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

8 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 8, and therefore exhibit more perfectly the proportion of the several kinds. Garnets are unequally scattered throughout the mass. Fragments of fossil wood are frequently met with. A great many of the pebbles are found in a fragmentary condition, owing, probably, to the action of the large boulders with which they are associated. Shafts have been sunk into these deposits to a depth of more than 30 feet.

In some places immediately below this gravel there is a thin layer (a few inches thick) of a very fine tough yellowish clay, almost like an ochre ; and below this are the ancient rocks that form the floor of the valley of the Vaal.

This boulder-gravel extends for a considerable distance, almost parallel with the course of the river, forming a terrace-like ridge, which finally slopes down towards the bank. This is the deposit in which a large number of diamonds have been found ; the largest have all been within 2 feet of the surface.

The same gravel is found on the opposite side of the river at Klip Drift, but in much smaller quantity. The component pebbles are shown by those sent. See Nos. 7, 8, 9, Appendix.

Fragments of fossil wood are more plentiful here and at Hebron than in any other portion of the diamond-deposits yet examined.

The upper portion of the gravel, which diminishes in some places to a thickness of 5 feet, and in others even to a single foot, is placed upon a thin laminated layer, from 6 to 8 inches thick, spread over the floor of many of the " claims," adapting itself to the inequalities of the deposit on which it lies. On breaking through it, a yellowish and rather ochreous clay is found ; and, although shafts of 10 feet have been sunk into it, its thickness is not at present known. These deposits evidently rest upon the rocks before mentioned.

Islands in the river here are of frequent occurrence : most of them are composed of gravel, which is apparently of a far more recent origin than that of the Kopjes and the terrace -like accumulations of Gong-Gong and Pniel. Some of them have trees growing them, as the island near the last-mentioned place. Several of these islands have been examined for diamonds, but with little success. The rapids, which also are very numerous, seem to be caused by felspathic dykes crossing the bed of the river.

Section III. At a distance of about twenty-four or twenty-five miles further up the river are the Hebron diggings. I have been able to obtain a more complete section of this locality than of the others. In this there is an evident sequence in the gravels ; and it throws considerable light upon the true position of those we have been considering.

Hebron and Diamondia. — This section (fig. 2), commencing at Diamondia, passes through the " lagoon " in the ancient river-bed (see Map), then shows the lower gravel, thence crosses the river to the upper kopjes on the Hebron side.

The uppermost deposits are those numbered Nos. 5, 6, and 7.