﻿Vol. 66.~] GEOLOGICAL STRUCTURE OF SOUTHERN RHODESTA. 375 



makes up the characteristic concentrates of this and all other 

 diamond pipes. It is of a deep green when fresh, and sometimes 

 shows a well-marked parting parallel to {100}. This becomes so 

 pronounced on weathering that the mineral might easily be 

 mistaken for a mica. 



Analysis of augite (chrome-diopside). Colossus Mine. 

 IV. 



Si0 2 5393 



AU0 3 190 



Cr 2 0, 0-70 



Fe 2 3 597 



FeO 267 



CaO 13-11 



MgO 20-08 



H 2 1-63 



Total 99-9 9 



The mineral is, therefore, a chrome-diopside, showing an excep- 

 tional freedom from alumina, and remarkable, moreover, for having 

 magnesia, not merely in excess, but in large excess of the lime. I 

 have to thank Dr. A. Hutchinson, F.Gr.S., for very kindly selecting 

 material and arranging for the analysis of this and the previous 

 specimens. 



EXPLANATION OF PLATE XXVIII. 



Geological map of part of Ehodesia, on the scale of 14£ miles to the inch. 



[Acknowledgments are due to the Government of Southern Rhodesia for 

 permitting the use of their farm map as a basis for the topography.] 



