﻿Vol. 66.'] GEOLOGICAL STRUCTURE OF SOUTHERN RHODESIA. 357 



contact-alteration products, one may observe minerals such as anda- 

 lusite and sillimanite (fibrolite). The latter occurs at Hillside, near 

 Bulawayo. Salisbury, the capital of Southern Rhodesia, is situated 

 on the Banded Ironstone Series, and andalusite (or rather, perhaps, 

 chiastolite)-schist underlies most of the eastern part of the town 

 near the granite ; while the more typical, less altered rock crops out 

 at the ' Kopje ' to the westward. 



It is interesting to note that in the Transvaal there is a thick 

 succession of unaltered shaly beds known as the Pretoria Series. v 

 Though possibly themselves of Archaean age, they are quite soft 

 and of normal appearance for the most part, but in places they 

 become highly ferruginous and can scarcely be discriminated from 

 the Ehodesian Banded Ironstones. 2 The latter are also absolutely 

 indistinguishable from parts of the Lower Witwatersrand Beds 

 of which, indeed, they may be exact equivalents, although this 

 vexed question need not be entered into here. 



A peculiar structural feature of the Banded Ironstones may be 

 briefly touched upon. The banding is usually vertical or nearly so, 

 and runs in straight and parallel lines. Intercalated, however, 

 among an apparently normal sequence of these beds, there will 

 often be found extraordinarily contorted layers, not usually ex- 

 ceeding a foot or so in thickness. Remarkable examples of this 

 may be seen near Bulawayo and Salisbury ; but especial mention 

 must be made of a magnificent section on the banks of the Sebakwe 

 River, about a mile east of the Globe & Phoenix pumping-station 

 (see fig., p. 356). The contorted portions there merge into crush- 

 breccias, and the phenomena do not appear to indicate shearing, 

 but rather concertina-like folding due to lateral pressure. The 

 way in which these intensely hard and resistant rocks have usually 

 yielded, without breaking, to the forces acting upon them, is suffi- 

 cient evidence of the depth at which the movements must have 

 taken place. 



(3) The Conglomerates. 



The next set of rocks to which attention may be drawn is the 

 Conglomerate Series. This comprises a succession of beds, appa- 

 rently of enormous thickness, but probably much folded. Near 

 Bulawayo they could scarcely be estimated as less than 10,000 feet 

 thick, supposing them to be a normal sequence. It is in the 

 somewhat remote Lomagundi district that these beds have attracted 

 most attention, owing to the important occurrences of gold among 

 them, and they are universally known as the Rhodesian « banket.' 

 I have had occasion to make a minute study of these beds in the 

 course of two lengthy visits to the district, and have recently paid 



i I have to express iny indebtedness to Mr. H. Kynaston, Director of the 

 Transvaal Geological Survey, for kindly conducting me over some of the 

 exposures of these rocks in 1908. 



'■* Compare also the Canadian slates and ironstones of Lower Devonian age 

 in Nova Scotia, Ann. Kep. Canad. Geol. Surv. for 1904 (1905), pt. A, p. 300. 



