Morgan. — Wate)- in Rock-mogmas. 401 



If in rock-magma a distinction between solution and chemical com- 

 bination cannot be made then classes I and II must be merged. In any 

 case, there cannot well be any hard-and-fast lines between the three classes. 

 Transition temperatures will depend to some extent upon conditions of 

 pressure and of chemical composition. 



Hypothetical Distribution of Magma in the Earth's Crust. 



If the influence of pressure, which is practicaly constant for a given 

 depth and that of chemical composition, which is of minor importance in 

 the present discussion, be discarded, the presence or absence of liquid rock 

 at a stated depth in the earth's crust depends within certain limits both 

 upon the temperature and upon the presence or absence of water. Pro- 

 vided sufficient water (or other "' mineralizer " or flux) is present, we may 

 have aqueo-igneous magma at quite moderate depths. Below this comes 

 igneo-aqueous magma, followed by fusion magma. Assuming that tem- 

 perature conthiues to increase towards the centre of the earth, and that 

 ordinary physical laws are not essentially modified by immense pressure, 

 we may suppose, with Arrhenius, that the fusion magma ultimately becomes 

 gaseous.* Into this zone, however, there is no present need to venture. 

 With a patchy and limited distribution of water in the earth's crust we may 

 have at relatively shallow depths disconnected reservoirs of aqueo-igneous 

 or igneo-aqueous magma surrounded by solid rock, with fusion magma at 

 some greater depth. Where water is absent we shall have fusion magma 

 only, confined by a solid roof of considerable thickness. It is quite possible, 

 and, indeed, probable, that the influence of increasing pressure is sufficient 

 to prevent the formation of fusion iriagma, except in those parts of the 

 earth's crust where the temjierature gradient is somewhat above the 

 average. 



Hypothetical Formation of Magmas, 



In order to narrow discussion, let us consider a segment of the earth's 

 crust assumed originally to have a temperature gradient of 1° C. in 200 ft., 

 and to contain appreciable amounts of water to a depth of ten or fifteen 

 miles, but little or no water at greater depth. Such a segment will be solid 

 to a depth of forty miles, below which may be ordinary fusion (universal) 

 magma. If now, through earth-movements or other cause, the temperature 

 of the segment rises, the fusion magma, owing to melting of the overlying 

 rock, will extend upwards. Even though the arguments in favour of a solid 

 earth be considered valid, yet it will doubtless be admitted that a sufficient 

 rise of the temperature gradient will cause melting in places, and this is all 

 that is here postulated. Sooner or later aqueo-igneous magma will begin to 

 form at a depth of, say, fifteen miles, and will gradually work its way up- 

 ward. Two cases, dependent on amount of heat supplied, are conceivable. 

 In the less -mportant of these, the temperature gradient failing to reach, 

 say, 1° per 100 ft., the fusion magma does not eat its way upward into con- 

 tact with the aqueo-igneous magma. The latter in this case will probably 

 cease its upward movement six or seven miles below the surface. On the 



* There are, of course, well-known astronomical reasons for believing that the earth 

 as a whole is rigid, and in discussion upon this paper Mr. G. Hogben pointed out that 

 the transmission of earthquake-waves through the earth seems to preclude the pos- 

 sibility of a liquid or gaseous interior, at least to a depth of eight hundred miles. Hence 

 appears an evident weakness in the above assiunptions. 



