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consisting mainly of lydite, jasper, agate, and quartz, with 
ochre and rotten felspar, mingled with large and small blocks 
of the felspathic amygdaloidal trap-rocks that constitute the 
hulk of each hill. Indeed this basement-rock is undergoing 
decomposition, some of it breaking up into rotten ochreous 
felspar, with frequent concretions of chalcedony (agate and 
carnelian), whilst other portions, less decomposable, remain in 
angular and exfoliating pieces. 
At different spots, however, the gravel varies in its constitu- 
tion. Eock-crystal is plentiful in some places, and amethyst 
also occurs. Garnet, peridot, mesotype, natrolite, and calcite 
abound here and there ; but the exact conditions under which 
they are met with have not been noticed as yet. Brown mundic 
(hepatic pyrites), ilmenite, specular iron-ore, diopside, tour- 
maline, and diamond are rarer minerals in the gravel. Of 
nearly all the above-mentioned minerals there are water- worn y 
freshly broken, and perfect crystals. The chalcedony, also, and 
jaspers occur in both the worn and the unworn state. 
It has been suggested by the late E. N. Eubidge and others, 
that large areas of this part of South Africa have been at a long 
distant period (and yet recently as regards geological time) 
covered with alluvia, derived from the operations of water and 
weather on the vast region drained by the head-waters of the 
Orange and Vaal, and now represented in part by the great 
Quathlamba or Draakenberg. This mighty range and its 
southern spurs supply by far the majority of the sources of 
the present Orange Eiver system, and still yield to the 
upper valleys agate gravel in abundance, from their amyg- 
daloidal volcanic rocks. To the north, however, in the Trans- 
vaal, some of the head- waters of the Vaal (Ky Gfariep) rise in 
the Gats Eand and other mountains, which consist of a different 
rock-system. To these allusion will again be made when we 
consider the probable origin of the diamonds. (See my paper 
on the Geology of the Diamond-fields of South Africa — 44 Geo- 
logical Magazine,” February 1871 — for technical details and 
full references to published papers and other materials.) 
The ancient alluvial plains, above alluded to, were probably 
terraced, according to our authors, by the successive subsidences* 
of lake and river ; and here and there they were at times 
coated with beds of calcareous tufa, derived from aggregations, 
of fresh- water and land shells ; and this still lies thick on many 
parts, and serves as a source of lime to the Colonists of the 
Interior. The work of natural denudation, or the remodelling 
of the surface by rain and rivers, progressed, whether aided or 
not by ice-action (as suggested of late by Mr. G. W. Stow for 
the surface-modification of the Stormberg and Queenstown 
country further south), and ultimately the levels of the present 
