Vol. 65.] PROF. J. H. L. VOGT ON LABRADOEIXE-NORITE. 99 



When some plagioclase has separated out, the proportion between 

 anorthite and albite in the solution changes, auorthite decreasing, 

 albite, on the other hand, increasing. Consequently there is no 

 longer equilibrium between the solid and the liquid phase. If the 

 conditions for equilibrium exist, an exchange takes place between 

 the solid and the liquid phase, some anorthite leaving the already 

 solidified plagioclase-crystals and some albite entering into them. 



When there is a total want of equilibrium during the process 

 of crystallization stages of plagioclase succeed each other with a 

 very considerable difference between the first (a) and the last (w) 

 mixed crystal (see fig. 5, p. 98). 



With complete equilibrium there results as the last product of 

 crystallization a plagioclase (P x ) with the same proportion An : Ab 

 as in the solution (see fig. 6, p._9S). 



The first case is met with, although perhaps never in its extreme 

 form, in the relatively quickly solidified dyke- and effusive sur- 

 face-rocks, the difference between the first (a) and the last (w) 

 plagioclase being occasionally as much as that between bytownite 

 and andesine-oligoclase. 



In the case of the labradorite-norite here described there has 

 been a partial — and only a partial — equilibrium between the 

 solid and the liquid An-f- Ab-phase. Our rock contains : — 



23 per cent, of phenocrysts with 61 An : 33 Ab : 6 Or ; 



77 per cent, of ground-mass, containing G1'88 per 



cent, of plagioclase with 52 An : 42 Ab : 6 Or ; 



the average proportion in the whole rock thus being 55 An : 39 Ab : 6 Or. 



This also represents the An : Ab : Or-proportion in the magma 

 before the crystallization of the rock. The small quantity of 

 orthoclase may be left out of consideration in the following investi- 

 gation. We then get : — ■ 



In the solution 58 - 5 An : 4T5 Ab, 



,, ,, phenocrysts 65 An : 35 Ab, 



„ ,, ground-mass 55'3 An : 44 - 7 Ab. 



As the solidification of the rock here described proceeded very 

 slowly, the crystallization of the first mixed crystal must have 

 taken place with but little, if any, supersaturation. Of the solution 

 58*5 An : 41*5 Ab (or, if Or be considered, 55 An : 39 Ab : 6 Or) a 

 mixed crystal of the composition about 78 An : 22 Ab (or about 

 77 An : 20 Ab : 3 Or) has first separated out. This, as well as the 

 following mixed crystals, has — in the ' porphyritic ' period, during 

 the crystallization of plagioclase alone — been approximately in equi- 

 librium with the solution, the result being porphyritic crystals of 

 the composition 65 An : 35 Ab (or, if Or be considered, 61 An : 

 33 Ab : 6 Or). In their outer shell the porphyritic crystals are 

 somewhat poorer still in anorthite and richer in albite. 



During the crystallization of the ground-mass the equilibrium — 

 or approximate equilibrium — between the porphyritic plagioclase- 

 crystals and the plagioclase-components in the remaining solution 



h2 



