THE INTRUSIVE DOLERITES AND ALLIED ROCKS 273 



bert, Hopetown, Britstown and Carnarvon, are appar- 

 ently portions of a single sheet which has been intruded 

 in the Dwyka series at a horizon very close to its upper 

 limit. Those peculiar ring-shaped intrusions described 

 from Queenstown are represented in the north also. De 

 Aar is situated just outside one of these, the railway to 

 Prieska passing right across it ; another one occurs at 

 Deelfontein ; a third is at Ercildoune, close to Britstown, 

 and is interesting because a pan is found occupying the 

 centre of the depression. 



As a rule the dolerites are remarkably uniform in 

 composition ; a number of analyses were made by Prof. 

 Cohen, 1 of which the following may be taken as typical : 



Silica, 52'7 ; alumina, 11 '4; ferric oxide, 9'0; ferrous 

 oxide, 3'7 ; lime, 11'6; magnesia, 7'4; potash, 0'7 ; 

 soda, 2'3 ; water, 1-4 ; total, 100'2. The average specific 

 gravity of the dolerite is very nearly 3'0. 



The constituents are plagioclase felspar, augite, 

 olivine, and magnetite, in the relative order of their 

 abundance ; but olivine is not infrequently absent. In 

 addition, biotite may be present and sometimes original 

 hornblende, either independently or in close connec- 

 tion with the augite. The pyroxene is in some varieties 

 more or less altered to diallage, while the conversion 

 of the olivine to greenish serpentine is a common 

 enough feature. 



In the more acid types quartz and occasionally ortho- 

 clase felspar are present, often in the form of micro- 

 pegmatite. 



1 E. Cohen, " Geognostisch-petrographische Skizzen aus Siul-Afrika," 

 N. J.fitr Min., etc., 1887, Beil., Bd. 5, p. 195. 



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