Waihi Borough Council Diamond Jubilee Booklet 1902-1962
Compiled from notes taken at a lecture given by the late A. H. V. Morgan.
The rocks of the Waihi district consist of andesites and rhyolites of the Tertiary Age (Miocene and Pliocene). Sedimentary rocks are almost entirely absent. The first outburst of volcanic activity appears to have taken place about 12 million years ago, and it was followed by several other outbursts alternating with intervals of rest during which time the lava sheets were carved into hills and valleys. Two distinct flows of andesitic lava and three layers of rhyolite can be distinguished.
The oldest rocks consist of a series of andesitic lavas of unknown depth, and it is in this series that the quartz reefs lie. These reefs were formed by extremely hot solutions containing dissolved quartz (silica) ascending through cracks and fissures in the rock. As this liquid cooled the quartz was deposited, and it so happened that in Waihi a quantity of gold and silver was also in solution and was deposited with the quartz, thus giving the lodes great potential value.
Overlying this reef-bearing quartz andesite and separated from it by an old land surface is a younger series of andesites into which the veins do not extend. The wooded hills to the north and west of Waihi consist of this younger andesite. Fortunately this lava flow ceased before it overwhelmed the outcrop of quartz on Martha Hill or Waihi gold may have remained undiscovered for ever.
Overlying this hard black younger andesite and again separated by an old land surface, is a grey coloured rhyolite, often spherulitic. This rock forms the hills east of Waihi and extends southwards to Mt. Hikurangi. The greater part of the Waihi Plains is covered by yet another rhyolite, grey to purple in colour and containing included fragments of andesite and other rocks. It extends to Waikino and Waitawheta and may be seen outcropping in Clarke Street and lower Rosemont Road. The well known Queen's Head landmark, a few miles from Waihi on the Paeroa road, consists of this rock which goes under the local name of "Wilsonite." The shafts on the eastern side of Martha Hill passed through 100 feet of it.
The youngest flow of lava consists of a fairly hard rhyolite of light colour, and it occurs in patches along a line east of Waihi and Waikino. It outcrops at the quarries near the Recreation Ground and the Gasworks.
In addition to the above there occurs several outcrops of volcanic rocks of doubtful origin and age, the most conspicuous of which is Black Hill, which consists of a horne-blende andesite.