Park. —Thermal Activity and Metalliferous Veins. 27 



of wood partly or wholly silicified and spangled with nests 

 and veins of iron-pyrites are of common occurrence through- 

 out the Hauraki region. 



The Martha Lode and its numerous ramifying branches, 

 the Silverton, Union, and Amaranth Lodes, at Waihi, are all 

 contained in an area of about a square mile. The huge lodes, 

 wide zones of silicified andesite, and extensive propylitisation 

 of the andesite, prove that Waihi was an area of intense 

 hydro-thermal activity some time prior to the eruption of the 

 later rhyolite- flows which now form the plains and wrap 

 around the isolated outcrops of andesite containing the 

 Martha and Silverton veins. The propylitisation has already 

 been shown by the Waihi Mine workings to extend to a depth 

 of nearly 800ft. below present water-level — that is, some 

 500 ft. below sea -level. Obviously the alteration of the 

 andesite was due to the action of ascending and laterally 

 moving thermal waters. 



At Thames and Coromandel some of the most productive 

 veins do not reach the surface of the enclosing rock, and the 

 mine-workings at Waihi have disclosed a similar feature in 

 connection with a few valuable veins in the Waihi Company's 

 property. * 



In 1888 Captain F. W. Hutton, as the result of a petro- 

 graphical examination of the Thames Mining District, con- 

 cluded that the veins were of hydro-tnermal action. f 



T. A. Eickard, a well-known American geologist who ex- 

 amined the same goldfield in 1891, when discussing Professor 

 Posepny's paper on " The Genesis of Ore-deposits," describes 

 the characteristic features of the district with the view of 

 adducing additional evidence of the association of thermal 

 springs and later eruptive rocks. \ He states that his ex- 

 amination of the ore-occurrences and vein-structure, though 

 incomplete, led him to conclude that the deposition of the 

 gold and its associated minerals had followed certain lines of 

 altered country rock which had been exposed to the effects of 

 dying but lingering solfataric agencies. 



Ohaeawai Cinnabar Deposits. 



The Ohaeawai Hot Springs quicksilver deposits, on the 

 mainland some distance north of the Hauraki Peninsula, are 



* P. C. Morgan, "Notes on the Geology, Quartz Reefs, and Minerals 

 of the Waihi Goldfield," Trans. Aust. Institute of Mining Engineers, 

 vol. viii, 1902, p. 168. 



f F. W. Hutton, " On the Rocks of the Hauraki Goldfields," Trans. 

 Aust. Assoc. Adv. Sci., vol. i, 1888, p. 245, and " Source of Gold at the 

 Thames," N.Z. Journal of Science, Vol. i, p. 146. 



\ T. A. Rickard, " The Genesis of Ore-deposits," Discussion, New 

 York, 1901, p. 222. 



