Yol. 55.] THE EHYOLITES OF THE HATJEIKI GOLDFIELDS. 449 



30. J^ToTES on the Rhyolites of the Hi-UKAKi Goldfields (]SrEw 

 Zealand). By J. Park, Esq., P.G.S., and F. RuiLEr, Esq., 

 E.G.S. With Chemical Analyses hy Philip Holland, Esq., 

 F.I.C., F.C.S. (Read May 24th, 1899.) 



Part I. — Characteristics of the Rhyolites hi the Field. 

 By James Park. 



[Plate XXXI— Map.] 



New Zealand, during the Secondary period, seems to have been singu- 

 larly free from volcanic disturbances, but after the advent of the 

 Eocene it again became the scene of outbursts which have prevailed, 

 with only short periods of cessation, up to the present time. The 

 eruptions of Tertiary date were generally more widespread than those 

 of recent times, their ejections sometimes affecting many hundreds 

 of square miles. 



The ejections of the early Tertiary eruptions were principally basic ; 

 those of the middle Tertiary, semi-basic ; of the older Pliocene, 

 acidic ; and of the newer Pliocene and recent, acidic, semi-basic, 

 and basic. 



The JPliocene acidic eruptions devastated a wide area in the 

 central portion of the ISTorth Island, smothering thousands of square 

 miles with rhyolitic lavas, tuffs, and pumice, which now form barren 

 plains and bare, Hat-topped ranges. The chief centres of eruption 

 lay in the Taupo, Rotorua, and Tauranga districts and the Hauraki 

 peninsula. It is in connexion with the rhyolites of the last-named 

 district that these notes are written. 



The Hauraki peninsula is situated in the province of Auckland, 

 and lies between the Firth of Thames, on the west, and the Pacific 

 Ocean on the east. Altogether it covers an area of about 1,000 

 square miles. Its surface is diversified with rugged, forest-clad 

 mountain-ranges, cut by deep ravines. 



The basement-rocks consist of Palaeozoic (probably Devonian) 

 slates and grauwacke, which generally occur in rapidly alternating 

 thin beds or layers. Near Cabbage Bay a small isolated patch of 

 richly fossiliferous marly clays, and limestone of Lower Eocene age, 

 rests directly upon the denuded surface of the Palseozoic slates; while 

 throughout the length and breadth of the peninsula a vast accumu- 

 lation of andesitic lavas and tuffs is piled on both the slates and 

 Eocene strata indiff'erently, to a depth in some places of 3000 feet. 



These andesites are the gold-bearing rocks of the Hauraki penin- 

 sula, the prevailing variety being an augite-andesite, in which the 

 augite is more or less replaced by hypersthene. In the Mercury 

 Bay and Waihi districts, the andesites are covered by heavy flows of 

 rhyolite and accumulations of rhyolitic ash. 



Both the andesitic and rhyolitic eruptions appear to have been 

 Q. J. G. S. No. 219. 2 g 



