TERTIARY AND RECENT DEPOSITS 415 



two formations, e.g., along the boundary of the Black 

 Eeef and the Campbell Band series in Vryburg ; at the 

 top of the latter formation at Heuning Vlei ; and along- 

 side a dolerite intrusion as at Strydenburg. Several 

 pans in Gordonia are surrounded by sand and no rock 

 is exposed within them. As far as can be judged the 

 erosive action of the prevailing wind is the most potent 

 factor in pan formation. Passarge 1 has, however, sug- 

 gested that pans may have been deepened or even per- 

 haps wholly formed through the agency of the vast 

 herds of wild animals which not so long ago used to 

 exist in Southern Africa. Mud was removed adhering 

 to their bodies when they had finished drinking and 

 wallowing, and at the same time substances contained 

 in solution, chiefly carbonate of lime, were removed. 

 The rainfall and sub-surface water gravitate into these 

 depressions and there evaporate, so that in time there is 

 a concentration within the pan of substances dissolved 

 out of the surrounding soil and rock. Hence in the 

 wet season the water in a great many pans is brackish 

 or even salt, while in the dry season the floor of such 

 a pan may be covered with a white saline incrustation. 

 Such salt-pans are quite common in the Colony, and 

 many of them have yielded vast quantities of salt with- 

 out showing any signs of exhaustion. Kautenbach's 

 salt-pan in Gordonia in three miles long and nearly a 

 mile wide, and the incrustation of salt forms a solid cake 

 from one to two inches thick lying on black sandy 

 mud. 



A most remarkable feature is that very often round 



1 Die Kalahari, chap. xvii. 



