228 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



Igneous rocks and magmas.— Among the aggregates the igneous 

 rocks are the most important, constituting as the}' do the original 

 matter from which the others have been derived. They are poly- 

 component systems with seldom fewer than six oxides, and it seems 

 out of the question at present to give a complete phase-rule discus- 

 sion of the chemistry of any such complex system. It does seem pos- 

 sible, however, to cover the field of actually occurring rocks by 

 partial systems based upon stable minerals, themselves combina- 

 tions of two or more oxides. 



Experimental work has progressed far enough to show that the 

 fusion and solidification phenomena of the igneous rocks are capable 

 of systematic treatment of the kind mentioned above, even when 

 the volatile components H 2 and C0 2 are included. 



Following this fundamental information on the fusion diagrams, 

 we must know the densities of the igneous rocks and their magmas 

 at all temperatures, with their changes of volume during solidifi- 

 cation, the textures and structures produced by various conditions 

 of solidification, the latent heats concerned in fusion of the magma 

 and in assimilation by it of other rocks, and many other physico- 

 chemical data. 



Some of the phenomena of differentiation of silicate rocks are 

 probably to be treated as results of simple crystallization backed up 

 by the effects of gravity in causing sinking or flotation of crystals. 

 This is the only method of differentiation that has been experi- 

 mentally proven, but the separation of two or more liquid phases, 

 and perhaps other phenomena also, may take part in this little- 

 known process. 



The phenomena of the movements of igneous magmas in the litho- 

 sphere must next be attacked. This involves studies of viscosity 

 and its changes with temperature; energy transfers accompanying 

 movement, including perhaps the conversion of potential gravita- 

 tional energy into heat energy by movement; differentiation due to 

 movement, and the effects of the separation of gaseous constituents 

 in causing differentiation, movements of magmas, and transfers of 

 energy. 



While the igneous rock is solidifying, processes of metamorphism 

 may go on as a result of the passage of its more volatile constituents 

 through the igneous rock itself and through its bordering rocks. The 

 rise of temperature in the inclosing rocks will also initiate reactions 

 and movements of material. The chemistry of these processes (al- 

 teration, serpentinization, contact 'metamorphism and replacement, 

 formation of pneumatolytic dikes and veins) will have been in- 

 cluded under the chemistry of the silicates with volatile components, 

 but the rates of the reactions and the transfers of energy accom- 

 panying them must be studied as a separate problem. The ques- 



