﻿Vpl., 66. J GEOLOSICA.L STRUCTURE OF SOUTHERN RHODESIA.. 361 



lime, arid farther east on Watson's Farm. At Concession Hill, 

 near Hartley, a grey limestone occurs in contact with the Banded 

 Ironstones, and the same is the case north of Belingwe. The dolo- 

 mites of the Graika Mine (Sebakwe) and of the Tebekwe Mine 

 (Selukwe) are both alteration-products of serpentine, the latter 

 retaining the microscopic structure of an olivine-derived serpentine 

 in a most remarkable manner. In the Gwanda district a dark-blue 

 to grey limestone with chert-bands occurs south of the Antenor 

 mine, and runs in the direction of the Colleen Bawn Mine about 

 12 miles away, where it is well exposed. East of the Colleen 

 Bawn, where the limestone is about 150 feet thiek, it is invaded 

 by the granite and is sometimes very coarsely crystalline, besides 

 snowing development of hornblende, mica, chlorite, scapolite, 

 sphene, magnetite, etc. Indeed, the band of amphibolite between it 

 and the normal granite is no doubt a reaction-product of the two 

 rocks. Near the Bucks Reef Company's battery, the same lime- 

 stone, which has here taken a sharp turn to the south, is converted 

 into a granular aggregate of green augite, apparently by an ordinary 

 dolerite dyke, although the Pandangwe granite mass is not far off. 

 At the Colleen Bawn Mine itself, graphite occurs in quantity, a 

 feature also noted at the Bushman Mine in Bechuanaland, and 

 at a granite-limestone contact, not far from the Rhodesian border. 

 This seems clearly due to the insolubility of carbonaceous matter 

 in a highly siliceous magma, so that in the process of absorption 

 by an invading granite the organic matter would tend to accumulate 

 along the margin of the granite in the form of graphite. This 

 process would also account for the streaks of graphite met with 

 in a marginal modification of the granite of the Victoria district. 



About 12 miles south-east of Bulawayo, where the rocks have an 

 approximately north-and-south strike, there are several exposures 

 of limestone, apparently a single band repeated several times by 

 folding and faulting. Thus, proceeding from Claremont Farm across 

 the Tuli Road, and then on past the Talisman Mine, one meets 

 successively from east to west : limestone, banded ironstone, con- 

 glomerate, limestone, conglomerate, schists, banded ironstone, 

 limestone, and, finally, a great intrusion of hornblende-porphyrite. 

 It is easy to draw sections to account for such a sequence in various 

 ways, but to interpret the true relations of the rocks is quite another 

 matter. Dips afford no assistance, as they are always vertical, or 

 nearly so : that is, to all appearance ; although, if mining opera- 

 tions necessitate following down a particular band of rock, its 

 true dip is rarely found to be so steep. Thus hundreds of feet of 

 sinking have shown the inclination of the Banded Ironstones at 

 the Antelope Mine to be about 60° northwards. In many cases the 

 apparent dip is not bedding, or even foliation, but merely a close 

 jointing allied to cleavage. 



To sum up regarding the relations of the various limestones, it 

 may be said that the Sinoia Series seems unquestionably younger 

 than the Conglomerates; whereas the other occurrences may in- 

 dicate quite different ages, and certainly seem at times to point to 

 close association with the Banded Ironstones. 



Q. J. G. S. No. 263. 2 c 



