GEOLOGY OF THE SOUTHERN TRANSVAAL. 4G1 



that this rcjxion was under gLacial influences at some time during 

 the lonj; period which interveued between the deposition of tlie 

 Megaliesberg formation, in probably the Devonian era, and of the 

 coal-bearing rocks of the High Yeldt, tho age of whicli is certainly 

 Oolitic. Mr. Dunn places the Dvvyka conglomerate just above the 

 Carboniferous. 



(b) Denmlat'wii of the present Surface. — Whatever agencies may 

 have effected the denudation of the older rocks of the Southern 

 Transvaal before the imposition upon them of the Secondary High- 

 Yeldt formation, those of later date were decidedly fluviatile. The 

 noteworth}' feature of this denudation is its apparent continuity 

 from the close of the Oolitic period until now. There are evidences 

 of an enormously prolonged period covered by the last subaerial 

 denudation. 



(c) During the earhj Part of the High-Veldt Coal-formation. — 

 Intervening between the two periods of great erosion {a and 6, 

 above) there was another denudation of this area, naturally fluvia- 

 tile, anterior to and during the deposition of the lower beds of the 

 coal-bearing formation. The alluvial deposits containing gold, re- 

 sulting from this minor degradation, will be found (I venture to 

 affirm) in the valleys of that period, many of these valleys being 

 more or less coincident with the present depressions. In their 

 lower portions there are still tongues or branches from the main 

 mass of the High-Yeldt Coal-formation which, in some cases, may 

 -overlie ancient alluvium ; indeed, that is the character of the " sand 

 and pebbles" mentioned in one of the preceding coal-sections 

 (p. 458) ; such deposits would somewhat correspond to the " deep 

 leads " of Australia. The last, greatly prolonged, fluviatile denu- 

 dation must surely have left numerous relics of its ancient river- 

 courses in the shape of gravels, which will be found to occupy the 

 summits and flanks of the hills within the larger valleys, although 

 very few have yet been detected. Those acquainted with the 

 Quaternary gravels of Europe cannot fail to be struck with the large 

 areas in South Africa, both hill and valley, exhibiting bare rock, 

 only here and there covered by a few inches, or possibly a few feet, 

 of blown sand. Sometimes there are deep patches of loamy soil, 

 but these, when cut into, have more the appearance of rainwash 

 than of fluviatile accumulations. 



The De-Kaap, Blydc, and Olifants Rivers have some good gravels 

 along their present courses, and the headwaters of the Limpopo very 

 few, whatever may be the case away to the north of the Megalies- 

 berg Range. The upper portion of the Yaal has muddy banks, with 

 gravel here and there, until it leaves the Transvaal ; then there are 

 large masses of terrace -gravel in which were first found the South- 

 African diamonds. 



Plate XV. 



Geological- Map of the Area between Klerksdorp and the De-Kaap Yalley, 

 and three sections. 



