VIII.

A GEOLOGICAL SKETCH.

From the work of the various explorers who have penetrated Africa, it is now certain that the interior of that Continent is occupied by a vast plateau from 4000 to 5000 feet above the level of the sea. In five separate regions--in the North-east, in Abyssinia, in the Masai country, on the Tanganyika plateau, and in the district inland from Benguela--this plateau attains a height of considerably over 5000 feet; while towards the coast, throughout their entire length, both east and west, it falls with great uniformity to a lower plateau, with an elevation of from 1000 to 2000 feet. This lower plateau is succeeded, also with much uniformity along both coast lines, by littoral and deltoid plains, with an average breadth from the sea of about 150 miles.

The section which I am about to describe, entering Africa at the Zambesi and penetrating inwards to the Tanganyika plateau, traverses each of these regions in turn--the coast-belt, the lower fringing table-land, the great general plateau of the country, and the third or highest elevation of the Tanganyika table-land. To deal thoroughly with so vast a region in the course of a single exploration is out of the question; and I only indicate here a few of the rough results of what was no more than a brief and hasty reconnaissance.

The first and only geological feature to break the monotony of mangrove-swamp and low grass plain of the coast-belt is the debris of an ancient coral-reef, studded with sponges and other organisms. This reef is exposed on the Qua-qua River, a little above Mogurrumba, and about fifty miles from the sea. It is of small extent, at no great height above the present sea-level, and, taken alone, can only argue for a very inconsiderable elevation of the coast region. Some twenty miles farther inland, and still only a few yards above sea-level, an inconspicuous elevation appears, consisting of sedimentary rocks. This belt is traceable for some distance, both north and south, and a poor section may be found in the Zambesi River, a few miles above the grave of Mrs. Livingstone at Shupanga. The rocks in question, which are only visible when the Zambesi is very low, consist of a few thin beds of red and yellow sandstones, with intercalated marly sandstones and fine conglomerates. Sedimentary rocks, in somewhat similar relation, are found at least as far north as Mombassa, above Zanzibar, and as far south as the Cape; and it seems probable that the whole of the plateau of the interior is fringed by this narrow belt. No organic remains have been found in this series north of Natal, but the fossils of the Cape beds may shed some light on its horizon. Associated probably with these rocks are the great beds of coal which are known to exist some distance up the river in the neighborhood of Tette.

A short distance above the junction of the River Shiré with the Zambesi the first hills of the plateau begin almost abruptly. They occur in irregular isolated masses, mostly of the saddle-back order, and varying in height from 100 or 200 to 2000 feet. Those I examined consisted entirely of a very white quartzite--the only quartzite, I may say, I ever saw in East Central Africa. At the foot of the most prominent of those hills--that of Morumballa--a hot-spring bubbles up, which Livingstone has already described in his "Zambesi." Hot-springs are not uncommon in other parts of the Continent, and several are to be found on the shores of Lake Nyassa. These are all of the simplest type, and although the temperature is high they leave no deposit anywhere to indicate their chemical character.

Two or three days' journey north and west of Morumballa, among the distant hills which border the valley of the Shiré, Livingstone marks a spot in his sketch-map where coal is to be found. After examining the neighborhood with some care, and cross-examining the native tribes, I conclude that Livingstone must, in this instance, have been either mistaken or misinformed. A black rock certainly occurs at the locality named, but after securing specimens of this as well as of all the dark-colored rocks in the vicinity, I found them to be, without exception, members of the igneous class. One very dark diorite was probably the rock which, on a distant view, had been mistaken for coal, for none of the natives along the whole length of the lower Shiré had ever heard of "a black rock which burned." Coal, however, as already mentioned, does certainly occur farther inland on the Zambesi; while, farther south, the Natal and Transvaal coalfields are now well known.

While speaking of coal I may best refer here to a small coal-bed associated with an apparently different series of rocks, and of special interest from its occurrence in the far interior of the country. On the western shore of Lake Nyassa, about 10° south latitude, coal was reported a few years ago by a solitary explorer, who penetrated that region prospecting for gold in the wake of Livingstone. The importance of such a discovery--a coal-seam on the borders of one of the great inland seas of Africa--cannot be over-estimated; and the late Mr. James Stewart, C.E., who has done such important work for the geography of Africa, made a special examination of the spot. From his report to the Royal Geographical Society I extract the following reference:--

"On the 29th we marched northwards along the coast, reaching, after three miles, the stream in which is the coal discovered by Mr. Rhodes. The coal lies in a clay bank, tilted up at an angle of 45°, dip west. It is laid bare over only some 30 feet, and is about 7 feet thick. It hardly looks as if it were in its original bed. The coal is broken and thrown about as if it had been brought down by a landslip, and traces of clay are found in the interstices. Yet the bed is compact, and full of good coal. I traced it along the hillside for some 200 yards, and found it cropping out on the surface here and there. It is 500 feet above the lake-level, and about a mile and a half from the shore. I lit a good fire with it, which burned up strongly. The coal softened and threw out gas bubbles, but gave no gas-jets. It caked slightly, but not so as to impede its burning."--_Proceedings_, vol. iii. No. 5, p. 264.

