F. P. Mennell — The Somahula Diamond Field of Rhodesia. 461 



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 shows 

 fragments 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 closely 

 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 great lake which must once have filled the adjacent 

 portion of the Zambesi basin, and draining an area chiefly occupied 

 by granite and the Archaean banded ironstone. 



The gravels of the Somabula are interesting from their unique 

 lithological character as far as Rhodesia is concerned, but their 

 chief interest naturally arises from their being the source of various 

 gem stones, particularly the diamond. The diamonds themselves 

 are peculiar as almost invariably of a green shade in the rough ; 

 this is, however, entirely lost in cutting. They occur in very good 

 crystals, principally octahedra, spinel twins of two octahedra, 

 twinned tetrahedra, twinned hexatetrahedra, dodecahedra, etc. 

 Etched triangles are characteristic of the tetrahedral faces. Worn 

 stones are almost entirely absent. The mineralogical associates of 

 the precious stones are not precisely similar to those of the 

 Kimberley diggings or of the more recent Transvaal discoveries, 

 but they nevertheless present a general resemblance to those of the 

 localities named. Garnets are often common, but are not of the 

 blood-red Kimberley variety. Ilmenite too is uncommon. Both 

 magnetite and haematite, of which grains are numerous, are evidently 

 derived from the banded ironstone, while the source of the zircon 

 and of the mica (muscovite) is equally clearly the granite ; these 

 minerals have no necessary connection with the original matrix of 

 the diamond. This too is probably the case with the beryls which 

 also occur. The typical minerals of the deposits are (besides the 

 diamond) enstatite, chrysoberyl, kyanite, and sapphire. Enstatite 

 is the commonest of the minerals popularly grouped together under 

 the name of olivine at Kimberley, but is here a remarkably hard 

 brownish variety. It is an abundant constituent of the sorted 

 material from the puddling machines. Some of the grains are 

 fairly clear, and might almost be taken for garnets on account of 

 their red-brown colour. Chrysoberyl is quite abundant for so rare 

 a stone. The prevailing variety is yellow, but the opalescent 



