THE GEOLOGICAL HISTORY OF THE COLONY 425 



tended over that wide area. Should the opinion of 

 Profs. Schenck and Passarge, that the dolomites of 

 Namaland and the N'Gami country belong to this group, 

 be correct, we shall have evidence of a still greater ex- 

 tension of this sea. 



The Campbell Kand limestones are succeeded con- 

 formably by the Griqua Town series, the peculiar 

 characters of which certainly indicate a complete change 

 in the conditions of sedimentation, though it is uncertain 

 how the change was brought about and what the new 

 conditions were. Their effect was to cause the deposi- 

 tion of thin layers of iron compounds and silica. The 

 thin but extensive layers imply deposition in quiet 

 water. Chemical changes took place subsequently, and 

 the resulting thin banded ferruginous jaspers and mag- 

 netic cherts are characteristic of the series in Cape 

 Colony ; the correlated Pretoria series in the Transvaal 

 is mainly an arenaceous formation, but only a small 

 part of the Griqua Town series in Cape Colony is of 

 that nature. In the Griqua Town series two other 

 events of great importance are recorded ; first the pre- 

 valence of glacial conditions, evidence of which has been 

 found in many places between the Mashowing in lat. 

 26 and the neighbourhood of the Orange Kiver in 

 lat. 29| ; the second event was the recurrence of vol- 

 canic activity in Bechuanaland, Griqualand West, 

 Prieska and the Transvaal. The lavas are andesitic 

 in character and differ much from those of the Pniel 

 and Zoetlief series. The lavas of the Ongeluk group 

 are followed, apparently conformably, by strata of very 

 much the same kind as those below the glacial beds, 



