742 H. A. BROUWER 



I am much indebted to Professor G. A. F. Molengraaff, of Delft, 

 who placed his collection of rocks at my disposal, while the text 

 has been much improved by his valuable suggestions; and also to 

 Professor A. Lacroix, of Paris, the rock material being studied for 

 the greater part in his laboratory. 



For assistance during my stay in South Africa, my best thanks 

 are due to the staff of the Geological Survey, particularly to Mr. 

 A. L. Hall, who furnished some rock specimens from Rietfontein 

 [451] and Spitskop [463]. I ( 



As a guide to the stratigraphy and the geological dates men- 

 tioned below, a table of the Transvaal formations is entered (see 

 Table I). 



IGNEOUS COMPLEX OF THE BUSHVELD 



The nepheline syenites of the Transvaal make part of a complex 

 of igneous rocks, which is intrusive as a laccolith or laccolithic 

 sheet between the upper strata of the Transvaal system and 

 the strata of the unconformably overlying Waterberg system. 



The laccolithic character of this complex was recognized by 

 Molengraaff, 2 who grouped them under the name '"plutonic series 

 of the Bushveld"; 3 it includes igneous rocks, which have a high 

 soda content as a common character. The name used by other 

 authors, "igneous complex of the Bushveld" or " Bushveld 

 igneous complex, " has the same signification and can be considered 

 as the official one, because the Geological Survey of the Transvaal has 

 adopted it. 



PLACE OF THE MAIN TRANSVAAL LACCOLITH 



The western boundary of the part of the complex in the Central 

 Transvaal, which has been uncovered by erosion, is found nearly 

 15 km. to the west of the Marico River; the eastern one, 

 nearly 25 km. to the west of Lydenburg; the medium breadth is 

 nearly 100 km. 



But probably it covers a much larger area; at least it appears 

 again between the Magalakwin River and the sources of the 



1 Throughout this paper numbers in brackets refer to official designations for 

 farms in South Africa. 



2 G. A. F. Molengraaff, "Geologie de la Republique Sud-Africaine du Transvaal," 

 Bull, de la Soc. Geol. de France, Serie 4, T. I, 1901, p. 13. 



3 G. A. F. Molengraaff, Geology of the Transvaal (Johannesburg, 1904), p. 42. 



