Bartrum. — The Conglomerate at Albany. 421 



7. Andesites. 



In general the amdesitic material is much weathered and altered, the 

 ferro-magnesian minerals in particular having been afiected. It was not to 

 be expected, considering the wealth of variety of our Auckland andesites, 

 that any new types would be revealed. Several varieties, however, were 

 discovered, some with plagioclase as the chief phenocryst, others with some 

 or other ferro-magnesian mineral more prominent in the first generation ; 

 but as a rule chlorite and other secondary aggregates greatly mask the 

 original nature of the rocks. Mention may be made of the following 

 varieties : — 



Hypersthene-andesite. — Macroscopically a particularly coarsely porphy- 

 ritic type with coarse phenocrysts of pyroxene and feldspar. The hyper- 

 sthene is accompanied by some augite and is largely replaced by calcite. 

 Some small serpentine pseudomorphs suggest olivine. The groundmass is 

 hyalopilitic. 



Pyroxene-andesite. — Another very coarsely porphyritic andesite, rather 

 light grey in colour. In section phenocrysts of colourless hypersthene are 

 both abundant and coarse, along with large equally colourless augite crystals. 

 Most of the rest of the rock is composed of much smaller phenocrysts and 

 intermediate lath-hke crystals of plagioclase, there being quite a minor 

 amount of the microcrystalline residuum of feldspar, pyroxene, and a little 

 magnetite. There is some coarse totally resorbed hornblende. 



Another interesting andesite, in which plagioclase is the only important 

 phenocryst, shows an abundance of branching growths of iron-ore in the 

 groundmass. The ferro-magnesian phenocrysts are small, infrequent, and 

 greatly altered. There are common inclusions of aggregated quartz grains, 

 often with epidote and chlorite, around which the iron-ore growths form a 

 conspicuously dense halo-like border. Coarse secondary epidote is frequent. 



8. Trachytes. 



These are rare ; only two types were found, both typical trachytes, so 

 far as macroscopical examination is concerned, but one characterized by a 

 little biotite and the other by a little hornblende.* In section both types 

 show many idiomorphic plagioclase phenocrysts, with more numerous but 

 smaller sanidine ones. 



In the hornblende-bearing type the amphibole is the green variety, 

 and is in scattered idiomorphic crystals. The groundmass is almost entirely 

 feldspathic : a few plagioclase laths are clear-cut and determinable, but the 

 main mass is of small ill-defined crystals lacking the albite lamination, and 

 presumably alkali-feldspar. Plate XXIX, fig. 4, shows the general structure 

 fairly well. 



Though the biotite of the biotite variety is apparent in the hand- 

 specimen, the only section cut shows no phenocr5^sts of that mineral, but 

 numerous small shreds of it in a groundmass built up almost wholly of 

 narrow laths of feldspar which lack the albite twinning and often show 

 good fluxional arrangement. f 



9. Rhyolites. 



Fragments of a curious white porcellanous felsitic rhyolite are frequent, 

 along with pumice and other material, in a thin band of grit-breccia 



* A third variety has been found since the above was written ; it is a silky rock 

 with phenocrysts of conspicuous greenish- brown hornblende. 



t A third type has been found recently ; it has fairly conspicuous phenocrysts of 

 brown anaphibole. 