I examined this section pretty carefully, and fear I must differ slightly from Mr. Stewart in his geological and economical view of the formation. The 7-foot seam described by Stewart is certainly a deception, the seam being really composed of a series of thin beds of alternately carbonaceous and argillaceous matter, few of the layers of coal being more than an inch in thickness. With some of the most carefully selected specimens I lit a fire, but with disappointing results. Combustion was slow, and without flame. Although there were what can only be called _films_ of really good coal here and there, the mineral, on the whole, seemed of inferior quality, and useless as a steam-coal. From the general indications of the locality I should judge that the coal existed only in limited quantity, while the position of the bed at the top of a rocky gorge renders the deposit all but inaccessible. On the whole, therefore, the Lake Nyassa coal, so far as opened up at present, can scarcely be regarded as having any great economical importance, although the geological interest of such a mineral in this region is considerable. Sections of the coal have already been prepared for the microscope, and Dr. Carruthers of the British Museum has identified the macrospores of Lycopodaceous plants, which are identical with similar organisms found in the coal-fields of England.

The Geology of the great African plateaux, so far as my section from the Lower Shiré to the Tanganyika plateau is any indication of their general structure, is of such simplicity that it may almost be dismissed in a sentence. The whole country from the Shiré river, a hundred miles above its junction with the Zambesi, embracing the lower and higher central plateaux, the whole Shiré Highlands from the river to the westward shores of Lake Shirwa, the three hundred miles of rocky coast fringing the western shore of Lake Nyassa, the plateau between Nyassa and Tanganyika for at least half its length--with one unimportant interruption--consists solely of granite and gneiss. The character and texture of this rock persist with remarkable uniformity throughout this immense region. The granite, an ordinary gray granite, composed of white rarely pink orthoclase felspar, the mica of the biotitic or magnesian variety, rarely muscovite, and neither fine nor coarse in texture; the gneiss, the same rock foliated. Of the relation of these gneissose and granitic rocks to one another I was unable to discover any law. Sometimes the gneiss would persist over a large area, sometimes the granite; while frequently the two would alternate perplexingly within a limited area. Mr. Joseph Thomson's section, drawn inland from Zanzibar and joining mine at the northern end of Lake Nyassa, and thence onwards by a more easterly route towards Tanganyika, reveals a somewhat similar petrographical structure; and, from scattered references in the journals of other explorers, it is plain that this gneisso-granitic formation occupies a very large area in the interior of the African Continent. Associated minerals with these rocks, as far as a very general survey indicated, were all but wholly wanting. At Zomba, on the Shiré Highlands, a little tourmaline occurs, but of the precious metals I could find no trace. Veins of any kind are also rare; and even pegmatite I encountered in only one instance. Intrusive dykes throughout the whole area were like-wise absent except in a single district. This district lies towards the southern border of the Shiré Highlands, immediately where the plateau rises from the river, and there the dykes occur pretty numerously. They are seldom more than a few feet in breadth, and consist of ordinary dolerite or basalt. The black rock on the Lower Shiré, already mentioned in connection with Livingstone's supposed discovery of coal, may possibly be one of these dykes; but that there is any considerable development of igneous rocks in this immediate locality I should doubt. Farther up the Zambesi, however, coulees of basalt are met with at more than one place, conspicuously in the neighborhood of the Victoria Falls. The only distinct trace of volcanic action throughout my route appeared towards the extreme northern end of Lake Nyassa. One is warned beforehand by occasional specimens of pumice lying about the lake shore as one travels north; but it is not till the extreme end of the lake is reached that the source is discovered in the series of low volcanic cones which Thomson has already described in this locality. The development is apparently local, and the origin of the cones probably comparatively recent.

