600 PETROLOGY AND MINERALOGY 



readily soluble substance b. When such an overtaking does not take 

 place, and this is said to be true sometimes, the explanation may 

 be found in the varying degree of viscosity of the magmas and the 

 corresponding changes in rate of crystallization. If increasing vis- 

 cosity, i. e., increasing internal friction, opposed the crystallization 

 of a and b in equal degrees, the initial advantage of the former could 

 not be so easily overcome by the latter, if at all. 



Other physico-chemical questions in this province are as follows: Is 

 the order of crystallization influenced by the relative amounts of con- 

 stituents, and to what extent? What is the role of the as yet but 

 little studied Impfkrystalle ? Are certain uniformly fine-grained ag- 

 gregates, consisting of two minerals mixed in definite proportions, the 

 product of what Guthrie has termed "eutectic mixture," analogous to 

 the cryohydrates ? During the solidification of a magma, what role 

 is played by the mineralizers, les agents mineralisateurs , those sub- 

 stances, in part of a gaseous nature, which seem, by their presence 

 in the magma, to exert a purely catalytic influence upon the crystal- 

 lization, i. e., they seem to aid the latter process without either suf- 

 fering change themselves or entering into the composition of the 

 substances which are formed in their presence? Thanks to Iddings, 

 we are somewhat better informed as to the causes of the frequently 

 observed magmatic corrosion and resorption of already crystallized 

 constituents by the remaining magma. Apparently we here have to do 

 with a shifting of the condition of stability between the solid and fluid 

 phases of the magma. 



Physical chemistry will also render much-needed help in reaching 

 the explanation of the differentiation of magmas. This widespread 

 phenomenon, characteristic both of extensive eruptive masses and of 

 broad dykes, consists in a splitting-up of the original magma into 

 two submagmas, one of which is acid, predominatingly alkaline and 

 rich in alumina, the other basic, rich in iron and magnesia silicates, 

 but poor in alumina and alkalies. The former submagma almost 

 always has a central position, the latter submagma appearing as a 

 basic marginal facies at the periphery. These submagmas must have 

 originated by diffusions acting in two opposite directions during the 

 fluid condition of the original magma, and thus there arise two ques- 

 tions: First, What forces can have produced such a separation into 

 two submagmas so diametrically opposite in character, collecting the 

 bivalent metals with a small amount of silicon into one group, and 

 assembling the monovalent metals with somewhat more silicon into the 

 other group? Second, Why did the acid submagma assume a central 

 and the basic submagma a peripheral position? There are several ob- 

 jections to the direct application of the law proposed by Soret and am- 

 plified by van't Hoff , which states that those constituents with which 

 a solution is almost saturated tend to collect about the colder points. 



