Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/34

 junction of the two rivers, a distance of upwards of 100 miles below Pniel.

The agencies that have worked out the one basin have without doubt operated in the formation of the other, both being valleys of denudation ; and the whole of such deposits as those found upon the lower plains, and along the banks of the rivers, must have originated within the boundarj-line that has here been pointed out as the great watershed of the rivers running into the Atlantic and the Indian Ocean. It is therefore within this limit that we must seek for the origin of the diamantiferous gravels and ether deposits of the Vaal.

Rocks and Fossils of the Upper Drainage-area. — Before proceeding, however, it will be well to consider the indications we possess of the character of some of the formations that form different portions of this outer boundary -line. The evidence in my possession is scanty, but of great interest. Commencing on the northern boundary, we find that in the Magalies Berg and Pretoria there are micaceous rocks, vastly different from those belonging to the great Dicynodon- formation of the Colony, consisting of and containing large quantities of mica, with ores of cojDper and lead, the latter, it is said, very abundant. See Nos. 19, 20, 21 in the Appendix.

From this spot to the sources of the Umzimvoobo, on the southern side of the Quathlamba, I have been unable to obtain rock-specimens ; but from this last locality I was fortunate enough to secure several, collected by Mr. Whitmore from the gravels along the watercourses. See No. 22 in Appendix.

These form a most interesting collection, and might be easily mistaken, both from their shape and character, for some of the pebbles obtained from the diamond-bearing gravels of the Vaal. From the Zeitza, a small stream rising in the same mountains, similar specimens were obtained (Nos. 23 & 24 in Appendix). Also from the heads of the T'somo, a river rising among the branches of the mountains that form the junction of the Quathlamba and Stormberg ranges, rocks very similar to some of the fragments found in the valley of the Vaal. See Nos. 25, 26, and 27 in the Appendix.

Origin of the Gravels. — The conclusion to be drawn from the consideration of these facts taken collectively (the whole of these specimens being procured from the southern side of the mountains, from the northern side of which both the Orange and Yaal take their rise) is, I think, that zones of fossil wood are found stretching along these mountains, and that they contain rocks which are similar to many of those found in the Vaal-River gravels. It is therefore highly probable that the fragments of the fossils found in them, and of the various, yet very similar, rocks, have both come from the same source, namely the elevated ridge that forms the watershed of the rivers that flow into the two different oceans, and that during the vast denudation to which the strata have been subjected large portions have been carried away in the one or the other direction. On the north they have been promiscuously intermingled