252 E. B. Bailey Si G. W. Grabham—Plagioclase Felspars. 



reason, common to find phenociysts with a core of cloudy secondary' 

 albite surrounded by a broad or narrow rim of clear primary felspar of 

 more basic composition (PL X, Fig. 1). 



The identification of the albite is a simple matter. Its felspathic 

 nature is obvious, since the replacement has been molecular, and the 

 twinning of the original basic felspar has been retained. The refractive 

 index is less than that of balsam, and symmetrical extinctions range up 

 to 16°. It is also the general rule in a composite crystal, cut so as to 

 give symmetrical extinctions, that the extinction, for any particular 

 twin lamella, changes sign where the lamella continues across the 

 boundary separating the albite from the basic plagioclase. Definite 

 cases were investigated in detail according to the optical methods 

 perfected by Michel Levy, and it was found that the secondare/ fekpar 

 is albite in the strict sense., and that in some cases it replaces felspar 

 originally containing, according to Michel Levy'' s stereograms, no less than 

 80 per cent, of anorthite. The albite in general does not appear to 

 contain cavities corresponding with the disappearance of the anorthite, 

 for in a great number of instances the total bulk of its inclosures is 

 quite trivial. So much is this the case that there are slides in which 

 it is an open question whether the albite is primary or not, and others 

 (PI. X, Eig. 1) where a single phenocryst betrays its history by 

 retaining a tell-tale scrap of the original basic plagioclase. There can 

 thus be no doubt in many examples of a marked introduction of soda 

 into the altered felspar crystals. 



When we inquire into the source of this introduced soda, we find 

 one fact which stands out with special significance. We have seen 

 that, within the limits of any particular felspar crystal, the more basic 

 zones are the more liable to albitization, and that, within the limits of 

 any particular lava, the more basic felspar crystals are preferentially 

 attacked ; but now we find the seeming paradox that, within the 

 limits of any particular volcanic district, the more basic lavas have 

 escaped ivith less alteration than their neighbours.' If, with Termier, we 

 ascribed the change to the action of percolating surface waters, the 

 exact reverse would have been anticipated, for the basic basalts, on the 

 whole, contain the more basic varieties of felspar. This relative 

 immunity of the more basic lavas is especially well exemplified in 

 Arthur's Seat, where the lower basic portion of the volcanic sequence, 

 and also the well-known basic intrusions of the vent, have escaped 

 unaltered, while the upper, less basic portion of the lava group, 

 consisting originally of highly felspathic basalts and raugearites, has 

 been verj- extensively affected. In this selective principle we notice 

 at once an analogy between albitization and silicification, and it is 

 reasonable to suppose that the change is of the nature of self-digestion, 

 or. to borrow a convenient term from physiology, of autolysis. The 

 soda introduced into the altered basic felspars probably belongs to the 

 lava itself, and so far as this base is concerned the revolution is 

 a matter of internal politics. 



The inquiry cannot cease here, for it is of importance to determine, 

 if possible, the stage in the history of the rock when albitization 

 occurred. If it be conceded that the soda introduced into the altered 

 felspars belongs properly to the lava containing these felspars, then it 



