324 Reviews — Geology of New Zealand. 



VI. — New Zealand Geology. 



New Zealand Geological Survey. — In Bulletin No. 10 (New 

 Series), 1910, Mr. Colin Praser describes " The Geology of the 

 Thames Subdivision, Hauraki, Auckland". The area forms part of 

 the promontory of the North Island east of Auckland and south of 

 Coromandel, and it includes "Waihou on the south and Hastings on 

 the north. The prevailing rocks of the area are Tertiary andesites 

 and rhyolites, and they indicate three periods of volcanic activity, 

 referred (with doubt) to Upper Eocene, Miocene, and Pliocene. These 

 rocks form the hilly and mountainous ground of Hastings and the 

 country east and south-east of Thames Town. The mountains, which 

 form a part of the Cape Colville range, rise to elevations of 2,000 feet 

 and more ; indeed, Table Mountain, formed of a dyke of andesite, is 

 reckoned to be about 2,800 feet above sea-level. The volcanic 

 accumulations rest ou an eroded floor of Jurassic and older sediments, 

 Avhich are exposed only in the northern portion of the area on and 

 near the coast of the Firth of Thames. The pre-Jurassic strata have 

 Yielded no fossils, and the Manaia Hill Series, referred to Upper 

 Jurassic, has yielded only Inoceramus haasti and Belemnites. 



The southern portion of the area consists for the most part of the 

 broad flood-plains of the Rivers Thames (or Waihou) and Piako, with 

 raised sea-beaches and high-level river-terraces. Volcanic activity 

 ceased about the close of the Pliocene period, and after the later 

 earth-movements there has been much infilling of the bays by 

 fluviatile agencies. 



Special attention is directed to the Thames goldfield on the borders 

 of Thames Town. There gold-silver quartz veins with ore-shoots 

 are worked in the andesitic and dacitic rocks of the earliest Tertiary 

 volcanic period. These quartz veins also penetrate the Jurassic and 

 pre-Jurassic sediments, but this floor of older rocks is more than 

 1,000 feet below sea-level at the Thames goldfield. In the opinion 

 of the author (pp. 3, 45) most of the ore-shoots " appear to show 

 a considerable dependence on depth below the existing land-surface, 

 since, irrespective of the elevations of the outcrops of the veins, the 

 upper zone, extending to a depth of from 400 to 500 feet, has proved 

 much the more productive . . . 500 feet has proved a critical depth 

 on this field . . . The downward limit of the pay-ores mined appears, 

 therefore, to conform roughly to surface -contours ". He considers 

 " that the principal veins at the Town of Thames occur in the near 

 vicinity of the vent of an ancient volcano ". Very little gold has 

 been obtained from creek gravels, for, as the author remarks, while 

 there has been much denudation of the ore-bearing veins, " most of 

 this aurifei'ous rock-waste was discharged by the streams on to an old 

 foreshore now depressed hundreds of feet below sea-level, and overlain 

 by recent alluvium." 



Auriferous tellurides occur in a few localities. The author draws 

 attention also to the widespread alteration of the volcanic rocks by 

 hydrothermal action. The work is well illustrated with maps and 

 sections, mining plans, and pictorial views. 



