4:28 Transactions. 



already described as exposed about two miles from Riverhead alongside 

 the Albany-Riverhead Road, and again near the paper-mills at Riverhead 

 itself, where the fragments are much coarser. The finer portions of the 

 grit-breccia are largely angular quartz grains with flakes of biotite, small 

 grains of the whitish rhyolite, and some of fine argillite. 



The rhyolite exhibits a few corroded quartz phenocrysts of moderate size, 

 and plentiful small rather rod-like flakes of brown biotite in a dense ground- 

 mass of minute microlites of feldspar, which are arranged more or less in 

 parallelism with the biotite flakes, and are enwrapped by a small amount 

 of irresolvable base from which they have only imperfectly separated. 



Significance of the Material of the Albany Conglomerate. 



The probable origin of the sedimentary material included in the con- 

 glomerate has already been sufficiently considered (see p. 424),' for it raises 

 no point of especial interest. The igneous constituents, however, present 

 a very different case. 



There is very general agreement amongst New Zealand geologists 

 (Fraser and Adams, 1907, table' facing p. 22) that the eruptions of 

 andesite which have contributed so largely to the building of Coromandel 

 Peninsula and other northern parts of Auckland Province probably began 

 before the Miocene, and thus before the period of formation of the 

 Waitemata beds, which are commonly regarded as Upper Miocene in age. 

 Andesitic eruptions have evidently been common since pre-Jurassic times, 

 for andesitic pebbles are found in JuraSsic rocks in the Cape Colville 

 Peninsula (Fraser and Adams, 1907, p. 52j, at Port Waikato (Bartrum, 

 1917, p. 422), and elsewhere. The writer has observed andesitic debris 

 capping hills adjacent to the hill route between Riverhead and Helensville, 

 not many miles from Riverhead, which may represent an extrusion of 

 pre- Waitemata time. 



Rhyolites are first known in the Auckland Province from the sup- 

 posedly pre-Jurassic sediments (Tokatea Hill series) of Coromandel Pen- 

 insula (Fraser and Adams, 1907, p. 43), and have considerable importance 

 from early in the Tertiary (Fraser and Adams, 1907. table facing p. 22). 

 Their only special interest as concerns this paper is that they have not been 

 recorded previously from any of the other conglomerate or grit bands in 

 the Waitematas.* 



The presence in abundance of gneissic rocks in the Albany conglomerate 

 raises an interesting question regarding the earlier geological history of 

 the North Island of New Zealand, but so large a one that it is inadvisable 

 to deal with it at all fully in a paper such as this, devoted mainly to 

 petrographic description. Briefly, the facts are these : — 



1. Gneissic plutonic rocks occur in the North Island of New Zealand 

 in conglomerates at Alexandra, in the King-country (Park, 1893) ; at the 

 gorge of the Waipaoa River, Poverty Bay (Sollas and McKay, 1906, pp. 175 

 et seq.) ; in Cretaceous or early Tertiary beds in the Whangaroa district 

 (Bell and Clarke, 1909, p. 50) ; in " Maitai " conglomerates in the 

 Hautotara Mountains of south-east Wellington (Sollas and McKay, 1906, 

 p. 185) ; and, as now recorded, at Albany. 



* This statement requires some modification: C. E. Fox (1902, p. 462) records 

 fragments of pumice. Their presence is, however, no necessary indication of the vicinity 

 of a rhyolitic terrain, for pumice can be naturally transported immense distances by 

 water. An interesting example of this fact is furnished by the presence of abundant 

 pumice in sub-recent shore-deposits at the Big Omaha, south of Cape Rodney, which 

 has probably been carried by the coastal drift around Cape Colville from the east coast. 



