F. P. Mennell — Tico Points in African Geology. 547 



locked up in the pyrites crystals could not have been there before 

 the pyrites itself was formed. Eickard' has explained the origin of 

 the gold in the auriferous sulphide reefs of Australasia and Colorado 

 as being due to the presence of organic matter. It would be 

 interesting in this connection to investigate the occurrence of a black 

 mineral, resembling and generally supposed to be graphite, in the 

 Eietfontein and Buffelsdoorn Mines. It has been suggested above 

 that the presence of pyrites in the slates is perhaps due to car- 

 bonaceous matter being present in the original mud from which they 

 were formed. The same mode of origin might be applied to the 

 pyrites in the banket, and would also account for the precipitation of 

 the gold in the metallic state.' I admit that it is difficult to under- 

 stand why other conglomerate beds (Kimberley Series, Bird Eeef 

 Series, etc.), which otherwise greatly resemble the auriferous 

 bankets, and also contain pyrites, should be poor in gold ; but the 

 conditions governing the circulation of underground waters, and the 

 nature of the precipitating agencies likely to have been contained in 

 the beds, are very little known to us, and there are possibilities of 

 infinite variation in these conditions. 



I have elsewhere ■' dwelt on the large amount of secondarily 

 deposited silica which the matrix of the conglomerates contains, 

 partly in the form of minute crystalline quartz, partly of 

 opaline nature. It is this secondary quartz which knits the 

 originally loosely compacted mass into a hard and almost glassy mass, 

 almost approaching vein quartz in texture, in which the margins of 

 the pebbles are almost obliterated, the rock breaking aci'oss matrix 

 and pebbles indiscriminately. This silica must have been deposited 

 from water, and is additional evidence in favour of hydrothermal 

 agencies as a factor in the origin of our gold-reefs. 



VII. — Notes on Two Points in African Geology. 

 By F. P. Mennell, F.G.S., Curator of the Rhodesia Museum, Bulawayo. 



ME. J. E. S. MOOEE'S interesting work on the "Tanganyika 

 Problem," reviewed on p. 418 of the September number of 

 the Geological Magazine, raises some interesting points in con- 

 nection with African geology, though, as your reviewer rightly 

 remarks, we are indebted to Mr. Moore rather for a statement of 

 the problem " than for proposing an adequate solution of it." At 

 the same time it must be admitted that Mr. Moore's general 

 geological conclusions appear to me, as a worker in Africa, much 

 more in accordance with the facts as I see them in the course of my 

 every-day life, than the hypotheses of certain eminent geologists 



' T. A. Rickard, "The Origin of the Gold-beariug Quartz of Bendigo Reefs": 

 Trans. Amer. Inst., vol. xxii (1893), pp. 314-315. "The Enterprise Mine, Rico, 

 'Colorado": Trans. Amer. Inst., vol. xxvi (1897), pp. 977-979. 



^ The possibility that water- worn pellets of iron pji-ites (referred to by Mr. Denny) 

 might also occur is not excluded ; yet it may be that these pellets are not water- worn, 

 but are of concretionary origin, in which case they would probably show a radial 

 fibrous structure. 



3 Quart. Jouru. Geol. Soc, vol. liv (1898), pp. 80 and 81. 



