252 SCIENCE PROGRESS 



able modifications in the petrographical character of the 

 resulting rock. In the first place the porphyritic elements 

 include felspar as well as quartz. Secondly, the total amount 

 of the porphyritic crystals is greater than it would otherwise 

 be, and the amount of ground-mass is correspondingly reduced. 

 Thirdly, the quartz crystals are corroded, as a normal incident 

 of the progress of consolidation and without any discontinuity 

 of external conditions. 



An important question relative to the solution theory of 

 rock-magmas is that of the ionisation or electrolytic dissociation 

 of the several constituents of the magma. In an aqueous solu- 

 tion of salts the water plays a special part, inasmuch as it serves 

 as a medium in which the other constituents become more or 

 less dissociated into ions, while the water itself is little, if at 

 all, affected. We cannot point to any constituent of an igneous 

 rock-magma as playing this passive role, but it cannot be 

 doubted that there is in such a magma a notable degree of 

 dissociation of some or all of the constituents. This follows 

 from the fact, established by Barus and Iddings, that rock- 

 magmas are conductors of electricity. They showed further 

 that the conductivity is increased by rise of temperature, and 

 that, of the different magmas tested, the most acid one is the 

 best conductor. Beyond this we have no direct knowledge of 

 this subject. From analogy we may probably infer that, for a 

 given electrolyte at a given temperature, dissociation is more 

 advanced in proportion as the amount of this constituent in 

 the magma is less. The particular ions into which a given 

 electrolyte breaks up may be set down with some confidence 

 for the simpler rock-forming compounds. We may assume, 

 for instance, that the positive ion of enstatite is magnesium, 

 that of orthoclase potassium, etc. 



Electrolytic dissociation is one factor in determining the 

 mutual lowering of freezing-point and the eutectic ratio between 

 two bodies in mutual solution. In a solution containing three 

 or more constituents the interaction depending on dissociation 

 is of a more complex kind, and it may result that the eutectic 

 ratio between two constituents is very notably altered by the 

 presence of a third. In particular, the solubility of any con- 

 stituent is diminished by the presence of another which has 

 one ion in common with it. Now most rock-magmas contain 

 a considerable number of minerals, and it frequently happens 



