400 Transactions. 



appropriate to the phenomena of solution. The well-known experiments 

 of C. Barus show that at 185° C. and upwards water has a strong solvent 

 action on soft glass. Lemberg shows that at 210° C. water slowly dissolves 

 anhydrous powdered silicates (1, pp. 308, 643, 770). If the solubility of 

 silicates does not materially diminish as 36.5° C. is approached, then, pro- 

 vided the pressure is adequate, a silicate solution will probably continue to 

 exist as a solution at higher temperatures. We may assume, for example, 

 a solution of silicic acid and hydrous silicates in an excess of water above 

 that chemically combined, the whole at a temperature of 400° C, or even 

 500° C. Such a solution may correspond to some aqueo-igneous magmas. 

 If the temperature of this hypothetical solution be raised it will reach 

 such a point that the water not chemically combined becomes potentially 

 gaseous — that is to say, it ceases to act as a solvent, and tends to separate 

 itself from the magma, but is held (more or less) in solution by the prevail- 

 ing pressure. Viscosity of the magma will also tend to prevent mechanical 

 separation. There may now be an inclination for the water combined with 

 silicates to break away from this union, but probably much or all as yet 

 remains chemically combined, and the hydrous silicates still mutually dis- 

 solve one another, notwithstanding that the temperature is lower than that 

 required for the ordinary fusion of anhydrous silicates. If the temperature 

 still rises, presumably in the end the combinations with water are broken 

 up. Since micas and amphiboles containing water form in both plutonic 

 and volcanic rocks, and since analcite and, it is believed, calcite occur as 

 primary minerals in various igneous rocks, there seems to be no difficulty 

 in supposing that the ordinary magma contains dissolved or combined water 

 at temperatures reaching or exceeding 1000° C. '' Dissolved water " in such 

 a magma is really dissolved steam, but the solution is a liquid, not " an 

 uncombined emulsion of rock and steam." At a temperature of, say, 1200° 

 to 1500° C. chemical combination of water with silicates, as indicated above, 

 may cease. Some steam will still be held in solution by pressure, but some 

 probably separates in the form of gaseous bubbles. Magma at a high tem- 

 perature (" superheated magma " of Daly) may be unable to dissolve more 

 than a trace of steam, and, if so, bubbles of aqueous gas or steam will rise 

 in the magma until either they reach a cooler portion where they can be 

 redissolved, or are stopped by the solid rock that forms the upper boundary 

 or roof of the liquid mass. Here the gaseous water and other volatile sub- 

 stances present may act on the roof rock, thus forming new magma. This 

 magma will necessarily contain water. 



From such considerations as those just stated it follows that there is 

 probably a continuous passage from so-called aqueo-igneous magmas at tem- 

 peratures not far above 365° C. to high-temperature magmas not requiring 

 the presence of water as a flux. The writer, with considerable hesitation, 

 divides magmas into three classes : — 



I. Aqueo-igneous magmas, in which much water is present, some 

 chemically combined, some in solution, and acting energetically as a 

 solvent. Temperature range, say, 350° C. to 700° C. 



II. Igneo - aqueous magmas, in which the water present is mainly 

 chemically combined. Any water not chemically combined is a gas, held 

 in solution by pressure, but probably not appreciably aiding in the fluxing 

 of the silicates forming the main part of the magma. Temperature range, 

 say, 700° C. to 1000° C, or more. 



III. Fusion magmas, which maintain a liquid form without the assist- 

 ance of water. Any water present is potentially gaseous, uncombined with 

 silicates, and held in solution by pressure alone. 



