﻿Vol. 66.] GEOLOGICAL STRUCTURE OF SOUTHERN RHODESIA. 371 



diamond, ruby, sapphire, oriental amethyst, yellow chrysoberyl, 

 catseye, alexandrite, beryl (aquamarine), blue and white topaz, etc. 

 Of these gems only the diamond has so far been found in situ 

 in Rhodesia, although common corundum is known from several 

 localities, and the ordinary yellow topaz occurs at Essexvale, 

 as already noted. 



(e) Superficial Deposits. 



In Rhodesia there are no drift deposits, like those of England, 

 connected with the recent river-system. The coarse angular rock- 

 debris seen in the beds of the rivers is very different from the alterna- 

 tions of gravels, sands, and brick-earth observed in Europe, besides 

 being insignificant in amount. Even great rivers like the Zambezi 

 and Limpopo are almost devoid of alluvial deposits. As if to make 

 up for this, we have a variety of formations due largely to chemical 

 action operating during the disintegration of the rocks upon which 

 they rest. These comprise laterite, calcareous tufa (surface lime- 

 stone), and compacted sands (surface ' quartzite ') . None of these 

 deposits appear to form actually on the surface of the ground ; they 

 are rather sub-surface in origin, accumulating under the soil. 

 The iron or alumina of the laterite, the lime of the tufa, and the 

 silica cementing the sands are obviously derived from the rise 

 towards the surface, by capillarity, of solutions containing the sub- 

 stances named leached out of the decomposing silicates of the under- 

 lying rocks. Laterite is particularly widespread in its distribution, 

 and may originate from rocks which seem little calculated to afford 

 the necessary amounts of iron and alumina. I have described the 

 features of the Rhodesian laterites in a recent paper, 1 and so 

 they need not further detain us. The tufaceous limestones are 

 usually found on the epidiorites and similar rocks of which the lime 

 content is considerable. The decomposition of the ancient crystal- 

 line limestones, curiously enough, gives rise to deposits of a lateritic 

 nature, instead of these tufas. The ' surface quartzites ' are rare 

 on the whole ; but good examples, sometimes resembling veryancient 

 rocks, may be seen close to Hillside near Bulawayo, and also in the 

 Victoria Falls region. They frequently have a very cherty appear- 

 ance, but are often quite conglomeratic in character, like so man) 

 of the laterites. 



V. Various Igneous Hocks. 



Besides the immense granite masses already mentioned, there 

 are a great variety of igneous intrusions of later date. They are 

 probably of very different ages, some being evidently connected with 

 the volcanic activity of the Forest Sandstone period, and some 

 certainly earlier. Perhaps the most remarkable of these rocks is the 

 great mass of coarsely crystalline picrite extending for nearly the 

 whole distance across Rhodesia from north to south, with an outcrop 



1 Geol. Mag. dec. v, vol. vi (190U) pp. 350-5] . 



