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.