0. JV. Fenner — Crystallization of a Basaltic Magma. 217 



Art. XYII. — The Crystallization of a Basaltic Magma from 

 the Standpoint of Physical Chemistry; by Clarence E\ 

 Fenner. 



Introduction. 

 Scope of the article. 

 Part I 



Process of crystallization as demanded by the laws gov- 

 erning entectiferons solutions. 

 Petrographic description of the Watchnng basalt. 

 Part II. 



The crystallization of a magma as affected by the law 



of mass-action. 

 Displacements of equilibrium within a solution effected 

 by changes of temperature and pressure — van't 

 Hoff's law. 

 Eesorption of olivine in the "Watchung magma. 



Introduction. 



Within the last few years petrographers have recognized 

 the important aid which might be rendered to the interpre- 

 tation of the structure and history of igneous and metamorphic 

 rocks by an application of the principles of physical chemistry. 

 The discoveries which have been made regarding the laws gov- 

 erning the crystallization of solutions, the application of the 

 phase-rnle of Gibbs and of the law of mass-action, and the new 

 conception of the phenomena of solid solutions, are believed to 

 be capable of rendering very great assistance in interpreting 

 the meaning of the structures with which petrographers have 

 become familiar. 



Although the applicability of these principles is generally 

 conceded, very little has yet been done in applying them to 

 specific cases. 



In making a study of certain peculiar phases of the basalt 

 which forms the Watchung Mountains in New Jersey, it came 

 to be recognized that the rather unusual conditions which had 

 attended its solidification had produced results which illus- 

 trated certain laws of the crystallization of solutions more per- 

 fectly than could be hoped for from the most elaborately 

 devised laboratory experiments. 



In a previous article* the author has shown that the Watch- 

 ung sheets were surface flows poured out over areas in which 

 Triassic shales and sandstones were accumulating under con- 

 ditions of continental sedimentation in structural valleys. f At 

 most points the basalts present the dense, noncrystalline 

 texture normal to this type of rock, but in certain areas the 

 flows appear to have spread over the sites of shallow lakes, and 



* Features Indicative of Physiographic Conditions Prevailing at the Time 

 of the Trap Extrusions in New Jersey. Journ. of Geol., vol. xv, No. 4, 

 May- June, 1908. 



f See also J. V. Lewis, Annual Eeport N. J. State Geol. Survey for 1906. 



Am. Jour. Sci.— Fourth Series, Yol. XXIX, No. 171.— March, 1910. 

 15 



