460 F. P. Mennell — llie Soinahula Diamond Field of Rhodesia. 



afford some idea of the geological and mineralogical features of the 

 field, of which I hope to give a much more detailed account at an 

 early date. 



The diamond area may be described as a tongue of the Somabula 

 Forest stretching along the central plateau of Khodesia from the 

 Uvungu River for about seven miles in the direction of Gwelo. 

 The beds of which it consists are undoubtedly younger than the 

 Forest Sandstones, as shown by their numerous pebbles of agates 

 derived from the lavas interbedded with those rocks, but they are 

 probably very different in age, and may perhaps be regarded as the 

 uppermost portion of the Forest Sandstone series. They directly 

 overlie the granite of the watershed, on to the apex of which they 

 extend, but further down the Uvungu River the ordinary Forest 

 Sandstones are met with. The general sequence appears to be — 



5. Surface rainwash, etc., chiefly redistributed gravel and 



sand (often absent) ... ... ... ... ... say 10 feet 



Somabula ('*■ I^ed and white sands say 40 feet 



*" Beds ) '^' ^''"'"'"^^ "^"^^^ parting's of clay, etc. say 40 feet 



12. "White micaceous sand ... ... ... ... ... 30 feet 



resting- unconformably upon 

 1 . Granite. 



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 

 bed-rock 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 some- 

 times clavey 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 16 feet was passed through before reaching the under- 

 lying gravel. It may be stated, however, that so heavy an over- 

 burden is met with on few parts of the diamantiferous area. 



The gravel itself is composed of beautifully rounded pebbles in 

 a matrix of sandj'^ 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 rarely by silica. The pebbles are mostly of quartz, 

 frequently rock crystal, but they also include jaspery banded 

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

 large and small pieces of silicified wood, as well as fragments of 

 granite and chloritic schist. Large boulders are comparatively rare. 

 The silicified wood, though distributed about in all sorts of positions, 

 may possibly have been 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, 

 shows 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 work has shown that my estimate of 40 feet 

 is probably a moderate one for the maximum thickness. 



