igii-l IDDINGS— PROBLEMS IN PETROLOGY. 293 



ent beyond our competence. Abstract conceptions of some of the 

 simpler relationships, based on partial knowledge of the factors 

 involved, serve to point the way along which quantitative investiga- 

 tion may be profitably pursued. 



The stoichiometric character of the chemical compounds that con- 

 stitute rock minerals relates them as definite functions to the chemical 

 constituents of the liquid magma from which they crystallized. The 

 existence of mixed crystals, and of solid solutions, introduces the 

 treatment of series into the problem of the expression of the relation- 

 ship between the mineral composition of a rock and the chemical 

 composition of its magma. In such an expression the fixed compo- 

 nents alone are involved. But there are definite quantitative rela- 

 tionships to be expressed regarding those chemical components of a 

 magma which may act only catalytically in producing the actual 

 mineral combination constituting the rock. Such actions may be 

 chemical, in the sense that compounds form that subsequently dis- 

 appear, as should H.O combine with SiOo to form hydrogen ortho- 

 silicate, 1^48104, and subsequently resolve itself into water and quartz 

 or tridymite. Or they may be physical, in the sense that increased 

 molecular mobility in the magma liquid may affect the character of 

 the crystallization by changing the freezing point and the nature of 

 the compounds stable under the conditions obtaining at the time. In 

 the broadest sense, then, the mineral composition of an igneous rock 

 is a function of the chemical composition of the magma. 



Since the physical conditions attending the solidification of rock 

 magmas affect the chemical equilibrium of the constituents, as well 

 as the physical character of the liquid, its temperature and viscosity, 

 and also influence the chemical composition with respect to the gas- 

 eous components capable of being held in solution under pressure, 

 the mineral composition of an igneous rock is also a function of the 

 physical conditions attending its solidification. 



To a notable extent is this also true of the texture of such rocks, 

 their degree of crystallization, size of grain, and the shape and 

 arrangement of the individual minerals. In the expression of these 

 relationships the treatment of serial functions must be a pronounced 

 feature. The gradual variations of temperature and pressure are as 



