27 



to decomposition, the whole was measured as felspar and the 

 relative proportions computed from the mode given above. 



Quartz 36'9 



Microcline ... ... ... ... 52'1 



Albite lO'l 



Muscovite ... ... ... ... *2 



Iron ore ... ... ... ... '7 



Th€re is thus a satisfactory agreement between the 

 calculated and measured constitutions. 



(6) MINOR POTASH APLITES, ETC. 



These are the rocks occurring as pipes and narrow. veins 

 through the granite. Most of the examples to be described 

 were collected at Port Elliot. 



Mineralogically the rocks exhibit a general similarity, 

 consisting mainly of quartz and felspars, microcline predomin- 

 ating. There is usually subordinate biotite present, but one 

 dyke (No. 1) cutting through the porphyritic and even- 

 grained o^ranites near the Victor Harbour end of Port Elliot 

 is quite free from biotite and resembles very closely the red 

 aplite. It differs in being a little finer grained and in having 

 a larger proportion of albite among its felspar. 



A partial analysis is as follows: — ■ 



SiO, 76-54 



Na.b 3-02 



K.O 6-05 



and as the albite is practically pure this would imply a 

 probable approximate percentage composition by weight 

 of : — 



Quartz Se'O 



Microcline ... ... ... ... 5r0 



Albite 13-0 



The specific gravity is 2*607, which reflects the increase 

 in soda felspar as compared with the red aplite. It seems 

 quite probable indeed that these two rock-types are closely 

 connected genetically. 



Some distance round from this dyke towards Middleton 

 Beach there is another dyke (No. 2) which is interesting in 

 that it contains very little albite and that it is characterized 

 by the presence of a fair amount of micrographic fabric, in 

 contrast to the allotriomorphic fabric of the other aplites. 

 When intergrown with felspar (either microcline or albite) the 

 quartz assumes the skeletal habit found in regular graphic 

 granite, otherwise it occurs in small, rather angular but 



