Pakk. — Thermal Activity and Metalliferous Veins. 21 



temperature 210° C. slowly dissolves anhydrous powdered 

 silicates. It is probable that at great depths the pressure 

 will be sufficient to hold the water in the form of a liquid in 

 a superheated condition.* At high temperatures both water 

 and steam possess a great capacity for dissolving mineral 

 substances. 



solfataric— i.e., formed by thermal solutions aided by 



Steam and Gases. 



It is well known that during and after volcanic eruptions 

 there are emitted enormous volumes of steam, also hydrogen- 

 sulphide, sulphur-dioxide, carbon-dioxide, as well as com- 

 pounds of chlorine, fluorine, and boron. These gaseous and 

 aqueous emanations come from the same source as the igneous 

 magma, accompany the magma in its ascent, and may pos- 

 sibly be one of the contributing causes of the eruption. 



Volcanic phenomena can be studied in many parts of the 

 world, but perhaps nowhere with more advantage than in 

 New Zealand. In the volcanic region of the North Island 

 there are thousands of square miles in which volcanic activity 

 can be seen in every stage and phase ; there are active, 

 intermittent, and extinct volcanoes, besides innumerable 

 geysers, fumaroles, and hot springs, active, decadent, and 

 dead. The active and intermittent volcanoes discharge 

 their lavas and fragmentary matter from single pipes, or 

 from lateral vents apparently connected with the main pipe, 

 and from fissure rents. The volcanic eruption at Rotoma- 

 hana in 1886 was from a fissure rent over six miles in length, 

 extending from the summit of Mount Wahanga southward 

 into the basin of Lake Eotomahana, and thence across the 

 rhyolite plateau to Lake Okaro.f The whole length of the 

 rent was the scene of great activity for some weeks after the 

 first great outburst. The geysers, hot springs, and fumaroles 

 occur in isolated groups, or along a line of fissure which often 

 runs along the floor of a valley, or lower flanks of a range of 

 hills. The geysers deposit siliceous and calcareous sinters, 

 mostly the former ; and the fumaroles native sulphur. Every- 

 where the air is pervaded with the smell of sulphur-dioxide. 

 The solfataric action is active, waning, or dead. With the 

 latter the vents are closed up by crustification. Where the 



* C. R. van Hise, " Some Principles controlling the Deposition of 

 Ores," Trans-American Institute of Mining Engineers, vol. xxx, p. 27. 



f (1.) Sir James Hector, "On th<' Recent Volcanic Eruptions at Tara- 

 wera," N.Z. Reports of Geol. Explorations, 1886-87, p. 243. (2.) S. Percy 

 Smith, " The Eruption of Tarawera," Wellington, 1886. (3.) Prof. P. W. 

 Hutton, " Report on the Tarawera Volcanic District," Wellington, 1887. 

 (4) Prof. A. P. Thomas, "Report on the Eruption of Tarawera," Wel- 

 lington, 1888. 



