480 SCIENCE PROGRESS. 



their constituent minerals. A difference, though smaller in 

 degree, is noticeable in the mineralogical constitution of 

 many plutonic rocks and the minor intrusions belonging to 

 them. In other words, the minerals developed by the 

 crystallisation of a given magma depend not only on the 

 composition of the magma, but also on the conditions 

 attending its consolidation. A classification founded prim- 

 arily on chemical principles, therefore, will sometimes bring 

 together rocks which in a mineralogical scheme would be 

 separated. We may note in passing one conclusion 

 to be drawn from such facts as those cited by Iddings — 

 though it is not the conclusion drawn by that author — 

 namely, the necessity of a " hypabyssal " division of rocks 

 to receive such types as the lamprophyres. 



A complete chemical grouping of igneous rocks would 

 probably take account of the proportions, firstly of silica ; 

 secondly of the alkalies with reference to the silica, and of 

 soda relatively to potash ; thirdly of lime, magnesia, and the 

 iron-oxides, relatively to one another. Having regard to 

 these points, it would be possible to construct a system 

 which would for the most part agree with a mineralogical 

 one. Where the two differed, the chemical system would 

 more accurately represent fundamental relationships of the 

 rocks, regarded as consolidated rock-magmas. 



Adopting the point of view just indicated, viz., that the 

 total chemical composition of an igneous rock is the prime 

 datum, we may inquire what other character or circumstance 

 is to be taken as a supplementary factor in such a scheme 

 of classification. And we are led logically to the conclusion 

 that the only remaining "independent variable" is the 

 ensemble of conditions governing t ] le consolidation of the 

 magma supposed given, or in other words the geological 

 environment, which is now represented for us by the mode 

 of occurrence and geological relations of the rock. We 

 have seen that this may influence, not only the structural 

 and textural characters, but also the mineralogical con- 

 stitution of a rock of given composition. The abyssal, 

 hypabyssal, and superficial (volcanic) conditions afford a 

 three-fold division, which is doubtless susceptible of further 



