PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 583 



Amphibolyte.] 



Mic. The original feldspar is entirely changed. That which now takes its place 

 was formed later than the hornblende, even later than the more recent portion of 

 the hornblende, since it encloses the hornblende crystals. It is triclinic and quite 

 fresh. It has n s in the acute axial angle; hence, its optical sign is +. It has the 

 twinning characters of albite. The lamellae are irregular in width and in length, 

 and interrupted before reaching the limit of the crystalline grain, and may therefore 

 be accepted as albite, at least provisionally. 



The augite is changed to green hornblende, which shows its prismatic cleavages 

 conspicuously, as well as its high polarizing colors. The crystals show two epochs 

 of growth, this being evinced by a colorless portion surrounding or irregularly 

 intergrown with the green portion, i. e., in irregular patches or areas, though always 

 limited in their growths by the geometric form of the mineral. In some cases the 

 geometric forms are not completed, and then the extremities of the section are 

 jagged or " fringed," as described by Williams.* The view of Van Hise,f however, 

 that these are secondary growths rather than the frayed ends of dynamically 

 deformed crystals, seems here to be applicable, since these growths, which are fresh 

 and colorless, and extinguish coincident with the original nucleus of the' crystals, 

 sometimes pass into zoned increments. These colorless enlargements are simply 

 later growths of the same mineral. The parts of these enlarged crystals do not 

 polarize with the same colors although they extinguish in unison. For instance, 

 the older portion, which always is green in common light, perhaps assumes a greenish 

 yellow, and the colorless part becomes lemon yellow. Sometimes the green mineral 

 becomes a dirty dark yellow and the colorless increment a purplish red, or again the 

 green part becomes brownish yellow, and the colorless part blue. This does not 

 indicate a difference of mineral species, but is due to the difference of the initial 

 coloration. That is, since the older hornblende is green in common light, and the 

 later is colorless, the points from which the double refraction is reckoned are different. 

 It is not to be presumed that the second growth followed immediately after the 

 first. It is apparent that the original crystal suffered' a long history, and was 

 coiToded and decayed. The uninterrupted growth of a crystal could hardly be 

 distributed to its various parts so capriciously as the colorless parts are distributed 



in the colored. This capriciousness may be shown by the 

 accompanying figure, in which a shows where colorless 

 areas appear in one crystal, b where they are in a second 

 crystal, and c where they are in a third. In the slide 

 FI ' ^ARY S GROTv I TH < s N iN > ^oRN- are various other combinations, the second growths, in 



several instances, being rather intimately ingrown with 



*Greenstono schist areas of the Marquette and Menominee areas in Michigan. U. S. Geol. Survey, Bulletin lxii,p. 126, 

 fig. 10, 1890. 



fSecondary enlargements of hornblende crystals. American Journal of Science(3), xxx, 231, 1885, 



