TERTIARY AND RECENT DEPOSITS 385 



in any of the pans yet examined in Gordonia, though 

 they are a marked feature in Heuning Vlei in 

 Bechuanaland and Waterpan in Vryburg. No explana- 

 tion of this peculiar distribution has been offered. 



These northern siliceous rocks fall in two distinct 

 classes, 1 those which consist of grains of quartz and 

 other minerals cemented by a siliceous matrix without 

 evidence of the latter having replaced calcareous mat- 

 ter, and those siliceous rocks which bear evidence of hav- 

 ing been originally calcareous, the whole or part of the 

 calcareous matter having been replaced by some form 

 of silica. The latter class of rock is by far the more 

 abundant of the two in the north of the Colony, and 

 as they always occur in intimate association with the 

 surface limestones they will be described with the latter. 



To the directly silicified rocks belong the outcrops on 

 the eastern slope of the Langebergen at Paarde and 

 Koodeman's Kloofs, the quartzites along the Genesa 

 Laagte,' 2 and some of those along the Molopo in Mafe- 

 king. 3 



3. THE NEWER GRAVELS AND ALLUVIAL DEPOSITS. 



At various levels between the high level deposits just 

 described and the beds of the present rivers in the 

 southern, eastern and western parts of the Colony there 

 are more or less well-marked terraces covered with 

 gravels and alluvium. Several such terraces can be 



1 These coincide with the two broad divisions of " eingekieselte " and 

 " verkieselte Gesteine " of Prof. S. Passarge, in Die Kalahari, 1904, a 

 very important book dealing chiefly with the geology of the Central and 

 Northern Kalahari. 



2 G. C , xii., p. 109. 3 Ibid., p. 149. 



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