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There is little deviation from the horizontal in the various strata, from the Cape Peninsula, onwards in a line to Leydenburg in the Transvaal. From this point to Delagoa Bay there is again a gradual terraced descent. From Leydenburg to Spitzkop we have schistose sandstone, thence to within 50 miles from the coast granite. From this to the coast felsite porphyry and, nearer the coast, tertiary sandstone.
Trap dykes occur all the way from the Cape Peninsula in a line to Delagoa Bay. They are, as determined by Cohen, Diabase. These dykes and vast sheets of trap are the characteristic features of the Upland Plateau. The scenery is a never-ending succession of cone-shaped mountains, table mountains, and flat uplands. The trap rocks have come to form the prominent features of the country from the long process of denudation to which South Africa has been subjected. The strata, through which they were thrust being softer, have yielded to wear and weathering, leaving the harder trap as protecting masses. In many parts the trap assumes the character of an aphanitic amygdaloid as in the Vaal Region; of a trap conglomerate as in a remarkable belt, which extends through the Western Province of the Cape Colony and Natal; or of a porphyritic trap as in Zululand. This is in all probability of the same age as the great trappean outbursts of the Upland Plateau.
The feature however of greatest interest to the geologist in the Upland Plateau is the presence near its inland boundary of the remains of volcanic throats or pipes. They are easily detected. Circular spots of a whitish soil are noticed. The soil on being examined is found to glisten with mica; masses of rocks foreign to the locality, garnets, and pieces of illemanite are turned up. In five at least of these throats diamonds have been got and are mined for. They are the Kimberley mine, Old de Beers, Dutoitspan, Coffeefontein, and Jagersfontein. The Kimberley mine now dug out to the depth of upwards of two hundred feet is the most famous diamond producing of these throats. Several of the throats in other parts of the Plateau have been examined, but no diamonds have turned up. The indications are all alike in every one I have observed; yet it seems as if the diamonds have no necessary connection with them.
These volcanic throats have nothing to do with the
trappean formations to which we have referred, and must have been formed subsequently. The walls of the Kimberley throat are the ordinary shales of the Upper Karoo, but there is a trappean layer conformable to these shales, which has been broken through also. The de Beer's mine has walls exclusively of Diabase. Dutoitspan has walls of alternate shale and trap. These facts are sufficient proof of my general position that the trap formations of the Upland Plateau are older than the volcanic throats or pipes. I have seen these pipes on the summits of trap covered hills in the neighbourhood of Hanover. The tuff contained in them has been determined to be palagonitic, and it has imbedded in it pieces of trap, sandstone, shale, trap conglomerate, granite, gneiss, mica schist, and chloritic schist. The argument for the relative age of the throats is therefore that the rocks of South Africa lie upon each other according to age, and that the older rocks are deeper down, and that the volcanic tuff in being thrust up got fragments of these rocks imbedded in it. It is not now absolutely necessary to believe that this tuff came up in a very hot condition. The walls of the throats are not much metamorphised, as they would have been in that case.
There can be no question, but the Karoo beds belong to the early part of the secondary epoch, and they are referred by Professor Owen to the Triassic Age. Similar fossil lizards have been discovered in beds in the South of Russia, which are undoubtedly Permian. Now the Permian is the last of the palaeozoic series, the trappean outburst of course happened subsequently to the Triassic Age. It was probably during this great upheaval that the mountain masses of South Africa were rent by huge fissures, through which the drainage of the inland regions is carried transversely across them. No more singular feature in the physical geography of the country is there than the narrow and tremendous gorges in mountain ranges, through which some of the South African rivers make their way transversely to the sea. No denudation theory can explain their abrupt and precipitous character. Denudation exhibits its work in gradual slopes, picturesque glens, and in open valleys. Cataclysm can alone explain the rending of mountain masses, such as has taken place in the ranges, which bound the great Upland Plateau of the Cape. In the
course of ages silently and gradually, denudation may remove continents. While we allow all this to modern uniformitarian geologists, such violations of the general laws of drainage in the river system of South Africa, by means of which rivers find their way through the heart of mountains, require some other explanation. The great trappean upheaval according to this view brought the period of inland lakes, which existed throughout the Triassic Age, to a termination. The waters of these lakes secured outlets by the enormous fissures of the mountains made by the upheaval.
The age of the volcanic throats may now be considered. The older trappean formations, which on the margins of the great inland lakes of South Africa assumed the character of a conglomerate from taking into the outburst fragments of rocks and pebbles, as we have seen, disturbed the strata of the Karoo series very little. The eruptions doubtless came from great depths, and while they broke up some parts, in sinuated side-flows between strata. Extensive layers of rock would thus be only raised to a higher level, the trap forming a stratum-like layer conformable to the previous strata. The volcanic throats however have the Karoo beds tilted up. In the Kimberley mine they are tilted to an angle of from 30° to 75°. The inference is that they belong to a comparatively recent date geologically speaking. Where trappean outbursts are examined the horizontality is preserved, and we can only explain this fact by holding that denudation and drainage have carried away all trace of the disturbed strata, and that a long geological period has elapsed since these trappean eruptions. The argument therefore is in favour of the volcanic throats having been formed very much more recently, and possibly in the tertiary epoch.
There has only been one great upheaval since that of the secondary epoch, viz., that which has given to South Africa its scanty tertiary beds, already referred to. There is collateral support to the opinion, that this was probably the same as that identified with the outbursts of the volcanic throats. In the progress of the alluvial diggings along the Vaal in search of diamonds, the older gravels some distance from the Vaal and elevated above the river were the best and most in favour. Indeed in such a spot as the celebrated Union Kop diamonds were
not got on the surface, but at between four feet and ten feet in depth. Such an alluvial deposit could not be considered as recent, situated as it was some 80 feet above the river. The diamantiferous part of it even was obviously older than the surface four feet. Other examples could be quoted in support of the general position I have assumed.
And now that the diamonds have been traced to what may be considered as their head-quarters in certain volcanic throats, I think it will be conceded that the diamonds of the alluvial fields came originally from similar spots now obliterated by the Vaal river in its erratic courses through that region. These old gravels of the Vaal cannot be of more than tertiary age, and surely younger than the Eocene. The inference is therefore that the volcanic throats to some of which diamonds have been ultimately traced belong to the tertiary age, and possibly to the same period as the elevation of the sparse tertiary beds of South Africa.