340 Professor S. J. Shand — Saturation in Petrography. 



case we may find for example olivine and silica together in the same 

 rock ". Such a rock would simply be classed as an aberrant variety 

 of the stable type towards which it tends. 



3. That the criterion of degree of saturation would make the 

 existing mineralogical system more definite than it is, while by no 

 means barring the way to future improvements. 



Mr. Scott replies that " penological affinities would be obscured". 

 I am tempted to ask whether some alleged affinities could possibly be 

 made more obscure than they already are? Jesting apart, it is my 

 opinion that most of the supposed affinities between loosely defined 

 rock types are partly subjective, and that in any case they are 

 incapable of quantitative treatment and must remain so until the rock 

 names themselves are more precisely defined. 



Mr. Scott also argues that the occurrence of solid solution may 

 make it impossible to determine the degree of saturation of a rock 

 and asserts that "many minerals can take up free silica ... in 

 solution ". A truer statement would be that " a few minerals have 

 been plausibly assumed to hold traces of silica in solid solution". 

 None of the known forms of silica is isomorphous with any other 

 rock mineral, and in the absence of isomorphism or chemical relation- 

 ship the amount of solid solution that can take place is very small. 

 Day and Shepherd 1 show that artificial pseudo-wollastonite (which, 

 by the way, is pseudo-hexagonal and so nearer in form to quartz than 

 any rock mineral is) may hold an amount which is "certainly less 

 than 2 per cent " of silica in solution. It remains to be shown that 

 any silica at all would be occluded if the magma contained another 

 molecule, such as nepheline, which combines with silica. It is safe 

 to say that the influence of solid solution in concealing traces of 

 quartz or of unsaturated minerals is too insignificant to effect even 

 the smallest subdivisions of a quantitative mineralogical classification. 



4. That rock names which transgress the boundary between the 

 saturated and the undersaturated rocks are inadequately defined and 

 should be avoided. I presume that my critic disagrees with me on 

 this point. 



The only positive proposals which I have made towards the frame- 

 work of a classification are as follows (my first paper, p. 513) : — 

 Class I. Oversaturated rocks. 

 ,, II.- Saturated rocks. 

 ,, III. Undersaturated rocks. 



(a) Monad metals undersaturated. 



(b) Dyad and triad metals undersaturated. 



(c) Both monads and dyads undersaturated. 



The scheme seems to me to offer a definite gain in significance and 

 precision of nomenclature at the cost of a very small rearrangement 

 of ideas. Subdivision of the classes will proceed according to mineral 

 ratios, and this maybe effected either in a purely arithmetical way, 

 as is done by Iddings, or with regard to eutectic proportions and the 

 order of crystallization. 



In concluding, I should like to thank my critic for his comments ; 

 adverse criticism is always helpful in " clearing the air". 

 1 Journ. Amer. Chem. Soc, 1906. 



