Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/21

1871.] STOW VAAL-RIVER DIAMOND-GRAVELS. 5 In the accompanying Map (Pl. I.), which shows the Vaal River from the Plaatberg to its junction with the Reit River, all the localities where the diggers are most thickly congregated are indicated.

My friend Mr J. Graham, who has furnished me with the results of some observations he made on the level of the river, found by the aneroid that its fall was exceedingly small. Between Hebron and Klip Drift, a distance, along the course of the river, of more than 25 miles, it was only 22 feet. The stream is divided into long level reaches; and these in most cases are joined by a succession of small rapids. Three of those below Pniel are marked as Nos. 1, 2, and 3, on the map. The sinuosities of the river are very remarkable. Whatever may have been the agency that occasioned the vast accumulations of gravel and boulders that we have to treat of, it must have been something more powerful than a current like that of the present Vaal.

The country to the south of the river consists of immense gently undulating flats, with scarcely any eminences worthy of being called hills. South of the Plaatberg the only elevation forming a range is between Bult-Fontein and Robinson's; and this only rises to a height of 400 or 500 feet.

A large portion of the country to the south of the Vaal is covered, as has been mentioned, with calcareous tufa, hidden in many places by a thin coating of light sandy soil, just sufficient to support a somewhat scanty herbage. Local depressions in the flats are very common; and most of these have no outlets to any lower level, although the drainage of a large extent of land slopes from every side towards them. In colonial phraseology they are styled " Braak- Pans." The origin of the depressions has yet to be explained; but the water of the "pans" appears to be due to its accumulation after heavy rains in these hollows. This is again rapidly evaporated in dry seasons, and leaves the soil impregnated with the saline particles dissolved and carried down thither from the higher slopes by the rain-water. The water, from the peculiar formation of the country just alluded to, has no other means of escape; thus the constant addition during a long course of ages has had such a sterilizing effect that the growth of all vegetation in the central portion, or " pan," is prevented by the extreme brackishness of the soil. Some of these "pans" are of large size — from 2 to 3 miles in length. Du Toit's Pan is one of this description. Between Jacobsdal and the Campbell-grounds there are two such salt-pans. About twelve years ago one of these ceased yielding salt, and continued in the same state for ten years; but during the last two years it has recommenced depo siting a large quantity of saline matter.

Occurrence and geological place of the Diamonds. — Diamonds, up to the present time, have been found principally: —

1st. In an unstratified gravelly drift, containing immense numbers of huge boulders, with a red, clayey, ferruginous or ochreous matrix. Pniel is an example of this kind.

2nd. Unstratified gravel, with boulders most irregularly inter-