[COLEMAN] SOUTH AFRICAN IRON FORMATIONS SI 



Mr. F. P. Mennell, of Bulowayo, describes the Banded Ironstone 

 series of southern Ehodesia as of Eparclia;an age, and says of it " the 

 characteristic feature of these beds is tiie peculiar banded flinty rock, 

 which appears under the microscope to be in all probability an altered 

 fine-grained mechanical sediment, silicified and highly charged with 

 ferruginous material, arranged in parallel bands. They alternate with 

 sheared conglomeratic and arenaceous beds, slates (phyllites) and 

 gneissic bands, which may result either from the crushing of acid in- 

 trusions or of tuffs. These beds are usually almost vertical to all appear- 

 ance, but this may be due to folding at right angles to their real direc- 

 tion, or to repetition over and over again by faulting; in any case it is 

 possible to travel over them for many miles in a direction at right angles 

 to the apparent strike." ^ He correlates them with the Griqua Town 

 series of Cape Colony and the Hospital Hill series of Johannesburg. 



Except for the immense width of the banded ironstones his descrip- 

 tion might apply Yerj well to many outcrops of the iron formation in 

 Canada, but his statement that the silica is altered from a fine grained 

 mechanical sediment seems doubtful in the light of American investi- 

 gations, which favour chemical sedimentation or deposit from solutions 

 obtained from basic eruptives. A thin section of a specimen from 

 Salisbury shows the silica as completely interlocking anhedra with no 

 hint of water rolled grains. In appearance it is exactly like a thin sec- 

 tion of similar iron range rock from near the Helen mine, Ontario. 



Banded ironstones are widely found in northern Cape Colony, 

 especially in the Prieska region, as described by Mr. A. W. Rogers of 

 the provincial survey. They occur mainly in the Griqua Town series 

 consisting " of peculiarly heavy green slaty rocks with quartzites and 

 jaspers containing large quantities of magnetite. Much of the rock 

 is banded, the thin layers having slightly different colours of which deep 

 red, bright red, brown and black are the most i^sual. The black layers 

 are almost entirely composed of minute crystals and grains of magnetite 

 with a little quartz between the grains, every intermediate stage between 

 almost pure magnetite and pure quartzite can be found." " The jaspers 

 are very fine grained rocks which break with a smooth conchoidal frac- 

 ture. They are made up of extremely minute crystalline particles of 

 quartz, and are coloured by oxides of iron of various degrees of 

 hydration."2 This account might be applied without change to our 

 American Iron Formation. 



Mr. Eogers has been good enough to send an interesting set of 

 specimens of these jaspery rocks, and also of the crocidolites found 



^ Geo!, of Southern Rhodesia, Rh. Mus., Special Rep., No. 2. 

 * Geology of Cape Colony, pp. 73-4. 



