﻿372 ME. FBEDEEIC P. MENNELL ON THE [-A-Ug. I9IO r 



about 4 miles wide as seen at Lomagundi, Selukwe, and Belingwe. 

 It is almost certainly a gently inclined sheet injected along a line 

 of weakness which is probably a thrust-plane, and it can be traced 

 from the TJmvukwe Hills on the east of the Ayrshire Mine in 

 Northern Mashonaland, right away to the Umsingwane Eiver at a 

 point about 30 miles south of the West Mcholson Mine in Southern 

 Matabeleland. It is predominantly an enstatite-rock, with only 

 a very little felspar in places ; but there are more acid varieties, as 

 well as others rich in olivine and merging into almost pure olivine- 

 rock. These specially basic types are usually more or less serpen- 

 tinized, and often associated with chromite. 



Probably allied in age and origin with this rock are the gabbros- 

 and the great dykes of ophitic dolerite found in many parts of 

 the country, especially in its central portion. The gabbros usually 

 contain enstatite, but not olivine, while the dolerites often contain 

 the latter, besides merging into varieties rich in micropegmatite and 

 therefore comparatively acid. Some of these are very like the well- 

 known rock of Penmaenmawr. Certain intrusions ofgranophyre, 

 such as the large boss at Gwelo Police Camp, are possibly to be 

 classed with these rocks, although they may be offshoots from the 

 granite masses. Very interesting in this connexion, and as bearing 

 on certain theoretical problems, are the associations of dolerite with 

 granophyre which has evidently been produced by the melting or 

 injection of granite into which the dolerite is intrusive. Splendid 

 examples of this may be seen near the Antelope Road, about 3 

 miles from the Matopo Station, and on a larger scale north of 

 Kahlele's old kraal on the road from Fort Usher to Gwanda. There 

 is every gradation between a typical granophyre and an obvious 

 ' mixed rock ' composed of phenocrysts or rather xenoliths of 

 granitic quartz and felspar embedded in a doleritic ground-mass. 

 In a similar way, a doleritic dyke near the railway-station at 

 Bulawayo, which contains apparently original quartz, is seen at 

 other points along its course to contain countless large and small 

 xenoliths of half-melted quartz derived from a reef cropping out near 

 by and dipping towards the dyke. 



The intrusions associated with the Forest Sandstone basalts are 

 of similar composition, although they can usually be distinguished 

 microscopically from the lavas. They often show, like the latter, a 

 glomeroporphyritic structure, and are never ophitic. I only know 

 of two that show olivine, one from the Deka River (Wankie district), 

 and another from Yan Wyk's Vlei (Tuli district) : the latter has con- 

 spicuous phenocrysts of olivine. 



It only remains to mention the diamond-bearing rocks, of 

 which Rhodesia, like the rest of South Africa, can now boast 

 examples. Two occurrences of 'blue ground,'" or * kimberlite,' are 

 known, about 5 miles apart, between the Bembezi and the lnkwekwezi 

 Rivers, 35 to 40 miles north-east of Bulawayo. One seems to fill a 

 large volcanic vent or ' pipe ' of the usual type ; the other was not 

 sufficiently opened up at the time when I saw it for one to be able 



