2go Report S.A.A. Advancement of Science. 



The top of the upper sandy beds is not seen, but the thickness 

 given is probably well within the mark. The levels of the granite 

 bedrock also vary considerably, and the beds themselves tend to 

 assume a lenticular shape, so that it is impossible to give more than 

 a rough idea of their proportionate development. 



The Upper Sands, which have been entirely removed by 

 denudation from some of the ridges, and from all the lower ground, 

 are sometimes clayey and stained red by iron oxides, and sometimes 

 fine and white. In a shaft at one spot on the slope of a ridge an 

 actual thickness of i6 feet w-as passed through before reaching the 

 underlying gravel. It may be stated, how^ever, that so heavy an 

 overburden is met with on few parts of the diamondiferous area. 



The Gravel itself is composed of beautifully-rounded pebbles 

 in a matrix of sandy clay, sometimes ferruginous. There are some 

 concretionary masses of iron, cemented sandstone, and the gravel is 

 converted in places into a hard conglomerate by infiltrated iron oxides 

 or more rarelv by silica. The pebbles are mostly of quartz, fre- 

 quently rock crystal, but they also include jaspery banded-ironstone, 

 chert, agate, hard sandstone or quartzite, and occasional large and 

 small pieces of silicified w-ood, as well as fragments of granite and 

 chloritic schist. Large boulders are comparatively rare. The silici- 

 fied wood, though distributed about in all sorts of positions, may 

 possibly have formed in situ, the granite and schist last mentioned 

 are the only other constituents of the deposit that are not well 

 rounded. The presence of the agate, as already mentioned, shews 

 the deposit to be newer than the lavas of the Forest Sandstone series. 

 In one shaft, 25 feet of gravel had been passed through at the time 

 of my visit, without any indications of approaching the base, and 

 more recent w^ork has shewn that my estimate of 40 feet is probably 

 a moderate one for the maximum thickness. 



The Lower Sandy Beds have obviously derived most of 

 their materials from the underlying granite. They are micaceous 

 throughout, the upper and lower parts South of the Railway being 

 clayey with a bed of clean sand in between. The base shews frag- 

 ments of decomposed granite, and it is difficult to fix the point where 

 the bed rock really begins. 



It is clear that in these sands and gravels we are not dealing 

 with the insignificant accumulations of the present-day river system. 

 This is no less evident from the character and distribution of the 

 deposits than from their position on the crest of what is now the 

 main watershed of the country. Their extent is quite in keeping 

 with a lacustrine origin; on the other hand, they correspond clos-^ly 

 with the alluvial deposits of rivers which have eroded their valleys 

 practically to the lowest possible level, and have for long been chiefly 

 occupied in widening them and spreading the materials furnished by 

 the process evenly over their flood plains. The Somabula beds may 

 therefore be set down, provisionally, as due to the action of an 

 important Tertiary River or river system, probably a feeder of the 



