Morgan. — Water in Rock-magmas. 399 



reference to so-called pneumatolytic action, Vogt quotes Arrhenius as saying, 

 " The solution in aqueous gas now gradually cools, and one substance after 

 another gradually separates from it. By reason of the great mobility of 

 the solution, and its consequent strong capability of diffusion, the minerals 

 (provided the cooling be not too rapid) are segregated in large crystals, such 

 as characterize a so-called pegmatitic structure. Gradually also the con- 

 stituents which longest retain a gaseous form — such as water and carbonic 

 acid — -escape '" (1, p. 644). 



If we consider the water in a magma to be chemically combined, no 

 difficulty arises as to its physical state until the temperature is sufficiently 

 high to destroy the combination. The more general view is to assume that 

 the water is in solution. Again, there need be no discussion as to its physical 

 state so long as the water or aqueous gas is truly in solution ; but the tem- 

 perature may become so great that steam separates from the magma in the 

 form of bubbles. In this latter condition it can no longer be regarded as 

 acting the part of a solvent for the containing magma. There is certamly 

 need both for a more exact terminology in order to prevent confusion of 

 thought, and for experimental work in order to furnish a better foundation 

 for magmatic hypotheses. 



The difficulties involved in the assumptions made by some writers are 

 thus stated from a chemist's point of view by Dr. James Moir : " All sorts 

 of geological authorities accept the belief that water can be made red-hot 

 and yet preserve its solvent properties. Now, every chemist and physicist 

 knows that above the critical point water can only exist as steam, entirely 

 devoid of solvent powers except for other vapours ; and this however high 

 the pressure. ... As rock-magmas are certainly not vapours, there is 

 no possibility of anything except an uncombined emulsion of rock and 

 steam " (7, p. 4). Dr. Moir relies upon the experimental work of Andreas 

 Smits and J. P. Wuite (5, abstracts, ii, p. 985), who found that the solubility 

 of sodium sulphate in water became zero at 365° C. This result, however, 

 cannot reasonably be extended to silicate solutions without experimental 

 proof. We know that the boiling-point of water containing dissolved solids 

 is above 100° C. at ordinary atmospheric pressure, and also that water at 

 temperatures above 200° C. exerts a strong dissolving power on silicates. 

 In the light of these facts it appears probable that an aqueous solution of 

 silica or a silicate will not necessarily cease to exist as such at 365° C. More- 

 over, there is some doubt as to whether the so-called critical temperatures 

 of w .ter and other volatile bodies are really constants irrespective of pres- 

 sures exceeding the critical pressures (2. pp. 460-61), and therefore it is 

 to be hoped that experimental data showing the solubility of silicates 

 in water at temperatures above 300° C. will soon be forthcoming. 



The whole matter may be regarded from another point of view. The 

 most enthusiastic supporter of aqueo-igneous fusion would not ask for the 

 presence of more than 10 per cent, of water in any magma. Is there any 

 real objection to regarding this amount of water as chemically combined 

 with the silica and silicates of the rock-magma ? The immense pressures 

 prevailing in magmas may bring about a real chemical combination such as 

 would be impossible at atmospheric pressure. 



Hypothetical Classes of Magma. 



A clear definition of the difference between water acting as a solvent 

 and water chemica ly combined in magmas must be left to the physical 

 chemist. In the following statements a return is made to the nomenclature 



