J. A. Thomson. — Igneous Intrusions of Mount Tapuaenuka . 3L3 



idiomorphic outlines to the feldspar, which generally surrounds them as 

 tabular prisms of smaller size, but at times is moulded on them in large 

 crystals. The iron-ores are clearly moulded on olivine and augite, but 

 appear to be anterior to the feldspar. Biotite and hornblende are of late 

 crystallization, and clustered around the iron-ores, olivine, and augite. 

 An interesting feature of the rocks is the presence, in some cases, of a 

 reaction-rim between olivine and feldspar. 



Biotite-dolerites. 



This type, also fairly abundant in the Dee gravels, differs from the former 

 not only in the absence of olivine, but also in the greater amount of feldspar 

 compared to the augite, and the richer development of biotite. The feld- 

 spar is more often zoned, and, on the whole, more acid (andesine to labra- 

 dorite). Apatite and iron-ores are more abundant, and zircon makes its 

 appearance as an accessory. Brown hornblende is only an occasioijal con- 

 stituent, but green uralite is very common. 



Structurally these rocks differ from the former in an inversion of the 

 relative order of the feldspar and augite. This is most marked in the more 

 feldspathic types. The feldspars are quite idiomorphic, and are frequently 

 enclosed by the augite, which occurs for the most part interstitial ly, without 

 forming large ophitic plates. The iron-ores are moulded both on feldspar 

 and augite, but are anterior to the biotite. In some cases, however, the 

 latter must have commenced crystallization at an earlier stage, since it is 

 found intergrown with the augite. 



One rock, somewhat similar in composition to the biotite-dolerites, 

 differs from them in the greater abundance of allotriomorphic magnetite, 

 and in the structure. Large crystals of augite are fairly common, but for 

 the most part this mineral occurs in quite small grains, of similar size to the 

 magnetite, and forms with it and basic plagioclase a fine-graiued ground- 

 mass. There are a few larger feldspars which might be considered as pheno- 

 crysts did they not enclose numerous small grains of augite and magnetite. 

 Biotite occurs in large honeycombed plates, and pilite pseudomorphs of 

 olivine are occasionally seen. Although at first sight the rock seems por- 

 phyritic, a more careful inspection suggests that this appearance is deceptive, 

 and that crystallization has been uninterrupted. 



Biotite-quartz-dolerite. 



Only one of the specimens collected is to be placed in this category. Its 

 augite is completely uralitized, but otherwise it differs mineralogically 

 from the biotite-dolerites only in the presence of a moderate amount of 

 primary interstitial quartz. Structurally an ophitic structure is better 

 exhibited, and the iron-ore is anterior both to the feldspar and the pseudo- 

 morphs of augite. The biotite has the same relationships as before, and 

 appears to have been intergrown with the original augite. 



So far as the writer is aware, this is the first recorded instance of a quartz- 

 dolerite (or quartz-gabbro) in New Zealand. 



~~~ Doleritic Rocks with Lamprophyric Affinity. 



Two rocks agree qualitatively in mineral composition with the olivine- 

 dolerites, but are distinguished by a very different structure. There are 

 a few large plates of augite, but this mineral occurs mostly in quite smaJ] 

 and often long prisms. The other minerals — olivine (represented by pilite 

 pseudomorphs), brown hornblende, biotite, and magnetite — -all occur in 

 small idiomorphic crystals of characteristic shapes. Large crystals of 



