Vol. 62.] THE GEOLOGY OF DUNEDIN (NEW ZEALAND). 421 



In the basanite, the nepheline and sodalite are much corroded, and 

 the pyroxene has a core of gegirine. It seems impossible that in 

 such rocks these alkaline minerals should have formed first, and 

 have suffered corrosion afterwards. For, in a rock with so high a 

 percentage of silica (49), and a comparatively-low percentage of lime 

 (7), a soda-lime felspar would have been formed in sufficient quantity 

 to use up all the available soda (4*7). Even in those rocks where 

 the silica-percentage is so low that there is more soda than can be 

 utilized in the felspar, it is, so far as specimens available for 

 examination in Dunedin are concerned, retained in the magma until 

 the last stage of crystallization is reached, when idiomorphic nepheline 

 separates out, and the augite becomes fringed with a mantle of 

 aegirine. This, which appears to be the normal order in such rocks, 

 is admirably shown in rocks from Auckland (N.Z.), also in those 

 from the Lobauer Berg. 



In the trachydolerites, the augite and olivine appear to be as 

 little true secretions from the magma of the rock in which they are 

 contained as the nepheline and segirine in the basanites. Not only 

 has resorption proceeded far while fringes of segirine have formed 

 round the minerals, but the augite must have been (with olivine) the 

 earliest constituent to form. In addition, the only plutonic frag- 

 ment found in the rock (it is 10 centimetres in diameter) contains 

 no olivine or augite, though all the other trachydolerite-constituents 

 are present. 



There can be no doubt that the dolerite at Papanui has obtained 

 large quantities of quartz now partly resorbed from the schist. In 

 connexion with this, it is only necessary to refer to marscoite, a 

 rock formed from a gabbro-magma that has taken up granitic 

 material prior to its intrusion and to the ' composite sills.' l 



In a word, all these minerals must be regarded as xenocrysts in 

 the rocks in which they are found. The question arises as to 

 whence these xenocrysts were derived. It is unlikely that they 

 were obtained from the melting of rocks previously solidified, 

 for the amount would have to be enormous, and the stage of melting 

 in the huge mass of trachydolerite, for instance, is the same all 

 through. Again, if any melting of adjacent rocks took place, it is 

 natural to expect that the quartz contained so abundantly in the 

 mica-schist, the fundamental rock of the district, would be well 

 represented. 



The only rational explanation appears to be that these rocks 

 have resulted from mixture of magmas before ejection. 



The nature of the magmas before this mixing cannot be more 

 than guessed at until further research is made, but the trachyte- or 

 phonolite- and dolerite-magmas appear more likely than the others. 

 It is possible that these two magmas owe their original separation 

 to magmatic differentiation, but whether that is so or not, the ultimate 

 cause in producing the numerous varieties of rocks would appear 

 to be magmatic mixing. 



1 A. Harker. ' The Tertiary Igneous Rocks of Skye' Mem. Geol. Surv. U. K. 

 (1904) chapt. xii. 



