56 THE KHEIS SERIES 



The Mary dale beds. 1 



These beds include lavas, tuffs, quartzites, arkose, 

 limestone, and schists of various kinds. They are found 

 dipping under the Kaaien beds between Brul Pan 

 and Stuurmans Puts in Prieska, and on this band the 

 village of Marydale stands. At Blaauw Puts again they 

 occupy a similar position, and here there is a remarkable 

 development of limestone and chert interbedded with 

 altered lavas. Elsewhere they form belts surrounded 

 by gneiss and lying parallel to the foliation planes of 

 that rock ; veins of gneiss and granite are abundant in 

 these isolated masses. 



The most striking features in the sub-group are the 

 intensity of the metamorphic effects in the lavas and the 

 presence of quartzites containing much magnetite ; the 

 latter rocks form prominent ridges and kopjes in the 

 Prieska Division between the Doornbergen and the 

 Kaaien hills, particularly the Groot Modder Fontein 

 ridge and the Zwart Kops. These magnetic quartzites 

 contain peculiar minerals of the amphibole group, but 

 crocidolite and its allies have not been found in them. 



The lavas are of various composition from altered 

 rhyolites to rocks probably at least as basic as olivine- 

 basalts. The acid rocks have not suffered very great 

 change, but the basic varieties, in some of which rem- 

 nants of the original structure are visible, form a series 

 from epidiorites to hornblende-schists and garnetiferous 

 hornblende-granulites. Amygdaloidal structure is oc- 

 casionally seen in all these rocks ; in the more basic 



1 Q. 



