BY H. I. JENSEN. 557 



Geological Survey, Post-Cretaceo-Tertiary. They exhibit numer- 

 ous similarities to those of East Africa described by G. T. Prior. 



The east of the Province of Otago forms part of a subsidence 

 area or senkungsfeld, the greater part of which is now submerged. 

 The west of the Province forms a highly folded complex of 

 Archaean and metamorphic Silurian rocks, west of which again 

 lies the great subsided continental area which joined problematical 

 Antarctica and the Sino-Australian area together, and which is 

 separated from the horst by a fault-line from which Tertiary 

 lavas have flowed. This fault is probably a continuation of the 

 great Samoa-Tonga-Taupo fracture. J 



The rocks described by Professor Marshall are hornblende 

 foyaite, augite diorite, tinguaites, ulrichite, trachydolerite, camp- 

 tonite, teschenite, trachyte, phonolite, kaiwikite, leucitophyre, 

 andesite, nepheline basanite, melilite basanite, dolerite, basalt, 

 etc. 



The conclusion is also arrived at by the author that in certain 

 cases there was a mixture of a basic calcic magma and an alkaline 

 magma before eruption. 



In comparing the analyses made by Dr. Marshall we can trace 

 many affinities between the different rocks, but we find only one 

 analysis which is near the mean of two others, viz., that of the 

 nepheline basalt which agrees closely with the mean of the 

 Papanui dolerite and the Leith Valley dolerite. If we take 

 together the two tinguaites analysed and the ulrichite, and take 

 the mean of the three analyses it will be found to agree closely 

 with the mean of the foyaite and camptonite. The camptonite 

 and foyaite may therefore be considered to be differentiation 

 products of the tinguaite-magma. 



Tasmania. — Now turning to Tasmania, we have rocks of 

 foyaitic magma represented by the Port Cygnet series and by 

 the Middle Tertiary nepheline basalts, melilite basalts and 

 melilite-eudialite basal ts.f 



* These Proceedings, 1906, pp.662, 665. 

 t R. M. Johnson, "Geology of Tasmania." — Twelvetrees, "Nomencla- 

 ture and Classification of Igneous Rocks," A.A.A.S. 1904. — Dr. Fred. 

 Parnell Paul, " Beitrage zur PetrographischenKentniss," Tschermak Min. 

 Mitth. 25th Band, iv.Heft. 



