Bartrum. — Rocks of Mount CJargill, Dunedin. 165 



on the weathering surfaces. This is hypabyssal trachydolerite. Near it 

 is also found abundant rubble of a basaltic or basanitic nature. 



In a quarry on the road running from lower Pine Hill north-eastwards 

 along the southern slopes of Mount Cargill is a type of trachytoid phono- 

 lite similar to that occupying a large area on Signal Hill, and known as 

 the Signal Hill type. This phono lite runs south-west along the hillside 

 from the uppermost forks of the North-east Valley Stream, just below 

 the steep southern face of the Main Peak. Near these forks begins a 

 winding road towards the Junction School. Along this road hypabyssal 

 trachydolerite in large rubbly blocks is first met ; a space covered solely by 

 basaltic debris intervenes ; and then there is an outcrop of basaltic scoria. 

 Fragments of the so-called " Junction basalt" are found plentifully all around, 

 and shortly the solid rock is exposed in a shallow quarry near this road. 



On a knoll about 1,200 ft. high, about a quarter of a mile north-east 

 of the scoria outcrop, a fresh basanitic rock is found in plentiful rubble. 

 No outcrop was discovered. 



Basaltic and basanitic rocks, and also a nephelinitoid phonolite, out- 

 crop in the valley of the North-east Valley Stream, below the forks men- 

 tioned above. The last-mentioned rock is of a peculiar type, and seems 

 to be the same nephelinitoid phonolite that occurs in the North-east Valley 

 quarry, about a mile and a quarter down-stream from the upper outcrop. 



Basaltic rubble is extensive on the hillside north-west of the North-east 

 Valley tannery. No recognizable outcrops were found, and its relation 

 to the Signal Hill phonolite cannot definitely be determined. 



PETROLOGY. 

 DESCRIPTION OF ROCK TYPES. 



A. Trachydolerites. 

 (a.) General Lava Trachydolerite. 



In hand-specimen this is a heavy greyish-black fine-grained rock show- 

 ing fairly prominent feldspar and, in places, pyroxene crystals. It breaks 

 with a rough fracture. Feldspar and pyroxene show up prominently on 

 weathering surfaces. This rock is described by Professor Marshall in his 

 paper on the Dunedin trachydolerites.* 



The microscope shows a base of irregular feldspar laths, with enmeshed 

 aegirine-augite and other crystals, enclosing moderately abundant pheno- 

 crysts. 



A porphyritic character is shown by the feldspar, and to a less extent 

 by the nepheline and pyroxene. v 



The phenocrysts are sanidine, augite (chiefly of various brownish tints), 

 resorbed hornblende, sodalite. nepheline, olivine, and occasionally aegirine- 

 augite and anorthoclase. 



The commonest phenocrysts are those of augite. 



The sanidine shows marked corrosion, and its margins are usually 

 dentate with aegirine-augite. Its twinning is by the Carlsbad law. The 

 extinction in many cases is parallel to well-marked cleavage, and in some 

 idiomorphic sections to the edge between the faces 100 and 010. The 

 crystals are usually small, but are found up to 5 mm. by 4 mm. in size. 

 Clear glassy crystals are character isitic, but both liquid and aegirine-augite 

 inclusions are common. 



* Trans. Aust. Assn. Adv. Sci., vol. 10, 1004, p. 183. 



