Dr. F. H. Hatch — Geological JSistory of South Africa. 161 



rise up above the general rim of cliffs, one of which contains 

 Crotty's Cave, and is a landmark for many miles. The eastern side 

 of the corrie is mostly formed by a less precipitous spur from the 

 mountain-side. The mouth is partially blocked by a conspicuous 

 rounded hill strewn with large boulders and rising about 100 feet 

 above the level of the water. On its outer or northern side this hill 

 descends steeply for about 450 feet to a broad, roughly semicircular 

 shelf with fairly level surface, probably representing an old moraine. 

 The latter has an abrupt edge which slopes down suddenly to the 

 plain about 300 feet below. 



On each side of the rounded hill at the mouth of the corrie there 

 is a possible outlet for the waters of the lake ; the one on the north- 

 west side would be over solid rock, and a rise of five or six feet in 

 the level of the water would cause the outflow to be by this channel. 

 On the other or eastern side the rounded hill is joined to the 

 mountain spur by a ridge of morainic material which blocks up the 

 wider and more natural outlet, and the lake discharges itself at this 

 point by the water percolating through the dam and issuing on its 

 further side in a series of springs which feed the swamps and give 

 rise to the streamlet running down a depression in a north-easterly 

 direction. It is noticeable that the rocky lip of the lake on the 

 west is lower than the top of the dam on the east, but the outflow 

 of the lake is nevertheless at a lower level on the other side of the 

 median hill, which indeed appears to consist entirely of morainic 

 material and to represent merely an unusually large and regular 

 morainic mound. N.o solid rock is visible in its composition above 

 the level of the lake, and there is no direct evidence that we have 

 to deal with a true rock-basin in this instance. 



{To he continued.) 



III. — The Geological History of South Africa. ^ 



By Dr. F. H. Hatch, F.G.S., M.I.C.E., 



President of the Geological Society of South Africa. 



{Concluded from the March Number, p. 104.) 



3. Geological History of the BocJcs. 



AFTER the granites, gneisses, schists, and sediments which make up 

 the Swaziland System had been elevated to form a continental area 

 extending over the northern and western portions of South Africa, 

 denudation began, and the material thus produced was carried to 

 the sea to form the Witwatersrand Beds. The nature of these 

 sediments — they consist of conglomerates, grits, and shales — 

 indicates a marine period with shallow-water conditions, which 

 continued almost uninterruptedly during their deposition. They 

 were accumulated first on a sinking, and then on a rising sea 

 bottom, for the lower beds are composed largely of mud and fine 

 sand, conglomerates only becoming abundant in the upper beds, 

 which were formed in the later portion of the period when the 



1 Presidential Address delivered by Dr. F. H. Hatch to the Geological Society of 

 South Africa, 29th January, 1906. 



DECADE v. — VOL. III. — NO. IV. 11 



