Vol. 65.] KARROO SYSTEM IN NORTHERN RHODESIA. 437 



as the head of a storm -course or kloof trending towards the river. 

 Shafts of 100 feet sunk in this formation often show a complete 

 rotting to that depth. 



These soft masses fall an easy prey to the meanderings of the 

 rivers, which quickly overtake the slow process of atmospheric 

 decay. On the river reaching the unaltered rock the true nature of 

 the Karroo strata can be seen, and it is difficult to realize that the 

 one is but the original form of the other. Coal-seams then form 

 conspicuous steps, and the sandstones wear into potholes and other 

 grotesque shapes. 



The transitions undergone by a coal-seam are remarkable. When 

 subjected to subaerial alteration, it appears as an evenly laminated 

 grey shale, of a faint amethystine colour. In the bedding is a 

 reddish or black dust, sometimes so arranged as to be suggestive 

 of plant-structure. The streak-powder is fine, and sharp to the 

 touch. Frequently the shaly structure disappears, and a brecciated 

 appearance takes its place. As the seam is followed from the 

 outcrop, improvement in commercial quality is marked by a 

 browner colour and a blackening of the bedded dust. Next a 

 yellowish efflorescence shows in the cracks, then an appearance of 

 specks of dull brittle coal, increasing into lamina?, with ' brassy ' 

 pyrite and vertical films of calcite. These improvements are but 

 gradual, and the good coal is only reached at 100 feet from the 

 surface by vertical sinking ; but the percolation, which is easier 

 along the dip of the seam than through the roof, extends along the 

 bed for over 300 feet from its outcrop. 



VII. Conclusion. 



In this paper I have tried to describe the local facies, at widely 

 separated localities, of the Karroo beds in Southern Rhodesia, 

 deeming that from them we may learn much of the powers that 

 have operated, and their results, over an extensive region of South 

 Central Africa. 



In conclusion, I desire to express my thanks to those who have 

 so kindly afforded me advice and help, especially in the discussion 

 of points by correspondence, namely, Prof. Grenville A. J. Cole, 

 F.G.S., Dr. A. Smith Woodward, F.R.S., and Mr. Herbert Kynaston, 

 F.G.S. ; but to Mr. G. W. Lamplugh, F.R.S., is my gratitude more 

 especially due. Mr. S. F. Townsend, Resident Engineer of the 

 Rhodesia Railways, very kindly permitted me to use maps and 

 altitudes ; and I have to thank Dr. Walcot Gibson, F.G.S., for his 

 recommendations in the drawing up of this paper. Dr. J. S. Flett, 

 M.A., and Prof. Grenville Cole have obligingly gone to considerable 

 trouble in examining my rock-slides, but much of the valuable 

 information given to me touches upon the pre-Karroo complex, 

 and is therefore felt to be beyond the scope of the present paper. 



Q.J.G.S. No. 259. 2g 



