90 ALFRED C. LANE 



I am not going to say all that might be said about such inter- 

 growths as perthite and sigterite (nepheline-albite), and other 

 pegmatitic iptergrowths as keys to eutectic proportions, because 

 I do not wish to write a treatise, but rather a review — I fear too 

 long — pointing out what seems to me inferences to be drawn 

 from Iddings's diagrams. But the loss of mineralizers might 

 change the eutectic. For instance, if the eutectic be Na^O • 

 AlgOg +6Si02 +6H3O • SiOg , a loss of H3O might mean the 

 replacement of the H3O by ^ (CaO "Al^ O3) which might be a 

 chemical quantitative change entirely worthy of recognition in a 

 quantitative chemical classification, if it proved of sufficient 

 importance ; as much so as any magmatic split. 



I think that the indications of Iddings's diagrams are that it 

 is not from a quantitative chemical standpoint of primary impor- 

 tance, although his figures take no account of the water. It is 

 not to be forgotton that in a holocrystalline rock the HgO of 

 the magma, while it may be in combination in biotite, analcite, 

 etc., is very likely to be concentrated in drusic and microdrusic 

 cavities, and not be noticed in the analyses at all, or, if at all, 

 then in the water given off below 110°; but a maximum idea of 

 this quantity may, however, be derived from the porosity, which 

 in the dike mentioned above is for the more crystalline part less 

 than a third of i per cent. 



While, therefore, Iddings's diagrams do suggest a natural 

 grouping and classification based on the various eutectics of 

 various magmas, yet the time is not yet ripe for such a perma- 

 nent arrangement. We do not know enough about the eutectics. 

 In the meantime, the new system of pigeon holes has some 

 advantages, and many, especially of the minor groups, may 

 endure. 



Still we can see that the old divisions of rocks might be 

 grouped around the average rock and given a more precise 

 chemical meaning, as follows : 



I. Acid, i. e., alkali-silica ratio 0.0 13 — , and silica percentage 

 0.58 + , or limited as in diagram, eutectic ratio toward which 

 crystallization takes place, alkali : SiOg : : i : 12. 



Granites and diorites, and many syenites. 



