172 Transactions. 



A nephelinitoid phonolite that has probably intruded earlier basanites is 

 found in a small quarry alongside a branch track that leaves the North-east 

 Valley to Junction School Road, and follows up the North-east Valley Stream. 



The phenocrysts, which are almost entirely sanidine in Carlsbad twins 

 and a little bright emerald-green to yellowish-green aegirine-augite, are 

 sharply idiomorphic. The groundmass is chiefly nepheline in small hexa- 

 gonal forms. Deep-green mossy aegirine-augite aggregates and flakes of 

 sodalite are also very plentiful. 



This rock is similar to, and possibly the same as. the nephelinitoid 

 phonolite that is quarried lower down-stream in the North-east Valley 

 quarry. 



C. Trachytoid Phonolites. 



No hard-and-fast line can be drawn between the nephelinitoid and 

 some of the trachytoid phonolites of Mount Cargill. These latter phono- 

 lites present in the Mount Cargill area fall under two types, named respec- 

 tively " Logan's Point " and " Signal Hill " by Professor Marshall in his 

 paper " Geology of Dunedin," referred to previously. 



The more important on Mount Cargill is the Logan's Point type, which 

 forms Mount Zion and other knolls, and through which the Mount Holmes 

 basalt and the trachydolerites have probably been forced. The Logan's 

 Point is probably earlier than the Signal Hill type of trachytoid phonolite. 

 Cotton, in a paper, " Geology of Signal Hill, Dunedin,"* brings forward 

 evidence that supports this view. 



The apparent succession of types in the Mount Cargill area will be dealt 

 vith later. 



(a.) Logan's Point Type of Trachytoid Phonolite. 



In the hand-specimen this is a dull leek-green fine-grained rock, showing 

 a few sanidine phenocrysts. Its field outcrop shows a platy structure. 



Under the microscope practically no phenocrysts appear beyond a few 

 poorly shaped corroded ones of sanidine, and a few of pleochroic aegirine- 

 augite and resorbed amphibole. Cossyrite and aegirine-augite, both in 

 the allotriomorphic mossy growths common in the allied Mount Cargill 

 rocks, are evenly and plentifully distributed in the groundmass in a clear 

 base of nepheline and feldspar. The pleochroism of both minerals is the 

 usual pleochroism noted already. 



The feldspar of the groundmass is typically allotriomorphic, and, as 

 well as enwrapping the aegirine-augite and cossyrite, encloses in its most 

 intimate meshes minute nepheline crystals that often are only distinguished 

 by staining-tests. In other sections an abundance of nepheline causes a 

 cellular structure of the groundmass. 



Occasionally the feldspar of the base shows good fluxional arrange- 

 ment ; the laths then prominent are polysynthetically twinned, and are 

 referred, on their extinction-angles, to oligoclase. When the greater part 

 of the base shows this structure the rock merges into the Signal Hill type. 

 Similarly, where the base is highly nephelinitoid the rock verges on the 

 nephelinitoid phonolites. This is particularly the case in sections from a 

 small conical knob near Butter's Peaks. 



In the Logan's Point rock magnetite is absent ; a few prisms of apatite 

 are included by aegirine-augite. 



In a section made from an outcrop of this type near Butter's Peaks 

 large phenocrysts of olivine and pinkish augite were found. Partial resorp- 



* Trans. N.Z. Inst., vol. 41, 1909, p. 111. 



