three days for crossing in canoes.

It is connected with another named Kalagwe (Garague?), farther north, and may be the Nyanja of the Maravim. From this lake is derived, by numerous small streams, the River Loapula, the eastern branch of the Zambesi, which, coming from the N.E., flows past the town of Cazembe.

The southern end of this lake is ten days northeast of the town of Cazembe; and as that is probably more than five days from Shinte, we can not have been nearer to it than 150 miles. Probably this lake is the watershed between the Zambesi and the Nile, as Lake Dilolo is that between the Leeba and Kasai. But, however this may be, the phenomena of the rainy season show that it is not necessary to assume the existence of high snowy mountains until we get reliable information.

This, it is to be hoped, will be one of the results of the researches of Captain Burton in his present journey.

The original valley formation of the continent determined the northern and southern course of the Zambesi in the centre, and also of the ancient river which once flowed from the Linyanti basin to the Orange River. It also gave direction to the southern and northern flow of the Kasai and the Nile. We find that between the latitudes, say 6 Deg. and 12 Deg. S., from which, in all probability, the head waters of those rivers diverge, there is a sort of elevated partition in the great longitudinal valley. Presuming on the correctness of the native information, which places the humid region to which the Nile and Zambesi probably owe their origin within the latitudes indicated, why does so much more rain fall there than in the same latitudes north of the equator? Why does Darfur not give rise to great rivers, like Londa and the country east of it? The prevailing winds in the ocean opposite the territory pointed out are said to be from the N.E. and S.E.

during a great part of the year; they extend their currents on one side at least of the equator quite beyond the middle of the continent, and even until in Angola they meet the sea-breeze from the Atlantic.

If the reader remembers the explanation given at page 109,*

that the comparative want of rain on the Kalahari Desert is caused by the mass of air losing its humidity as it passes up and glides over the subtending ridge, and will turn to the map, he may perceive that the same cause is in operation in an intense degree by the mountains of Abyssinia to render the region about Darfur still more arid, and that the flanking ranges mentioned lie much nearer the equator than those which rob the Kalahari of humidity.

The Nile, even while running through a part of that region, receives remarkably few branches. Observing also that there is no known abrupt lateral mountain-range between 6 Deg. and 12 Deg. S., but that there is an elevated partition there, and that the southing and northing of the southeasters and northeasters probably cause a confluence of the two great atmospheric currents, he will perceive an accumulation of humidity on the flanks and crown of the partition, instead of, as elsewhere, opposite the Kalahari and Darfur, a deposition of the atmospheric moisture on the eastern slopes of the subtending ridges. This explanation is offered with all deference to those who have made meteorology their special study, and as a hint to travelers who may have opportunity to examine the subject more fully. I often observed, while on a portion of the partition, that the air by night was generally quite still, but as soon as the sun's rays began to shoot across the upper strata of the atmosphere in the early morning, a copious discharge came suddenly down from the accumulated clouds.

It always reminded me of the experiment