FENNER, THE WATCHVXG BASALT 123 



ning repeated in two or three stripes. Terminal faces are often well . 

 developed. 



These features characterize the secondary plagioclase in most of the 

 slides in which it occurs. The identification of the plagioclase as albite 

 has been determined by numerous tests. The statistical method of 

 Michel-Levy, applied to a great number of crystals, gives maximum ex- 

 tinction angles of 15° to 16°. The index of refraction is slightly less than 

 that of balsam ; optical character, positive. Sections perpendicular to the 

 acute bisectrix Z give extinction angles of 13^° to 15°, measured from 

 the 001 cleavage. These tests are mutually confirmatory and indicate a 

 practically pure albite. 



An interesting feature observed in many instances is that the crystals 

 of vein feldspar are plainly in crystallographic continuity with the 

 plagioclase laths in the walls. It is notable, however, that among 

 those which project farthest, the larger and stouter forms show a ten- 

 dency to parallelism in a direction at right angles to the walls, as if their 

 growth had been favored at the expense of those which projected at more 

 acute angles. The average of size is decidedly larger than that of the 

 plagioclase in the unaltered basalt. It is what might be expected if the 

 original feldspars had grown until they occupied the space left by re- 

 ]noval of diopside and magnetite, and in some cases, the relations are 

 euch as to force the conviction that this has occurred. 



In the process of chemical readjustment by which crystals of a solid 

 solution place themselves in equilibrium with the surroimding medium, 

 when the composition of the latter becomes altered, two methods seem 

 possible. The crystal might wholly dissolve and new and distinct crystals 

 of the stable form might be deposited, or the identity of the crystal might 

 be preserved, while the excess of one constituent was removed by solution 

 and its place taken, by a sufficient amount of the second constituent to 

 supply the deficiency. The evidence of the thin sections described indi- 

 cates that the process was to a large degree at least of the latter nature, 

 the identity of the crystals being preserved. It is probable that this 

 method of substitution, however, can advance only when the liquid in 

 contact is very mobile and capable of penetrating within the interior of 

 the crystals. If of the viscous constituency of most magmas, the contact 

 would be merely at the surface and exchange between solid and liquid 

 could proceed only through the almost infinitely slow agency of osmotic 

 diffusion in the solid. 



Albite in much larger crystals and showing a very different form of 

 development appears in several slides. In these, the process of readjust- 

 ment by which labradorite was transformed into albite appears less evi- 



