292 IDDINGS— PROBLEMS IN PETROLOGY. [April 21, 



into the silicate compound. The physical conditions which control 

 the chemical equilibrium within magma solutions that yield these 

 mineral compounds are problems for the geophysicist, though their 

 nature may be inferred in a general way from the mode of occur- 

 rence of the rocks containing the minerals in question. 



Indications of a catalytic action of H^O within rock magmas are 

 furnished by the association of free silica with orthosilicates con- 

 taining magnesium and iron, such as the common occurrence of 

 quartz and biotite in granitic rocks ; the frequent association of 

 quartz and tridymite with olivine in lavas ; and of quartz, tridymite 

 and fayalite in lithophysae in certain highly siliceous lavas. 



The instability of these systems under changed conditions of 

 equilibrium is shown by the inversion of hornblende to an aggrega- 

 tion of pyroxene, magnetite and feldspar, in some lavas ; and by the 

 solution of quartz phenocrysts in some basalts, accompanied by the 

 formation of shells of metasilicates surrounding them. 



Already laboratory research has established the range of stability 

 of some of the rock minerals under laboratory conditions : the inver- 

 sion temperatures under atmospheric pressures of the various forms 

 of SiOo, quartz, tridymite, crystobalite ; of the simpler compounds 

 crystallizing as orthorhombic and monoclinic pyroxene, and the 

 corresponding amphiboles ; of a simple system involving aluminium, 

 magnesium calcium silicates ; and of other series of compounds. 

 The value of these definite contributions to the problems of the 

 mineral composition of igneous rocks is great. Much more is 

 needed. And the necessity for eventually approaching nearer to 

 the physical conditions obtaining in rock magmas is apparent, when 

 the probable efficiency, chemical and physical, of highly heated 

 gases under strong pressures is taken into consideration. Research 

 under such conditions is attended with great difiiculties, and some 

 risks. Enough has been mentioned to show a wide range for future 

 study by the geophysicist, the chemist, and the petrographer. 



2. The Mathematics of the Petrology of Igneous Rocks. 

 The study of igneous rocks involves the consideration of groups 

 of intricate relationships, the exact expression of which is at pres- 



