CHAPTEE IX. 



THE INTKUSIVE DOLERITES AND ALLIED BOCKS. 



THE dark-coloured heavy rock, blue- black when freshly 

 broken, and red, brown, black or yellow on weathered 

 surfaces, that occupies such great tracts of country in 

 Central Cape Colony, is popularly known by the name of 

 " yzer-klip " or ironstone to the people who live near it. 

 It probably got the name from the property it has of 

 ringing like a piece of metal when struck. 



The mineral composition shows that the rock belongs 

 to the basic group of igneous rocks, and may be termed 

 a dolerite. In this book, as in the Reports of the Geo- 

 logical Commission, this name is used in the sense 

 adopted by Allport l and Teall, 2 including rocks composed 

 chiefly of plagioclase and augite. The composition of 

 the South African rocks varies considerably, and in very 

 many localities rocks with obviously different composi- 

 tions can be seen to belong to one and the same mass. 

 The intrusions, however, as a whole can conveniently 

 be termed dolerites. 



The dolerites have all consolidated at some distance 

 below the surface of the earth and can be seen only on 

 the removal of the overlying rocks by denudation. 



1 Q. J. G. S., xxx., p. 529. 2 British Petrography, ch. vii. 



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