Prof. T. Rupert Jones — Diamond Fields of S. Africa. 57 



or from quartz-veins in Karoo beds/ in old schists,^ or in granite. 

 The abundant peridot or cbrysolite of the alluvium, however, is 

 referable perhaps to the amygdaloid s of the Karoo beds, if not to 

 the volcanic rocks of the Gats Eand, though it may have come 

 from metamorphic rocks ; ^ whilst garnet is found in both igneous and 

 metamorphio rocks.* Diopside occurs in crystalline rocks. Ilmenite 

 and hepatic marcasite are known only in metamorphic rocks. 



The great abundance of agate in the river- alluvium necessarily 

 points to the amygdaloidal lavas of the Karoo formation as the 

 source whence the Orange, the Vaal, and their tributaries, have 

 obtained a large proportion of their gravel (Eubidge, Quart. Journ. 

 Geol. Soc, vol. xi., p. 7; and Atherstone, Geol. Mag., Vol. VI., 

 p. 212). There must be much amygdaloid in situ, or imbedded, 

 judging from the unworn crystals of natrolite, peridot, etc. Agate 

 gravel ^ is traceable up the higher streams ; but whether or no these 



^ Dr. Eubidge stowed me a crystal of topaz in a piece of one of the Karoo 

 auriferous quartz-dykes near Smithfield, described in the Quart. Journ. Geol. Soc, 

 Tol. xi., p. 4, and vol. xii., p. 237- Should some of the gold-bearing quartz-veins, 

 such as occur on the Kraai River and near Smithfield, be continued northward to the 

 Vaal, they would be probable sources of rock-crystal, topaz, and gold; but as the 

 yield is very poor at the places named, we cannot reckon on the supply being great 

 elsewhere. Dr. Muskett (in a letter read at a meeting of the Port-Elizabeth Nat. 

 Hist. Society, October 29, 1868) expressed an opinion that the diamonds may have 

 been " set free by the disintegration of the highly ferruginous trap-rocks that abound 

 near the spot where they have been found." We learn from Mr. Higson (quoted 

 above at p. 52), that both trap-dykes and quartz-dykes of the Karoo series traverse 

 the Vaal. 



2 Dr. Grey has sent to England tremolite, amethyst, quartz with galena, a workable 

 variety of steatite, and massive prehnite, from the valley of the Vaal. (Exhibited 

 before the Geological Society, November, 1870.) 



^ Chrysolite is common in the Brnzilian diamond-drift, though only metamorphic 

 rocks appear to have yielded the debris. 



* The garnet-sand, washed for pyrope, near Bilin, in Bohemia, has been found to 

 contain diamonds. Bull. Geol. Com. Italy, 1870, p. 175 (this fact, however, is 

 doubted in Poggendorf's Annalen) ; and Geol. Mag., 1870, Vol. VII., p. 348. 



* Agate pebbles are known in the upper part of the Caledon Eiver ; and pebbles of 

 amygdaloid rock and rolled agates on the north-eastern branch of the Orange Eiver, 

 and in the Kraai. In 1856 Dr. Rubidge remarked that "the amygdaloid rock which 

 supplies the agate-gravel of the Orange, Caledon, Kraai, and Vaal Rivers appears 

 to exist in the ' Mont-aux-Sources ' (Ginnt's Castle), in the Draakenberg, as an un- 

 worn specimen was found in the Eland Eiver (a tributary of the Vaal), not more 

 than twelve miles from its source." — Quart. Journ. Geol. Soc, vol. xii., p. 237. 



In a footnote at p. 223, Trans. Geol. Soc, Second Series, vol. vii., 1858, mention is 

 made of some drifted ossicles of Encrinites, picked up, when Sir G. Cathcart returned 

 from his unsuccessful attack on Moshesh, by a soldier, near the most easterly branch 

 [Kraai .'] of the Orange Eiver, where they were associated with ferruginous casts of 

 small tm-reted shells \_Murehisonia ?], with fragments of agate, quartz, and fossil 

 wood, and with crystals of mundic The occurrence of Devonian strata at the base 

 of the Draakenberg is indicated in all probability by the fossils referred to ; and, whilst 

 the quartz and mundic may be referable to veins in such rocks, or to altered schists 

 of the same or greater age, the agate and fossil wood are sure local signs of the Karoo 

 beds. 



So also the river-gravel from near the mouth of the Orange River, referred 

 to in the same footnote, with its Encrinital joints, rolled amygdaloid, waterworn 

 agate, carnelian, cream-coloured chert, and greenstone, and fragments of micaceous 

 shale and copper-ore, points to the Metamorphic, Devonian, and Karoo strata, through 

 which the river runs, in escaping from the interior of South Africa on the western 



