president's address. 9 



of about the same age as the Mio-Pliocene beds of Bogenfels. 

 He looks upon them as the deposits formed under arid conditions 

 upon an early Tertiary land-surface on which erosion channels 

 had been formed. That the climate had been wetter before their 

 deposition he considers proved by numerous solution holes and 

 channels in the underlying dolomite which were filled in with 

 material forming part of the Pomona beds, and also, perhaps, 

 by the deeply weathered state of schists and quartzites under 

 those beds, but he utters a warning against accepting deep 

 weathering as proof of greater rainfall. 



The quartzites of the Pomona beds are very like the surface 

 quartzites of the western and southern districts of the Cape 

 Province, but hitherto the only organic remains found in the 

 latter"^ are obscure impressions of plants that have not been 

 determined. Where the underlying I'ock is seen, it, like the 

 schists mentioned by Professor Kaiser, is deeply weathered; 

 numerous instances of this are to be seen in Caledon, Swellen- 

 dam and Bredasdorp. The conditions under which those 

 quartzites were formed have not been satisfactorily explained. 

 Kalkowski's suggestions^ that the siliceous cement was derived 

 from siliceous grass and diatoms living in pans, has not been 

 substantiated by the discovery of their remains in the typical 

 quartzites, nor by the form of many of the quartzite deposits. ^^ 

 The residual clays immediately below the quartzites might be 

 suspected to contain free hydrated alumina instead of hydrous 

 silicates, in fact to be laterites, but this has not been proved. 

 Laterite containing free alumina is believed to be peculiar to hot 

 climates with marked wet and dry seasons; therefore, so far as 

 it goes, the failure to find laterite under the quartzites is evidence 

 against the existence of such a climate at the time of their 

 formation. It is difficult to surmise a source of the siliceous 

 cement elsewhere than in the underlying rock, for there is no 

 evidence of siliceous spring water having supplied it; the wide 

 distribution of the rocks, as well as the absence of sinter, puts 

 that explanation out of court. The process is probably analogous 

 to that by which tufaceous limestone is formed ; it is perhaps an 

 extreme example of the frequently -observed hardened crust on 

 rocks which are quartzites in depth, become comparatively soft 

 above ground-water level and again intensely hard on the out- 

 crop, though the chemical process by which the transfer of silica 

 is effected does not appear to be known. The bearing of these 

 large bodies of siliceous rocks on the climatic conditions at the 

 time of their formation is not clear. Passarge,-^ who investigated 

 many occurrences in the Protectorate, seems to refer them to 

 periods of increased moisture following aridity, and the fact 

 that they are so abimdant in the south and west of the Cape 

 and in the Namib. where, though the rainfall may be low, there 

 is heavy dew, while they are less frequent in the Great Karroo 

 and apparently unknown in the Upper Karroo, indicates _ a 

 dependence on recurrent alternation of arid and somewhat moist 

 conditions. Kaiser attributes the abundance and variety of 



