89 



In the Malmetjbuiy grain soils, as in those of the Cape Division, the 

 ^ lack is phosphatic material, and potash is almost as urgently 



needed ; at the same time it must be said that, not only in proportion 

 to the lime present, but also absolutely, the aonount of phosphoric oxide 

 is higher in the Malmesbury than in the Cape Division, and seems to 

 continue increasing in amount as one travels northward from Tygerberg 

 to Zwartland. 



It is noteworthy that the surface soils which are more or less in- 

 fluenced by the underlying lime deposits such as those at Drooge Vlei, 

 Geelbeksfontein, Oostenwal, and Uitkomst in addition to containing 

 more lime than the other soils, are also richer in potash. The ultimate 

 origin of the comparatively large amount of potash in the soils of this 

 .class is a point of some interest which it is worth while elucidating: it 

 does not seem improbable that it is caused by the debris of granitic 

 rocks being mixed with the compacted sand; fro-m the blown sand itself 

 the potash coiild certainly not have been derived.* 



Out of 68 soils overlying the rocks of the Malmesbury series, col- 

 lected in the Cape and Malmesbury Divisions, no less than 16 were de- 

 ficient in all three inorganic plant food constituents in an available form; 

 as many as 45 are poor in phosphoric oxide; and of these latter five 

 also lack potash, and eight lime. Of the remaining 23 soils, eight were 

 deficient in lime only, three in potash only, and five in both lime and 

 potash. There were, therefore, only seven soils, of all those 63, that 

 were not lacking in respect of one or other of the three mineral fertilising 

 constituents, and even out of these seven, six were no better than fail 

 all round. 



MIDDELBURG. 



(Privately collected.) 



No. Field CornetcY. Farm or place. Collector. 



1. Nauwpoort. Carlton. Dist. Railway Engineer. 



Amongst the characteristic features of the Beaufort geological series, 

 the occurrence of bands of dark purple mudstones and shales may be 

 noticed by any traveller by rail in those parts of the country where the 

 rocks of the Beaufort series appear at the surface, for instance, between 

 Nauwpoort Junction and Ludlow on the Midland Railways. References 

 to these mudstones and shales may be found, inter alia, on pages 33 and 

 174 of the Geological Commission's Report for 1903, on pages 78 and 

 81 of the Commission's Report for 1904, and on page 98 of that for 1905. 



These purple shales are exposed in various parts of the railway cut- 

 tangs on the west of the railway line south of Carlton, and especially 

 so just at the 262nd milepost. They frequently appear to be of a cal- 

 careous nature, and to investigate this point, some of the weathered and 

 finely disintegrated shale was procured for analysis. As the ahale IB 

 found intermixed with limestone bands and layers of sandstone, no op- 

 portunity has been available of procuring any of this purple material in 

 a sufficiently decomposed condition to form an actual soil by itself, but 

 the above sample, taken from the cutting south of Carlton, represents 

 the weathered shale in as finely divided a condition as it was possible 



* A somewhat similar problem arises in connection with the soils of the farm Hooge- 

 kraal, Xos. 33 and 34 of the Cape Division soils (fide pages 40 and 43 of this volume). 

 The point will be reverted to later when discussing the bearing which the geological 

 relation? of ril? have upon the proportions of plant food which they contain : see Part V. 



