422 ME. A. J. C. MOLTNEUX ON THE [Aug. 1909, 



The Basal Conglomerates. 



Any discussion on Karroo areas leads to a natural enquiry 

 concerning the existence of the Dwyka Conglomerate. So far, the 

 most northerly claim for such occurrence is that at Palapye, in the 

 Bechuanaland Protectorate, latitude 22° 40' S. 1 ; while Dr. E. T. 

 Mellor 2 has lately found it along the Limpopo Paver. In Rhodesia 

 hasal beds possessing glacial features have not yet been recorded, 

 and the contacts with the rocks of the complex are frequently found 

 to be made by grits, coal-measures, or Forest Sandstones. This 

 diversity shows the uneven nature of the floor on which the Karroo 

 sediments were laid down. But Rhodesia has been shown by 

 Mr. Newell Arber 3 to be included in the region of Suess's Gond- 

 wanaland, and it would be remarkable if this province alone can 

 show no signs of glacial activity when other parts of the same 

 ancient continent reveal them distinctly. Thus India has its glacial 

 boulder-beds ; Victoria, New South Wales, and Tasmania their 

 Bacchus - Marsh conglomerates and glacial deposits ; Brazil and 

 Argentina their boulder-beds ; and South Africa its Dwyka. 



Yet it is notable that both Northern and Southern Rhodesia 

 lie within 20° of the Equator, or in latitudes lower than that of 

 the areas of glaciation of the other provinces of Gondwanaland 

 mentioned. If, therefore, ice-action operated in these localities of 

 the torrid zone, we might gather some idea of the compensating 

 altitude above sea-level at which it occurred ; while if it be absent, 

 there would be the suggestion of a warmer climate than elsewhere, 

 and a consequent question as to the causes that brought about a 

 local amelioration of temperature. 



In Northern Rhodesia there are coarse basal deposits of some- 

 what restricted distribution, but locally swelling out into wide 

 masses. They never occur elsewhere than on the floor of the basin, 

 in which respect they correspond with the Dwyka ; and, although 

 there is insufficient evidence of glacial origin, they possess features 

 that distinguish them from true aqueous beds. It is probable that 

 they were formed as ' screes ' in the subaerial break-up of the 

 uneven surface of the pre-Karroo complex. 



A feature noticed by every one who has travelled in the Luano 

 Valley is the existence of a surface-shingle of smooth and shining 

 boulders and pebbles. They show in swelling mounds among the 

 mapani forest, or lie in gravel-deposits under river- wash. They are 

 conspicuous everywhere, and there is little of the area of the plain 

 away from the alluvial terraces that has not a layer of shingle 

 above or below the soil. Where this occurs on the surface, the 



1 A. J. C. Molyneux, ' A Contribution to the Geology of the Bechuanaland 

 Protectorate ' Proc. Rhodesia Sci. Assoc, vol. vi (1907) pt. ii. 



2 Mem. Transvaal Geol. Surv. No. 3, 1906. 



" 3 ' Catal. Foss. Plants of Glossopteris Flora in Brit. Mus. Nat. Hist.' London, 

 1905, p. xviii, &c. 



