ALKALI ROCKS IN THE TRANSVAAL 743 



Matlabas and to the north of the Palala Plateau, where it extends 

 to the north to near the Limpopo, nearly 30 km. to the south of the 

 northern boundary of the Transvaal. 1 



BOTTOM OF THE LACCOLITH 



The bottom of the laccolith is formed everywhere by the upper 

 strata of the Pretoria series, generally consisting of Magaliesberg 

 quartzite. 



Therefore the rocks of the laccolith in the Central Transvaal 

 are for the greater part surrounded by the upper Pretoria strata; 

 in the strata of the Transvaal system, which dip everywhere 

 toward the central part of the complex, the dip of the strata de- 

 creases when the distance from the complex increases. Because 

 the Magaliesberg quartzites of the Pretoria series have specially 

 resisted erosion we now see them as a ridge surrounding the complex. 



We see that the bottom of the laccolith is determined, but the 

 the place of the roof of the laccolith is uncertain. 



ROOF OF THE LACCOLITH 



The part of the laccolith, which has not been uncovered by 

 denudation, is covered by the strata of the Waterberg system and 

 partly by the younger strata of the Karroo system. Between the 

 basal conglomerate of the sandstone series of the Waterberg 

 system and the underlying rocks of the Bushveld complex we some- 

 times find a series of felsitic rocks, which other authors have con- 

 sidered as being directly connected with the deep-seated rocks of 

 the laccolith, but which the Geological Survey of the Transvaal 

 regards as a lower division of the Waterberg system. 



The Waterberg system then includes: 



Waterberg system 



Upper j Sandstones, grits, and 



Division \ conglomerates 

 Lower fFelsites and allied volcanic 



Division < rocks with interbedded 



(Volcanic series) shales 



1 G. A. F. Molengraaff , " Geologische Aufnahme der Siid-Afrikanischen Repub- 

 lik," J ahresbericht iiber das Jahr i8g8, Pretoria, 1900; G. G. Holmes, "Some Notes 

 on the Geology of the Northern Transvaal," Trans. Geol. Soc. South Africa, VII (1904), 

 S1-S6. 



