"Vol. 62.] GEOLOGY OF DU^EDIX (NEW ZEALAND). 413 



represented, and there are no pebbles of trachydolerite, so far as 

 known. 



18. Melilite-basanite. — A dark basaltic rock, exhibiting dis- 

 tinct grains of olivine in hand-specimens. Slices show some plagio- 

 clase, the extinction-angle of which is high enough for anorthite ; 

 augite, generally granular and very abundant: olivine in rather 

 long crystals. Alelilite was the last mineral to crystallize, and is 

 present in irregular plates containing inclusions of augite, anorthite, 

 and magnetite. jSTo perofskite has been detected. The presence of 

 felspar in these rocks appears to be exceptional: in consequence 

 of its presence, the rock should be classed as a basanite. 



It is only in the extreme northern part of the district that any 

 melilite has been found up to the present. The rock forms a lava- 

 fLow of some 50 feet thick, resting upon the calcareous sandstone 

 below and covered by trachytoid leucite-phonolite above. Field- 

 indications point to the probability that the rock covers a large area, 

 but this has not yet been proved by microscopical investigation. 

 Occurring as it does, right on the edge of this volcanic district, it is 

 difficult to say what position it would occupy in the complete series 

 of rocks : but it is probable that it is the equivalent of one of the 

 younger basalts, immediately below the trachytoid phonolite in the 

 main series. 



19. Port-Chalmers Breccia. — In many places the component 

 fragments are large, reaching 60 centimetres in diameter ; but 

 usually they are quite small, and in some large masses of rock it is 

 unusual to see any fragments of greater size than 10 cm. The 

 fragments are very variable in nature. Pieces of mica-schist, 

 diorite, and dolerite are not infrequent, j^epheline-syenites are 

 fairly frequent ; and tinguaites (porphyritic and dense), phonolites, 

 and basalts are abundant. In sections, much of the microscopic 

 matter is seen to be phonolite and trachyte. Occasional crystal- 

 fragments of felspar and augite occur. The rock is firmly coherent, 

 and the grains are held together more by the compact nature of 

 the fine material in which they are embedded, than by any definite 

 cement. 



The Port-Chalmers Breccia is a fine to coarse clastic rock, closely 

 cemented into a hard coherent mass by consolidation and cementa- 

 tion. It covers a large area near Port Chalmers, and is also found 

 to the west and south of Hoopers Inlet. It lies above the trachyte, 

 with which it is in contact, but is covered by trachytoid phonolite. 



The breccia evidently represents the detritus thrown out from 

 a crater in which violent spasmodic steam-explosions occurred. 

 All the rocks previously deposited in the neighbourhood were 

 shattered, and in the constant attrition attending their emission 

 and falling back were partly reduced to powder : this has formed 

 the cementing-material which has converted the fragmentary matter 

 into a hard coherent rock. It is a noteworthy fact that the majority 

 of the tinguaite-dykes radiate from a spot near the present greatest 

 thickness of breccia — Observation Point ; and it is probable that the 



Q. J. G. S. No. 247. 2 f 



