Bartrum. — Rocks of Mount Car (/ill, Dunedin. 



171 



The phenocrysts are of sanidine, of brownish-pink augite, of almost 

 •ompletely resorbed amphibole, of sodalite, and occasionally of nepheline. 



Sanidine is the most common ; it is usually markedly corroded, but 

 occasional good idiomorphs show up. Carlsbad twinning is common. 

 When nepheline occurs it is in very large crystals ; sodalite is in numerous 

 rounded and flaky forms. 



Pinkish-brown augite not infrequently forms an outgrowth to resorbed 

 hornblende. One or two deep-green pleochroic and idiomorphic aegirine- 

 augite phenocrysts are present. 



Under moderate magnification the groundmass exhibits a prominent 

 nephelinitoid, or cellular, structure. The nepheline of these clear cellular 

 portions is in minute hexagonal cross-sections. 



Highly pleochroic aegirine-augite and cossyrite aggregates are scattered 

 regularly and fairly plentifully in the nepheline base. All branching 

 portions of these aggregates are in crystalline continuity, and extinguish 

 together. The pleochroism of the cossyrite is from bright reddish-brown 

 to brownish-black, and of the aegirine-augite from deep grass-green to 

 greenish-yellow. The identification of the cossyrite is based on its descrip- 

 tion in this and similar rocks of the district by Professor Marshall. No forms 

 approaching idiomorphism were found on which to apply optical tests. 



In portions only of certain sections feldspar shows up well in minute 

 needles that have rough parallelism, but elsewhere it is relatively scarce. 

 There are a few scattered granules of magnetite. 



From the east end of Butter's Peaks one section made was found to 

 differ from the others, and to present an undoubted nephelinitoid phonolite. 

 It probably represents an unimportant local variation of the general basic 

 trachydolerite. Cossyrite is very scarce in this section ; it is in minute 

 dense growths. The aegirine also is very dense, and is of much less import- 

 ance than in the typical nephelinitoid phonolite. Nepheline forms almost 

 the whole of the predominant clear base of the groundmass. 



Chemical Characters. 

 The analysis made of this Mount Cargill nephelinitoid phonolite shows 

 a close agreement with that of the nephelinitoid phonolite represented 

 by analysis B. 



Si0 2 



A1 2 3 



Fe 2 3 



FeO 



MgO 



CaO 



K 2 



Na 2 



H 2 



CI .. 



Mn0 2 



P 2 5 



99-94 



99-41 



A. Nephelinitoid phonolite from Butter's Peaks, Mount Cargill. (An- 

 alysis, J. Bartrum.) 



B. Nephelinitoid phonolite from Hohentwiel, Hegau.* 



* Rosenbusch, " Elemente der Gesteinslehre," No. (i, p. 292, 1900 ed. 



