FENNER, THE WATCHVNO BASALT 131 



the nepheline-syenite pegmatite veins of southern Norway, notes a de- 

 velopment of secondary albite in a form of mieroperthitic intergrowth 

 with orthoclase along cleavage lines. He finds evidence that the forma- 

 tion of this secondary albite belongs to a period of mineral-formation 

 previous to zeolite-deposition, and continues: 



One finds also at times that the albite of the microperthite was later re- 

 moved again during the zeolite-phase, whereby quite cellular, spongy, porous 

 feldspar-remnants result. Perhaps a small part of the analcite of the vugs 

 may even have been formed at the expense of the dissolved albite. 



The process and results described by him appear to be closely analogous 

 (taking account of somewhat diiferent conditions) to those which oc- 

 curred in the Watchung rocks. 



W. LixDGREN,^^ in discussing the minerals characteristic of various 

 zones of vein-formation, considers sada-lime feldspars unstable in all 

 vein zones, while albite is found in contact-metamorphic deposits, in 

 deeper vein zones and in middle vein zones. The physical conditions 

 prevailing in the Watchung rocks during the short period of formation 

 of albite appear to have corresponded, so far as can be inferred, to those 

 of deeper or middle vein zones. 



The stability of the albite molecule, under conditions under which 

 anorthite breaks down into other compounds, gives rise at times to the 

 special effect which Sir Archibald Geikie^** terms "albitisation," 



a process in which, while the lime of the plagioclase is removed or crystallized 

 as calcite, instead of forming a lime-silicate like epidote or zoisite, the rest of 

 the original mineral recrystallizes as a finely granular aggregate or mosaic of 

 clear grains of albite. 



F. Becke*" describes an occurrence of secondary albite which offers 

 considerable similarity to the manner of occurrence in the Watchung 

 rocks. His explanation of its formation is interesting and suggestive in 

 this connection. Eeferring to changes in the rock after consolidation, 

 epidote and zoisite are recognized among the products, and in the same 

 category is placed albitic plagioclase, which, in the form of irregular 

 veins of crystallographically parallel orientation, penetrates the original 

 plagioclase crystals. He continues: 



I cannot recognize in mechanical effects upon the rock the origin of these 

 new forms. I would much rather believe that our rocks, after the magmatlc 

 consolidation was finished, were for a long time under other conditions, in 



^ "The Relation of Ore-Deposition to Physical Conditions," Econ. Geol., vol. 2, pp. 105- 

 127, 1907. 



" "Textbook of Geology," London and New York, vol. 2, p. 790, 1903. 



" "Petr. Stud, am Tonallt der Rieserferner," Tsch. Mltth., vol. 13, p. 420, 1892-1893. 



