IGNEOUS ROCK-MAGMAS AS SOLUTIONS 249 



to the mutual solution of the two constituents. Similar results 

 are obtained for olivine and melilite, augite and melilite, anor- 

 thite and melilite, etc. Thus, for several pairs of constituents, 

 the eutectic proportions are determined by two distinct methods ; 

 first by the order of crystallisation, and secondly by the 

 maximum lowering of freezing-point. That the results obtained 

 by these two methods show a satisfactory agreement, to such a 

 degree of approximation as can be expected, affords a strong 

 inductive proof of the validity of the solution hypothesis in this 

 application. 



Taking his investigations of slags as a starting-point, Vogt 

 has endeavoured to work out on broad lines the application of 

 the known laws of solutions to the crystallisation of igneous 

 rock-magmas. His presentation of the subject is to be regarded 

 as a first sketch, to be corrected and completed in the light of 

 further knowledge. At present serious limitations are imposed 

 by our ignorance of the thermal and other physical properties of 

 the silicate and other compounds which take part in the com- 

 position of the rocks, and experimental research in this direction 

 is the first requisite before the petrological inquiry can be 

 prosecuted farther. That the difficulties of such research, 

 though great, are not insuperable is proved by the work already 

 accomplished in the Geophysical Laboratory of the Carnegie 

 Institution at Washington. 1 



Meanwhile it appears that the data furnished from the petro- 

 graphical side are quite sufficient for testing some of the more 

 important principles, and for obtaining specific results which 

 have at least an approximate value. The interpretation of 

 the porphyritic structure will illustrate this. The porphyritic 

 crystals in lavas may often represent, as supposed by Rosen- 

 busch, the result of " intratelluric " crystallisation, prior to the 

 extrusion of the magma ; in which case these elements and the 

 general ground-mass belong to two distinct stages of crystal- 

 lisation, divided by an abrupt change of physical conditions. 

 Several petrologists, however, have pointed out that there are 

 grave difficulties in framing an explanation on these lines for 

 the porphyritic structure in intrusive rocks. Into the arguments 



1 Day and Allen, The Isomorphism and Thermal Properties of the Felspars 

 (1905) ; also in Amer. Jonrn. Set., (4) vol. xix. (1905), pp. 93-142 ; Allen and White 

 " On Wollastonite and Pseudo-Wollastonite," Amer. Journ. Sci., (4) vol. xxi. 

 (1906), pp. 39-108 ; with other papers already cited. 



