294 IDDINGS— PROBLEMS IN PETROLOGY. [April 21, 



essential factors in the consideration of the physical conditions of 

 rock magmas, as the variations in texture and in mineral composition 

 are universally characteristic features of igneous rocks. 



The existence of definite quantitative relations between the min- 

 eral composition and the texture of igneous rocks on the one hand, 

 and the chemical composition of the magma and the physical condi- 

 tions attending its eruption and solidification on the other, rests on 

 the obedience of the component elements to the laws of physical 

 chemistry. These laws are not fully established, or known, at this 

 time, and the relationships involved may be too intricate to be com- 

 pletely expressed in customary mathematical terms, nevertheless, the 

 definiteness of the quantitative relationships can not be doubted, and 

 approximate expressions of them become problems for petrologists 

 of the future. 



In tlie consideration and correlation of all known igneous rocks, 

 variability in composition and texture and the existence of continuous 

 series are the most conspicuous general characteristics. The varia- 

 bility in the composition of igneous rocks indicates heterogeneity in 

 magma solutions. This mav be inherent in them, and represent a 

 condition of existence before the initiation of eruption ; or, as is 

 more probably the case, it may result from difi^erentiation of homo- 

 geneous magmas during periods of eruptive activity, within more or 

 less extended regions. Dift'erentiation results from difl^usion of com- 

 pounds in solid molecules, or less complex ones, either at the time of 

 separation as cr} stals, or earlier, through convection currents, differ- 

 ences in density, or dift"erences in solution pressure. The resulting 

 magma solutions differ only in the quantities of various chemical 

 compounds : the amount of some in extreme instances reaching zero. 

 Subsequently formed compounds are not inherently different from 

 those in other magmas except by reason of the amounts of certain 

 chemical components, which may be concentrated in some differ- 

 entiated parts ; as in the concentration of the rare elements in some 

 pegmatites; or by different combination of chemical elements through 

 catalytic agents. There are no inherent, or inherited, characteristics 

 of form, organism, or immaterial traits, as in living beings. The 

 magmas are simply dift'erently mixed solutions of inorganic com- 

 pounds. 



