Geology and Natural History. 471 



developed in the massive beds of the Galena dolomite and they 

 play an important role in the deposition of the ores. 



The ore minerals are chiefly galena, sphalerite and smithsonite, 

 with which are associated marcasite and calcite. The ore deposits 

 may be grouped into two divisions according to their form : 

 first, those which occur in crevices, especially in the joints in 

 the rocks and which are in the nature of vein deposits ; second, 

 those which are disseminated in small jDarticles through certain 

 beds of the rock and which are in the nature of replacement 

 deposits. The ores have undergone the usual alteration through 

 the agency of descending surface waters and present three dis- 

 tinct mineral zones. At the top there is a zone containing large 

 masses of galena. Below this is a zone in which smithsonite is 

 the important ore. This last zone ends at about the level of 

 ground water and is succeeded below by a zone in which sphaler- 

 ite is the dominant mineral. The ore materials were derived 

 from the sedimentary rocks themselves, in which they existed in 

 a disseminated condition as original constituents, by the action of 

 underground waters, which dissolved them out from the mass of 

 the rock and later redeposited them in their present positions. 

 The ore deposits, at least in the majority of cases, were found 

 to lie in the synclinal folds of the rocks. 



The atlas accompanying the report contains IS topographic 

 and geological maps of the area. They are drawn on a large 

 scale and the formations are mapped on them in detail, w. e. e. 



9. The Diamond Pipes and Fissures of South Africa • by 

 H. S. Hakger. Trans. G-eol. Soc. South Africa, vol. viii, 1905, 

 pp. 110-134. — The author shows that in addition to the well- 

 known volcanic necks which have been exploited for diamonds, 

 great numbers of others existrin Central South Africa. They are 

 found penetrating the beds of the Karroo System. Some are 

 filled with basaltic rock and gave rise to overflows of lava, while 

 others are filled with fragmental material, contain the kimberlite 

 in which the diamonds occur and appear as the result of explosive 

 action. The location and characters of a number of these are 

 given, their minerals described and their origin discussed. 



The following analyses of the altered kimberlite, called "blue 

 ground " when soft, and " hardibank " when hard, from the Vogel- 

 fontein Mine are of interest : 





Si0 2 



A1 2 3 



Fe s O s 



FeO 



MgO 



CaO 



Na 2 



K 2 



I 



38-08 



2-46 



24-48 



2-59 



12-88 



4-14 



1-12 



0-84 



II 



33-42 



0-94 



23-84 



3-52 



10-80 



9-84 



0-97 



0-86 



III 



36'57 



5-09 



13-75 



4-68 



11-85 



8-49 



2-55 



0-64 





Ign. 



H 2 O 100 ° 



C0 2 



P 2 5 



Cr 2 3 



MnO 



Ti0 2 



S Total 



I 



10-14 



0-25 



1-67 



0-67 



tr. 



tr. 



tr. 



tr.= 99-32 



II 



7-83 



0-43 



5-88 



0-89 



tr. 



o-o 



o-o 



tr,= 99-22 



III 



6'31 



5-47 



4-61 



0-58 



tr. 







o-o 



tr. = 100-59 



I 



Hard 



, dark hardibank. 











II 



Soft, 



light h; 



irdibank. 











III 



Soft, 



blue ground. 