Apart from this local development of igneous rocks at the north end of Lake Nyassa, the only other break in the granitic series throughout the area traversed by my line of march occurs near the native village of Karonga, on Lake Nyassa. About a dozen miles from the north-western lake shore on the route to Tanganyika, after following the Rukuru river through a defile of granite rocks, I came, to my great surprise, upon a well-marked series of stratified beds. At a bend in the river a fine section is exposed. They lie throw against the granitic rocks, which here show signs of disturbance, and consist of thin beds of very fine light-gray sandstone, and blue and gray shales, with an occasional band of gray limestone. By camping at the spot for some days, and working patiently, I was rewarded with the discovery of fossils. This is, of course, the main interest of these beds,--for these are, I believe, the only fossils that have ever been found in Central Africa. The shale, naturally, yielded the most productive results, one layer especially being one mass of small _Lamellibranchiata_. Though so numerous, these fossils are confined to a single species of the _Tellinidæ_, a family abundantly represented in tropical seas at the present time, and dating back as far as the Oolite. Vegetable remains are feebly represented by a few reeds and grasses. Fish-scales abound; but I was only able, and that after much labor, to unearth two two or three imperfect specimens of the fishes themselves. These have been put into the accomplished hands of Dr. Traquair of Edinburgh, who has been kind enough to furnish the following account of them:--

EDINBURGH, 23d April, 1888.

DEAR PROFESSOR DRUMMOND--I have carefully examined the six specimens of fossil fish-remains from Central Africa, which you submitted to me, and though I certainly would have wished them to have been less fragmentary, I shall do my best to give an opinion upon them.

No. 1, the largest, is the hinder portion of a fish of moderate size, showing not only scales, but also the remains of the dorsal, anal, and caudal fins. The caudal is strongly heterocercal, and was probably deeply bifurcated, but the rays of the lower lobe are very badly preserved: only the posterior parts of the dorsal and anal are seen, nearly opposite each other, and composed of fine, closely placed, and closely articulated rays. The scales, displaced and jumbled up, are osseous, thick, and rhomboidal, with a strong blunt carina on the attached surface, while the exposed part of the external surface is covered with ganoine, and ornamented with rather sparsely scattered pits and punctures.

Belonging to the Order Ganoidei, this fish is with equal certainty referable to the family Palæoniscidæ, but its _genus_ is more a matter of doubt owing to the fragmentary nature of the specimen. Judging from the form and thickness of the scales, I should be inclined to refer it to _Acrolepis_, were it not that the dorsal and anal fins seem so close to the tail, and so nearly opposite each other; here, however, it may be remarked that the disturbed state of the scales affords room for the possibility that the original relations of the parts may not be perfectly preserved. I have, however, no doubt that, as a _species_, it is new; and as you have been the first to bring fossil fishes from those regions of Central Africa, you will perhaps allow me to name it _Acrolepis_ (?) _Drummondi_.

No. 2 is a piece of cream-colored limestone, with numerous minute, scattered, rhombic, striated, ganoid scales, which I cannot venture to name, though I believe them to be palæoniscid. Associated with these is a small portion of the margin of a jaw, with numerous minute sharp conical teeth. But also lying among these minuter relics is a scale of a much larger size, and clearly belonging to another fish. It measures 1-4 inch in height by the same in breadth; its shape is rhomboidal, having an extensive anterior covered area, and a strong articular spine projecting from the upper margin. The free surface is brilliantly ganoid, and marked with furrows separating feeble ridges, which pass rather obliquely downwards and backwards across the scale, and terminate in eight sharp denticulations of the hinder margin. A little way off is the impression of the attached surface of a similar scale, and there are also two interspinous bones, probably belonging to the same fish.

This is probably also a palæoniscid scale, resembling in shape those of _Acrolepis_, but it is rather thinner than is usually the case in this genus. It has also considerable resemblance to some of those scales from the European Trias, named by Agassiz _Gyrolepis_. Though it may be rather venturesome to name a species from such slender material, nevertheless we may, provisionally at least, recognize the scale as _Acrolepis (?) Africanus_.

Nos. 3 and 4 are small pieces of the same limestone, covered with the minute striated palæoniscid scales referred to above.

No. 5 is a piece of gray micaceous shale, with scales of yet a fourth species of palæoniscid fish. One conspicuous scale unfortunately, like all the rest, seen only from the attached surface, is 1-4 inch in height by nearly 1-6 in breadth; it is tolerably rectangular in shape, having a well-developed articular spine and fossette. Part of the scale is broken away at the anterior margin, the impression brought into view showing that the covered area is narrow, and indicating that the free surface is striated with rather sharp ridges passing obliquely across the scale. The posterior margin is finely denticulated.

Though this scale is in my opinion specifically, and possibly generically, distinct from those previously named, the outer surface not being properly displayed renders it impossible to give a sufficient diagnosis.

