168 Transactions. 



There is seen to exist a certain similarity in chemical composition 

 between the trachydolerites and the alkali basalts. This is not borne out 

 by the petrological and mineralogical characters, in which the trachy- 

 dolerites approach closely to the phonolites of the adjoining area. The 

 analyses, to be given later, of these phonolites show also how closely they 

 merge into the trachydolerites in chemical characters. 



(b.) Hypabyssal Type of Trachydolerite. 



Two areas show extensive rubble of this rock — the one on Pine Hill, 

 and the other near the headwaters of the North-east Valley Stream. It 

 was not found actually in, situ, but so great a heap of angular blocks as 

 there is on Pine Hill is not likely to have travelled far. 



The differences from the lava type are mainly textural. and are. 

 indeed, few. 



In hand-specimen large pyroxene, amphibole, and feldspar crystals are 

 conspicuous. The feldspar and soda-pyroxene are porphyritic. Olivine, 

 nepheline, and sodalite are less plentiful than in the lava type, but feld- 

 spar is more so. The corrosion of the feldspar is a noticeable feature. 



The pyroxene is chiefly a pleochroic green soda-bearing variety of 

 augite, or aegirine-augite. A little pink faintly pleochroic augite is also 

 present. The aegirine-augite is usually idiomorphic. 



Resorbed hornblende in places includes a little feldspar. 



In the groundmass the aegirine-augite is less important than the feld- 

 spar, and is less mossy than in the general lava rock. The plagioclase 

 feldspars — varying between oligoclase and acid andesine — also much increase 

 their importance. 



The main features of the type are the increased size of the phenocrysts 

 in general and the more open nature of the groundmass. 



(c.) Dense Basic Type of Trachydolerite. 



In hand-specimen this rock' is indistinguishable from the dense green 

 nephelinitoid phonolites and tinguaites common in the Dunedin district. 

 It has a leek-green very fine-grained matrix, in which are a few prominent 

 crystals of feldspar and pyroxene. Many variations of a minor nature 

 are exhibited by the rocks included in this class. 



Under the microscope the distinguishing features are — (1) scarcity 

 or lack of nepheline in the groundmass ; (2) abundance of aegirine-augite 

 and small amount of feldspar in the groundmass ; (3) the dense nature 

 of the groundmass ; (4) typical scarcity of phenocrysts. 



The relative importance of the different phenocryst minerals varies 

 from section to section. 



An interesting feature is the occurrence of small rounded leucite crystals 

 with characteristic radial inclusions of aegirine-augite. Another pecu- 

 liarity lies in the alteration (or, may be, corrosion) of the olivine pheno- 

 crysts. These have been more or less wholly replaced by a clear colourless 

 secondary mineral and magnetite dust. The fibrous nature and other 

 characters of this secondary mineral seem most characteristic of serpentine. 

 A bluish-green chloritic mineral is sometimes connected with this alter- 

 ation of the olivine. 



Sharply idiomorphic, fresh olivine crystals are, however, not uncommon. 

 There is an occasional corona of augite and magnetite to the olivine. 



Phenocrysts of feldspar are less common than those of the ferro- 

 magnesian minerals ; of them, sanidine is the commonest, but anorthoclase 





