F. E. Wright — Schistosity by Crystallization. 225 



the quantity 'of substance in the solution at one time being 

 extremely small." Through the superheated water as a 

 medium, adjustment by solution and deposition goes on con- 

 tinuously during the deformation. Under such conditions 

 minerals like the micas and amphiboles, which have a tendency 

 to grow most rapidly in one direction, develop with their 

 longer axes in the direction of least resistance, perpendicular 

 to the line of greatest stress ; and the texture of the resultant 

 rock will be characterized by a parallel arrangement of its 

 mineral components. 



Since in such metamorphic rocks the recrystallized silicates 

 belong chiefly to the micas, amphiboles and feldspars, all of 

 which cleave well, the cleavage of these rocks is due in large 

 part to the cleavage of their components. In short, the tex- 

 tures observed in metamorphic rocks of this type are charac- 

 terized by a definite orientation of mineral components recrys- 

 tallized at high temperatures under unequal stresses.* 



Beckef has applied the name Krystallizationsschieferung to 

 this process of recrystallization under stress and has reached 

 conclusions similar to those of Van Hise outlined above. 



A recent investigation by G. F. Becker and A. L. Day J on 

 the development of crystals under stress shows that although 

 crystals are able to grow T in a given direction in spite of a 

 counteracting force, their growth in the plane normal to the 

 pressure is vastly greater, the proportion being about 1 to 

 1000 or still larger. In their experiments, alum crystals were 

 used which are isometric and which have, therefore, no pecu- 

 liar direction of elongation. Had substances been tried which 

 crystallize in prismatic or tabular shapes, it is probable that 

 there, also, the direction of most rapid crystal growth would 

 have coincided with the direction of least resistance normabto 

 the active stress, as in the experiments below. 



In the Geophysical Laboratory of the Carnegie Institution, 

 several experiments were performed in imitation of this pro- 

 cess of nature, and results were obtained which roughly veri- 

 fied the preceding theoretical deductions. The problem which 

 confronted us was to produce crystallization from solution 

 under strain. Purely aqueous solutions could not be used, 

 jsince in them hydrostatic conditions obtain and stress differ- 

 ences are not possible. A glass, however, from a physico- 



* The arguments given in brief in this paragraph are essentially those of 

 Van Hise developed in extenso in his monograph, loc. cit. 



f Becke, F., Uber Mineralbestand und Struktur der Krist. Schiefer ; Sit- 

 zunsber. Wiener Akad., 7 Mai, 1903. This paper was unfortunately not 

 available to the writer. A brief statement of his conclusions, however, is 

 given by U. Grubemann in " Die Kristallinen Schiefer," i, (1904). 



% "The Linear Force of Growing Crystals," Proc. Wash. Acad. Sci., vii, 

 283-288, 1905. 



