402 Transactions. 



other hand, if the temperature gradient becomes relatively steep the lower 

 magma will ultimately junction with the upper. In view of the fact that 

 the lower magma may reasonably be expected to contain at least a small 

 amount of water and other mineralizers, either as gas or contained in an 

 aqueo-igneous differentiate, such junction means transference of water and 

 heat to the aqueo-igneous magma. Lateral extension of the fusion magma 

 will enable it to maintain a supply of water in gaseous form to the aqueo- 

 igneous magma. The latter, therefore, will work (or, in Daly's phrase, 

 "stope") its way upward at an accelerated pace. So the action will go 

 on until either thermal equilibrium is established or some new condition 

 connected with approach towards the earth's surface comes into play. 



Most geologists will probably concede that in some such way as that 

 indicated above solid rocks comparatively near the earth's surface may pass 

 into the liquid condition. The prominent role here assigned to water will, 

 however, be disputed, and doubtless strong arguments can be marshalled 

 against statements resting on so hypothetical a basis. 



Magmatic Differentiation. 



Under the assumptions made in this paper a magma during formation 

 will necessarily differentiate into two, or perhaps three, parts, distinguished 

 by differences in water-content. The preference displayed by water for 

 silica and alkaline silicates leads to the belief that the aqueo-igneous por- 

 tion of the magma will be relatively light and acidic, whilst the fusion magma 

 will be heavy, basic, and non-aqueous. The igneo-aqueous portion, inter- 

 mediate in position, will probably be intermediate also in chemical com- 

 position, but may incline to acidity. Further differentiation, it is easy to 

 imagine, will result through the cooling of portions of the magma to the 

 point at which crystallization begins. The opening of a passage to the 

 earth's surface, whereby part of the magma may be extruded, will also give 

 rise to differentiation, which under some conditions may be of a varied 

 nature. The absence of water probably limits _ differentiation very con- 

 siderably ; and if it be possible for a differentiated magma to rise as a whole 

 to a temperature above that required for an ordinary fusion magma, then 

 it may be assumed that convection currents will check differentiation, and 

 tend to bring about an approximately uniform composition. 



Formation of Granite. 



An aqueo-igneous magma will in general have the composition of a 

 granite or an acid diorite. Ultimate consolidation of a granitic magmi. is 

 largely influenced not only by cooluig, but also by conditions permitting the 

 escape of water, such as obtam when a rising magma approaches the sur- 

 face of the earth. The consolidation temperature of granite may be betw^een 

 575° C. and 800° C* (8, p. 342), but, according to some geologists of the 

 French school, a lower temperature is more probable. It may be observed 

 that under the assumptions made in this paper some granites probably 

 represent more ancient re-fused granites, gneisses, and rocks of sedi- 

 mentary origin, and thus their formation completes a cycle of change. 



* The quartz of granite shows by its etch figures that it was once fi quartz, into 

 which ordinary a quartz passes at 575° C. (552° C. according to Brun). The upper limit 

 is 800° C, because at that temperature quartz inverts to tridymite. It may be sug- 

 gested, however, that extreme pressure would modify these data. 



