FEATURES OF THE FELDSPARS. 237 



judging from the mass-analysis of the rock (page '241), must be albite. 

 .Many small areas of this inclosed feldspar occur with their axes in the 

 'same direction. Their material is not sharply defined from the surround- 

 ing eleolite, but appears to pass into it by insensible gradations. 



Of the feldspars the most abundant is the cloudy albite occurring in 

 the columnar crystals already mentioned. In the thin section these 

 possess long f|uadrangul ir forms, characterized by a series of remarkably 

 fine twinning lamellae, whose close study affords the best evidences of 

 the pressure to which the entire rockmass has been subjected. Indi- 

 vidual twinning plates often wedge out and disappear, while others spring 

 from the sides of cracks. Other lamella? are bent and bowed, some are 

 broken off sharply at cleavage cracks, while still others in the interior of 

 the grains are crossed by a second series of striations running nearly at 

 right angles to the first ones. There are also indications that some of 

 these grains are composed of two feldspars, for their resemblance to 

 Brogger's * pictures of cryptoperthite and microcline-microperthite is very 

 striking. The character of the two feldspars, however, has not been cer- 

 tainly established, though it is quite probable that albite and microcline 

 form one of the combinations. The specific gravity and composition of 

 these albites have already been given (page 234). Since they contain but 

 one per cent of K 2 it is quite clear that the potash molecule cannot 

 play a very great role in the intergrowths. 



The difficulty in determining the true nature of the constituent feld- 

 spars in these combinations is due principally to the fact that the large 

 grains are penetrated in all directions by jagged embayments of a pellucid 

 plagioclase with broader twinning lamellae than those of the turbid pheno- 

 crysts and without inclusions of any kind. Small areas of this glassy 

 feldspar occur all through the large albites, so that the latter appear to 

 be completely saturated with the former. The saturating feldspar often 

 has two sets of twinning striations. It polarizes in gray and blue tints, 

 and always has ragged outlines when it does not grade into the enclosing 

 albite. It seems impossible to assign any but a secondary origin to the 

 included material. The Large crystals arc so corroded by it that in some 

 cases but a slight film of the original substance separates the different 

 areas of the new substance from each other. ■ The different areas of the 

 new feldspar, moreover, are optically continuous with one another, as 

 are also d ill ei-en t poi-t ions of the enclosing albite, so thai the polarization 

 of the intergrowths is very like that of quartz and orthoclase in micro- 

 pegmatite. 



besides this saturating feldspar there are other feldspars occurring in 

 small grains, in some instances forming a sort of mosaic in which all the 



i W, C. Brogger: Z.n-. f. Kn-i b xvi, 1890, taf, xxii, fig 3,nnd taf. xxiii. liir. I. 



