TERTIARY AND RECENT DEPOSITS 403 



pure limestone that is so widely spread between the 

 main watershed of the Colony and the Orange Eiver 

 has chiefly been derived. Every heavy rain that carries 

 the products of decomposition from their place of origin 

 to the flat ground, and especially to the shallow pans, 

 brings with it some carbonate of lime which it leaves 

 behind on evaporation. To this source must be added 

 the slow creep of water towards the surface by capillary 

 attraction and the influence of plants. 



In the country north of the Orange Kiver thick layers 

 of limestone have been formed just underneath the sur- 

 face. 1 They are best developed in flat country traversed 

 by rivers draining the areas occupied by the Campbell 

 Rand series, but they are not confined to such country. 

 An interesting feature in connection with these lime- 

 stones is that the carbonate of lime is replaced locally 

 by silica in the form of opal and agate. The distribu- 

 tion of the silicified rock is peculiar ; it has not been 

 noticed on the Kaap Plateau, in the valleys running 

 southwards from the Maremane anticline, on the Kuru- 

 man River below Tsenin, on the Molopo (Hygap) below 

 its junction with the Nossob, along the western side of 

 Gprdonia, nor in any part of the country south of the 

 Orange River ; on the other hand the process has gone 

 on between Vryburg and Tsenin and along the Molopo 

 at least as far east as Kuis, and Dr. Passarge 2 has 

 proved that silicified limestones of recent or late Tertiary 

 age are found over an immense area in the Northern 

 Kalahari. 



1 G. C., x., pp. 202, 255 ; xi., pp. 69, 78, 126 ; xii., pp. 96, 187. 



2 Die Kalahari, 1904. 



26* 



