Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/46

22 intruded laterally, and between previously existing strata, has received their impress, and is now the great conservator of the country from the tremendous denudation going on in South Africa, with its excessively dry climate, its intense heat, and its occasional heavy floods.

The Vaal-River Diamantiferous Region is known to extend from near the town of Potchefstrom, the capital of the Transvaal Republic, down the whole course of the Vaal to its junction with the Orange River, and thence along the Orange River for at least 60 miles. Digging-operations are at present confined to the Middle Vaal, from the residence of the Griqua Chief, Barend Bloem, at Hebron, downwards on both banks of the river to the Bechuana Kraal of Sibonell. Along the whole course of the Vaal there is the same general development of trap, consisting of basalt, amygdaloids, trap-porphyry, trap-conglomerates, and associated metamorphic rocks. Granite exists nowhere in situ; but granite nodules occur in the trap conglomerate. Syenitic greenstone is developed in one of the localities near Klip drift, and seems to be the foundation rock there.

Sedimentary rocks, sandstones, limestones, &c. occur in some of the Kopjes (hills or heights) capped by basalt.

The soil on the summits of the rocky hillocks consists of a ferruginous loam and imbedded water-worn pebbles. These pebbles and the loose boulders exhibit the appearance of having been water-worn for a considerable time. They are mainly of basalt, amygdaloid, sandstone, agate, peridot, garnet, tourmaline, quartz, jasper of various colours, granite, binary granite, serpentine, malachite, gneiss, &c.

It is to be noted that the alluvial soil prevails everywhere on the tops of the rocky hillocks, considerably above the present river-bed, and extends inland from the river for two and three miles, and in some cases much further. The hollows are filled with drift sand; and underneath, the pebbly mass is imbedded in a stiff clay, which is most difficult to work by the ordinary washing processes carried on by the diggers, and accordingly has been neglected for the easily washed material of the sides and summits of the heights. The summits are in greater favour than the sides amongst the diggers; and no doubt the wealth of the sides has found its way into the hollows and probably into the gravelly bed of the present river-course.

The history of one digging-centre in the Vaal, Cawood's Rush, supports this opinion. This spot is a low-lying gravelly bank; and the river sweeps round the greater part of it, so as to form it into a species of peninsula. The old watercourse was undoubtedly across the isthmus, where large and very ancient trees still live, indicating the former bank of the river to have been there. The trees around the present bank are all young. When, therefore, it is understood that these trees only occur flanking streams, it is manifest that the peninsula of Cawood's Hope was formerly an island, and that at a very recent period it was the river-bed. It proved a place of unexampled success in the diamond-findings of South Africa. Shells of the genera Anodon and Cyclas, which live in the Vaal, were dug up in every part of the spot with the epidermis still on. The gravel