352 Journal of the Department of Agriculture. 



Dunn's map would therefore, excepting the dolerite- sheets, faith- 

 tuUy reflect the general geological features of the Witkop area. Dunn 

 would, for instance, appear to have mapped as volcanic beds a dolerite 

 sheet which occurs on the high ground in the neighbourhood of Yaal- 

 bank and Dankf ontein ; but since the chemical composition of the 

 volanic beds and dolerite seem to be very similar no great error will 

 be made for soil purposes if Dunn's map be accepted as correct. It 

 is, however, very probable that the volcanic beds weather more readily 

 than dolerite. 



Reference to the geological map (Plate III) shows that the strata 

 of the Witkop area belong to the Stormberg series of the Kan'oo 

 system of rocks, together with intrusive dykes and sheets of dolerite. 



Proceeding downwards, the Stormberg series consists of the 

 following groups : — 



4. Volcanic beds. 



3. Cave sandstone. 



2. Red beds. 



1. Molteno ])eds. 



Volcanic Beds. — The volcanic lavas " are all basaltic in com- 

 position, varying from coarse-grained doleritic types to highly vesi- 

 cular varieties, while there is a considerable development of volcanic 

 ash. ... At the boundary of Dankf ontein with Vaalbank the 

 cave sandstone is overlain by nearly 300 feet of volcanic ash, followed 

 by an almost similar thickness of lavas."* 



The actual chemical composition of these lavas would not appear 

 to be known. They contain, however, the following minerals: — 



Lime-soda Felspars. 



Augite. 1 



Olivine. iSilicates of lime and magnesia with more or less iron. 



Enstatite.J 



Magnetite. 



Apatite — Calcium phosphate with calcium fluoride or rliloride. 



Agates. 



Calcite — Calcium carbonate. 



Lime-soda zeolites. 



(The last three are found filling the steamholes of the amygda- 

 loidal lavas.) 



The petrological descriptions of the volcanic beds afford no indica- 

 tion as to the amount of potash they contain, but probably they 

 contain a similar quantity to dolerite, i.e. about 0.7 per cent. 



Soils derived from these beds would be somewhat clayey, but well 

 provided with pho^sphoric oxide, potash, lime, and magnesia. The 

 abundance of these food elements will have led, in the course of time, 

 to an abundant fixation of nitrogen by the soil. Soils derived from 

 the volcanic beds should therefore be of high fertility, provided 

 always that, in addition to food supply, the other factors necessary 

 for plant growth are normally present. As a matter of fact the 

 farmers of the area recognize the soils derived from the volcanic beds 

 as among' their best. 



Cave Sandstone. — The cave sandstone is a massive fine-grained 

 uniform rock, probably an aeolian deposit,! which consists very 



* A. L. du Toit, Geological Commission, 16th Annual Report. 

 t ^- ^- du Toit, in a private communication. 