No. 6 is a piece of the same shale, having the clavicle of a small palaeoniscid fish, which it is, however, impossible to name.--I am, yours faithfully,

R. H. TRAQUAIR.

These fossiliferous beds seem to occupy a comparatively limited area, and have a very high dip in a south-easterly direction. At the spot where my observations were taken they did not extend over more than half a mile of country, but it is possible that the formation may persist for a long distance in other directions. Indeed, I traced it for some miles in the direction in which, some fifty or sixty miles off, lay the coal already described, and to which it may possibly be related.

With one or two general remarks upon surface geology and physical geography I bring this note to a close. First, regarding the Lakes Nyassa and Shirwa,--there is distinct evidence, and especially in the case of the latter, that they have formerly occupied a considerably larger area than at present. Shirwa is an extremely shallow lake; though the eastern and southern shores are mountainous, it is suggestive rather of an immense bog than of a deep inland sea. For many miles before reaching the shore there are signs that one is traversing the site of a former and larger Shirwa, which may possibly at one time have been actually connected with the lower extremity of Lake Nyassa. To substantiate this conclusion, however, will require more detailed examination of the Shiré Highlands than I was able to give. The peculiarity of Shirwa is that the water is brackish to the taste, while that of Nyassa and of the other Central African lakes, with the exception of Lake Leopold, is fresh. The shallowness of Shirwa, and the precariousness of its outlet through Lake Cheuta to the Lujenda, amply account for this difference; for the narrow waters of Nyassa and Tanganyika are thoroughly drained and profoundly deep.

That Lake Nyassa is also slowly drying up is evident from the most superficial examination of its southern end. There it has already left behind a smaller lake--Lake Pomalombé--a considerable expanse of water, through which the Shiré passes a few miles after emerging from Lake Nyassa, but already so shallow that nowhere in the dry season does the depth exceed three fathoms. If the silting up of this lake continues for a few years it will render this sheet of water, which commands the entrance to Lake Nyassa, totally unnavigable, and thus close the magnificent water-highway at present open, with a portage of seventy miles, from the top of Lake Nyassa to the Indian Ocean at the mouth of the Zambesi.

Regarding the interesting question of the origin of Lake Nyassa and its great sister-lakes in the heart of Africa--the Victoria and Albert Nyanza and Tanganyika--I do not presume to speak. No follower of Ramsay in his theory of the glacial origin of lakes could desire a more perfect example of a rock-basin than that of Lake Nyassa. It is a gigantic trough of granite and gneiss, three hundred miles in length, nowhere over fifty miles in breadth, and sixteen hundred feet above the level of the sea, the mountains rising all around it, and sometimes almost sheer above it, to a farther height of one, two, and three thousand feet. The high Tanganyika plateau borders it on the northern shore, and the greatest depth is precisely where the glacial theory would demand, namely, towards the upper portion of the lake. On the other hand, the physical geology of the country in which these other lakes are situated, as well as several features connected with Lake Nyassa itself, lend no countenance to such a view; and probably the suggestion of Murchison and other geologists is correct, that all these lakes, colossal though they still are, are the remnants of a much vaster expanse of water which once stretched over Central Africa.

The only other point to which I need allude is the subject of glaciation itself. And I refer to this pointedly, because I have lately encountered allusions, and in quarters entitling them to respect, to the presence of glacial phenomena in the Central Lake district of Africa. I confess that my observations have failed to confirm these suggestions. It has been my lot to have had perhaps exceptional opportunities of studying the phenomena of glaciation in Europe and Northern America, and I have been unable to detect anywhere in the interior of Africa a solitary indication of glacial action. In Kaffirland, far to the south, there are features which one would almost unhesitatingly refer to glaciation; but in East Central Africa not a vestige of boulder-clay, nor moraine matter, nor striæ, nor glaciated surface, nor outline, is anywhere traceable. One would be curious to know to what extent the flora and fauna of the inland plateau confirm or contradict this negative evidence against the glaciation of this region.

Finally, the thing about the geology of Africa that strikes one as especially significant is that throughout this vast area, just opening up to science, there is nothing new--no unknown force at work; no rock strange to the petrographer; no pause in denudation; no formation, texture, or structure to put the law of continuity to confusion. Rapid radiation, certainly, replaces the effects of frost in northern lands--and the enormous denudation due to this cause is a most striking feature of tropical geology. The labors of the worm, again, in transporting soil in temperate climates are undertaken by the termite; but here, as elsewhere, every fresh investigation tends to establish more and more the oneness and simplicity of Nature.