27i PltOCEEDINGS OF SECTION" C. 



rocks appears to be dependenc on conditions similar to those controll- 

 ing the development of biotite, for they commonly accompany one 

 another in rocks of intermediate composition when either is present. 

 The particular kind of amphibole which forms in rock magmas 

 depends primarily on the proportion of elements present, and 

 secondaril}^ on attendant conditions which produce A^ariation in amj^hi- 

 bole from one magma. Tliis is strikingly illustrated in the two igneous 

 rocks from Gran, Norway, described by Brogger.* The magmas have 

 almost the same chemical composition, yet one crystallised into a 

 mixture of hornblende and lime-soda-feldspar, while the other crystal- 

 lised almost completely into hornblende, which contains all the com- 

 ponents of the feldspar and hornblende in the hrst-mentioned rock. 



That hornblendes are less stable compounds in igneous magmas 

 than pyioxenes and numerous other minerals is shown by the frequent 

 occurrence of paramoi'phs of other minerals after hornblende, com- 

 monly seen in so-cailed black borders, and the absence of correspond- 

 ingly changed crystals of other minerals. 



Another chemical principle involved in the ])roduction of pyro- 

 genetic minerals is tliat aii'ecting the formation of compounds that 

 possess conmion ions when in solution. It is known that when there 

 are in solutions ions capable of entering two or more compounds, the 

 concentration of the least soluble compound may be increased by the 

 entrance of ions derived from other compounds into its molecules, and 

 this may proceed to the complete incorporation of the common ions 

 within one compoimd, upon its separation in the solid phase. This has 

 sometmies been called erroneously '' mass action."' That compound 

 forms at the expense of another in any particular instance which is 

 the more stable under attendant conditions. Illustrations of this action 

 are found : in the case of the complex amphibole in the hornblendite 

 of Gran already mentioned; in aluminous pyroxenes (augites), which 

 contains corajionents capable of forniing lime>soda-feldspars, and in 

 numerous other rock minerals. This pujnciple, together with that, of 

 the crystallisation of isomorphous compounds, is probably concerned 

 in the production of the lime^soda-feldspars witli notable an^ounts of 

 albite molecvdes, as in andesine and labradorite, in magmas so low in 

 silica as to necessitate the production of leucite from the potassium 

 present, Avhen the more active potassium should have combined with 

 the silicon in a polysilicate (orthoclase), leaving the less active sodium 

 to enter orthosilicate (nephelite). 



Following out the discussion of all the probable compounds likely 

 to form under known chemical laws from molten rock magmas upon 

 cooling, and taking into consideration the relative chemical activities 

 of the several constituent elements in igneous rocks, it is possible to 

 dediice a probable mineral composition for anj^ given magma, under 

 given conditions of cooling. The mineral composition of ig-neous rocks 

 tlien becomes a necessary consequence of tlie chemical reactions likely 

 to obtain in molten rock magmas, and depends not only on the kinds 

 ond amounts of the elements present in each case, but also on the 

 conditions of temperature and pressure modifying the chemical 

 fictivities of the elements and the stability of the compounds. As 

 these conditions are known to vary with the experience of different 



* Brog-ger, W. C, Erupt. Gest. Krvst. Geb., vol. III.. 1899, p. 93, and Quart. 

 Jour. Geol. Soc, vol. L, 1894, p. 19. 



