198 president's address — section c. 



ftlong certain flow lines; recrystallization, probably due to secondaiy 

 silicilication, lias resulted in the formation of more coarsely crystal- 

 line aggregates of quai-tz and chalcedony with secondary felspar 

 sometimes showing. Another rhyolite from L. Karng shows no band- 

 ing but a striking perlitic structure. The ground mass was originally 

 glassy, but is now cryptocrystalline to microcrystalline. No fluidal 

 stiiicture can be seen, Ijut the jilienocrysts are corroded and partly 

 replaced by calcite. Tlie other type which I have not seen Thiele 

 refers to the typical quartz-porphyries, althouglx he says that the 

 giound mass in some sections resembles that of a pyroclastic rock. 



Analyses by E. O. Thikt.e, B.Sc. 



I. II. 



SiO, ... ... 78-(i4 ... 78-47 



AloOg - ... 0-85 ... 10-6& 



Fe„03 ... ... 0-£4 ... * 0-18 



FeO ... ... 2-00 ... 2-23 



MgO ... ... 0-10 ... tr. 



CaO ... ... 0-80 ... 0-66 



Na.,0 ... ... 203 ... 3-2i) 



K„0 ... ... b-W ... 4-15 



TiO„ ... . . 0-67 ... 0-5J* 



P2O5 ... ... tr. ... tr. 



H„0(comb.) ... 0-40 ... 0-20 



H2O (Hygroso.) ... 014 ... 00** 



Total ... ... 100 -SB ... 100-54 



I. Banded Rhyolite, southern plateau of Mount Wellington. 

 II. Quartz-jorphyrj , southern shore of L. Karng, Mount Wellington. 



TliE Basic Series (Melaphykes or Basalts). 



Interbedded witli the rhyulites and the Palaeozoic sediments of 

 the Mount Wellington area are a series of basic rocks described by 

 Howitt (46) as nielaphyres or ancient basalts. Using the former term 

 in the sense of an altered basalt it will be convenient to retain this 

 name for the series, as it will serve to distinguish them from the older 

 basalts of Kainozoic age. 



In the field they occur as thin flows, quite subordinate in import- 

 ance to the rhyolites, and are dense, black, and frequently much 

 altered. A section of a rock collected by Thiele from the Moroka Snow 

 Plain consists of a basic plagioclase, light-brown augite ophitically 

 enclosing the felspar, abundant opaque minerals mainly ilmenite, 

 and a considerable amount of chlorite possibly secondai-y after olivine. 

 The rock is considerably altered. Thiele states (92) tliat epidote 

 and calcite are common alteration products, and that amygdaloidal 

 varieties have the cavities filleil with chalcedony. The accompanying 

 analysis is by G. Ampt from the above rock. 



The Western District area. — No analysis of the acid rocks of this 

 district are available, but Dennant (63) in his paper states that the 

 silica percentage of a quartz-porphyry from this district is 74"7. 



