MAGXESIA IMPHKGXATKJ) SOILS. 175 



In reporting' upon tlie geological characters of the 

 specimens submitted to analysis, the late Mr. A. E. V. 

 Zealley* stated as follows : — 



No. 432. — The rock is an altered enstatitite. 



No. 795. — A medium grained oliTo-green rock, made up 

 of enstatite and felspar with a trace of mono-clinic 

 pyroxene. The rock is better classed with fels- 

 pathic enstatitites. The felspar probably is about 

 15 per cent, of the rock. 



No. 1022. — A medium grained felspar-rich norite, con- 

 sisting- of about tM'o-thirds by volume of labradorite- 

 bytownite and one-third diopside and enstatite 

 (together). 



It will naturally be expected that norite soils, on account 

 of their higher content of lime, will be more productive than 

 those derived from serpentine or enstatitite, and this has 

 undoubtedly i)roved to be the case. 



In classifying the soils of the Great Dyke according to 

 geological origin, one is at a disadvantage, as the Dyke 

 formation has so far only been systematicallj' surveyed in 

 one region, viz., the Selukwe Mineral Belt. From the report 

 of this sum^ey, which was published last year by the Southern 

 Khodesia (jeological Survey Department, it is seen that tlie 

 central portion of the Dj-ke in that belt is represented by 

 felspar-rich norite and the two sides by enstatitite, con- 

 sequently, if this arrangement prevails generally, it will 

 follow that the more productive soils of the Dyke Avill most 

 probably be located in the middle of the intrusion. 



As typical examples of the general chemical characters 

 of the more productive and the less productive soils of the 

 Great Dyke, the following cases are cited: — 



CoMrosiTioN oi^ Soils Occurking ox the Gkeat I^yke. 



Analysis of air-dried fine earth (portion passing through 

 3 mm. sieve). Samples treated with hydrochloric acid 

 (sp. gr. 1'12) and digested for fortv-eight hours on a water* 

 bath:— 



(uj LIBRARY U^j 



It- 



* " The Geology of the Selukwe Mineral Belt." Southern Rhodesia 

 Geological Survey, ]3ul. 3. 



