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Volume 38, 1905
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[Read before the Otago Institute, 13th September, 1904.]

IT is manifest that the whole series of eruptive after-actions will commence at the moment of intrusion of the magma, and continue until the igneous mass has become completely cooled.

Igneous magmas are now admitted by petrologists to contain more or less water together with many constituents of a hydrous or gaseous character. Hence the fusion of magmas is not believed to be pyrogenetic—that is, the result of dry heat alone—but hydato-pyrogenetic—that is, fusion by heat in the presence of water.

According to Arrhenius* water renders the magma more liquid. It has been shown by experiment that magmas which require a temperature of 3,000° Fahr. to produce dry fusion can be fused in the presence of water at 500° Fahr. According to the same distinguished physicist water in a rock magma acts the part of an acid, liberating free silicic acid and free bases.

The activity of water at high temperatures is very great. Barus has shown that water heated above 185° C. attacks the silicates composing soft glass with remarkable rapidity; and Lemberg has proved experimentally that water at a

[Footnote] * Svante Arrhenius, “Zur Physik des Vulkanismus,” Geol. Fören. Forgh., Stockholm, 1900.

[Footnote] † C. Barus, “Hot Water and Soft Glass in their Thermodynamic Relations,” Am. Jour. Sci. iv, vol. ix, 1900, p. 161.

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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.

[Footnote] * C. R. van Hise, “Some Principles controlling the Deposition of Ores,” Trans-American Institute of Mining Engineers, vol. xxx, p. 27.