Bartrum. — Rocks of Mount Cargill, Dunedin 



177 



Basanites. 



Mention has been made already of the so-called basanites. In the 

 hand-specimen they are fine-grained dense blackish rocks, showing few 

 conspicuous phenocrysts. 



Besides the occurrences already noted, a basanite is found as extensive 

 rubble near the hypabyssal trachydolerite on Pine Hill. 



Under the microscope these rocks show a typically dense base and a 

 paucity of phenocrysts ; augite is the commonest of the few phenocrysts 

 that are seen. It is most often of a pale-pinkish colour, but a green augite 

 with pink border and a purplish-violet variety are also present. 



In the basanite found north of the Junction School a strongly pleo- 

 chroic mica is prominent. It occurs, along with a little serpentine, as an 

 alteration-product of the olivine ; its pleochroism varies from deep brown 

 to bright golden-brown. It is thought to be anomite. 



Feldspar seldom is a phenocryst in the Mount Cargill basanites ; a few 

 very large feldspars show albite twinning, and seem to be o'igoclase. They 

 are, however, so crowded by augite granules and other inclusions that an 

 exact determination cannot be made. 



The groundmass is very dense, and is composed mainly of small grains 

 and squares of magnetite. 



In the anomite-bearing rock the magnetite is less important, and an 

 interstitial feldspar is the chief constituent, along with grains of colourless 

 augite. The augite is usually in fair amount in these rocks, but feldspar 

 typically occurs only in a comparatively few needle laths. 



No chemical analysis of any of the basanites was made. 



Relationships of the Trachydolerite and Phonolite Series. 



The analyses given in the subjoined table show how gradual a passage 

 there is chemically from the more basic trachydolerites to the phonolites. 

 Petrological characters also indicate that such a gradation is not a matter 

 of chance, but represents a differentiation of many types from the one 

 magma. In certain cases this is due to differences in the rate of cooling. 



All evidence from the Mount Cargill area would show that the Logan's 

 Point trachytoid phonolite is a portion of the main alkaline magma, and, 

 in fact, a modification of the trachydolerites and the nephelinitoid phono- 

 lites. Evidence from other parts of the district dispels any idea of its 

 contemporaneity with these latter rocks. 



98-83 98-64 98-72 99-94 98-66 



A. Trachydolerite, Main Peak, Mount Cargill. 



B. Trachydolerite, near Pine Hill, Mount Cargill. 



C. Basic type of trachydolerite. Butter's Peaks, Mount Cargill. 



D. Nephelinitoid phonolite, Butter's Peaks, Mount Cargill. 



E. Logan's Point trachytoid phonolite, Mount Zion, Mount Cargill. 



