Bartrum:. — Rocks of Mount Gargill, Dunedin, 



169 



also occurs in a few large crystals The feldspars exhibit the same round- 

 ing as in the general trachydolerite, but the edges are sharply defined. 



Pyroxene and brown amphibole also form phenocrysts. The pyroxene 

 is generally idiomorphic pale-pinkish to pale-greenish-pink augite. It is 

 commonly fringed by dust-like aegirine-augite. Aggregates of pinkish 

 augite are common. 



In some sections nepheline forms important large well-shaped crystals. 

 Sodalite in small flaky forms is moderately abundant. The augite includes 

 a few apatite prisms. 



The impenetrable nature of the groundmass is given it by the felted 

 dust-like granules of aegirine-augite. Typically, no cossyrite is present ; 

 but in a few sections, where the density of the groundmass is not so 

 marked as in typical sections, a few opaque-brown dendritic growths may 

 be of this mineral. 



A few minute feldspar needles are scattered throughout. Staining 

 detects nepheline in the mesh of aegirine-augite dust in minute rare hexa- 

 gonal and square forms. 



Magnetite is very scarce, unless it occurs with aegirine-augite as a 

 resorption-product of amphibole. 



Chemical Characters. 



An analysis was made of this type, and comparison with the two other 

 analyses appended shows how closely it agrees chemically with both the 

 trachydolerites and the trachytoid phonolites. 



98-64 99-47 100-46 



A. Basic type of trachydolerite, Butter's Peaks, Mount Cargill. (An- 

 alysis, J. Bartrum.) 



B. Trachydolerite from Columbretes, Spain.* 



C. Tracytoid phonolite.| 



(d.) Nephcliniloid Type of Trachydolerite. 



In hand-specimens this rock is indistinguishable from the preceding 

 basic type. 



Within a few yards in the field this type merges, in successive vari- 

 ations, from the general trachydolerite to true nephelinitoid phonolite. 



* Rosenbusch, " Elemente der Gesteinslehre," p. 355, No. 4, 1901 ed. 

 t Rosenbusch, loc. cit., p. 292, No. 4. 



