LATER STAGES OF EVOLUTION OF IGNEOUS ROCKS 43 



affinity of the various oxides for one another.^ Some molecules not 

 represented in the crystalline product, because relatively very 

 soluble, may have been present in the liquid in much greater con- 

 centration than those molecules which happened to separate as 

 crystals. Only a complete knowledge of the relative proportions 

 of all the compounds existing in the liquid could decide the question 

 of relative affinities. Thus it is sometimes stated that potash has 

 a greater affinity for silica than soda because certain rocks contain 

 orthoclase (KLAlSijOs) and nephelite (principally NaAlSi04), but all 

 that can be safely deduced from the association of these two minerals 

 is that among all the compounds in the liquid magma these two were 

 relatively insoluble under the conditions there prevailing. There 

 may have been much more combination in the hquid into the 

 molecules NaAlSijOg and KAlSi04. That there was at least some 

 combination of this kind is plainly brought out by the fact that the 

 orthoclase is never free from albite, nor the nephehte from kahophi- 

 lite. 



For a similar reason statements concerning the relative strength 

 of acids, e.g., silicic and carbonic acids, should be guarded. At 

 first sight the reaction BaCl2+H2S04=BaS04+2HCl might be 

 considered to indicate that sulphuric acid is a stronger acid than 

 hydrochloric acid, but we may immediately write the reaction 

 Ag2S04+2HCl=2AgCl+H2S04, which, on the same basis, would 

 prove the opposite. As a matter of fact, it is the extremely limited 

 solubility of barium sulphate and silver chloride which determines 

 the fact that both these reactions proceed practically to com- 

 pletion. If silicic and carbonic acids occur together in a solution, 

 all we know is that both will exist in a very great variety of combina- 

 tions; those with which we shall become acquainted in the crystal- 

 line state will be those which happen to exceed their saturation 

 limit under the particular conditions. 



It is clear then that in the discussion of the crystallization of a 

 magma references to relative affinities should, in general, be 

 avoided for the reason that the crystalline (least soluble) phases 

 give no evidence on that subject. The crystalHne product may, 



'See Johnston and Niggli, "The General Principles Underlying Metamorphic 

 Processes," Jour. GeoL, XXI (1913), 506-8, 513. 



