44 



mechanism of complementary differentiation has never been 

 satisfactorily explained, and it seems as if some sort of separ- 

 ation in the fluid magma must be postulated, perhaps by the 

 formation of two immiscible fractions, or perhaps by sinking 

 of earlier-formed crystals and their refusion at the base of a 

 magma column with a very steep temperature gradient. (^^^ 

 This might produce a basic fluid magma underneath and an 

 acid one on top. Wliatever the mechanism in the present 

 instance, the separation was very complete, seeing that the 

 lighter differentiates — the potash and soda aplites — are quite 

 free from ferro-magnesiau constituents. 



(6) COMPARISONS WITH SIMILAR ROCKS ELSEWHERE IN THE 



STATE. 



Rocks of very similar type to those described above are 

 found in other parts of South Australia. The assemblage 

 recently described by Mr. C. E. Tilley from Cape Willoughby, 

 Kangaroo Island, are in many respects identical with the 

 Encounter Bay rocks, and are unquestionably derived from 

 the same magma, although the development of distinctive 

 characteristics would show that they did not form part of the 

 same original intrusion. 



The igneous rocks — microclino-bearing granites and 

 albitites — reported by Jack from Eyre Peninsula, analyses of 

 which are quoted in this paper, are very similar chemically 

 and mineralogically to the series under discussion, and Mr. 

 Tilley informs me that in the Tate Museum of the University 

 of Adelaide there are two specimens from Alford, near 

 Kadina, one a granite and the other a porphyritic aplite, 

 which are hardly distinguishable from the rocks of Encounter 

 Bay and Cape Willoughby. 



Again, there are at numerous localities dykes of epidiorite 

 or uralitic dolerite similar to those of Rosetta Head and Port 

 Elliot, some of which have been already referred to, and 

 these are in some cases accompanied by intrusions of sodic, 

 and in one case potassic, dyke rocks; the rock associations, as 

 well as the chemical and mineralogical characteristics, are 

 quite analogous to those of the Encounter Bav occurrence. 

 Mention may also be made to the very similar rocks described 

 by Mawson occurring as dykes at Broken Hill. 



It is probable that further investigation will show that 

 these two rock series, the granite-syenite and the dolerite- 

 aplite series, are of much wider distribution in South Aus- 

 tralia than is at present known, and if the suggestion connect- 

 ing the dolerites with the granite magma is correct it appears 



(36) 67. Iddings, Igneous Rocks, vol. i., p. 269. 



