part 1] AXXIYERSART ADDRESS OF THE PRESIDENT. Ixix 



been igneous action during a season of relative quiescence, this too 

 has its own significant characteristics. 



We shall not, however, appreciate fulh^ the relation between 

 igneous action and crustai stress, unless we have regard also to the 

 petrographical facies of the rocks erupted at different phases of a 

 complete cvcle. It is becoming increasingly evident that magmatic 

 differentiation is closely bound up with progressive cr3'Stallization, 

 the essential factor being a separation, or partial separation, effected 

 between the crystalline and the fluid portions at one stage or 

 another of the process. Thanks to the researches of the Washington 

 chemists, this doctrine now rests upon a basis of laboratory experi- 

 ment, and Dr. Bowen has recently put forward a general theory of 

 primary differentiation founded upon this direct evidence. The series 

 of successive ' fractions ' begins with terms rich first in magnesian 

 and next in calcic silicates, and shows then a steady increase both 

 in alkalies and in silica. So far this agrees with what has been so 

 often observed in a series of related plutonic intrusions, following 

 one another in an order of increasing acidity with falling tempe- 

 rature. Bowen shows, however, that a rock rich in alkali-felspars 

 and free silica need not represent the final term. The continued 

 concentration of water and other gases in the residual magma gives 

 rise in the later stages to hydrolj^sis and analogous reactions, by 

 which the progress of crystallization is much modified. This 

 appears in the formation of orthosilicate micas in the presence of 

 free silica, and finally in the production of nepheline and other 

 basic alkaline silicates at the expense of the albite molecule. The 

 differentiation -series is thus extended by the addition of new terms, 

 still increasingly rich in alkali, and specifically in soda, but now 

 with a diminishing content of silica. The empirical law of in- 

 creasing acidity must therefore be replaced by one of increasing 

 alkalinity. An ideal complete series would begin Avith an ultra- 

 basic type, such as a peridotite, devoid of alkali, and end with 

 another ultrabasic type, such as an ijolite, ver}^ rich in alkali. 



This doctrine, for which much geological as well as chemical 

 evidence can be adduced, throws an interesting new light upon the 

 mutual relations of igneous rocks. For my present purpose it is 

 ■ sufficient to emphasize the general rule to be deduced from it. If 

 at any stage of progressive crystallization, or progressive fusion, a 

 separation or partial separation is brought about between the fluid 

 and the crystalline part, the former will be relatively lich in alkali, 

 and particularly in soda, while the latter reciprocal^ will be rich in 



