94 PROF. J. H. L. VOGT ON" LABEADOEITE-NOEITE [May I909,. 



changed. By continued cooling the separation of a is completed 

 along a curve P — Q in fig. 2a, which curve is shown by a straight 

 line P — Q in the projection (fig. 2b). At Q commences a simul- 

 taneous crystallization of a & b, which with falling temperature 

 proceeds along the curve Q — E a _ h _ c . At last, with constant 

 temperature at the ternary eutectic E a -b- c , the solidification of c 

 as well as of the rest of a & b takes place. 



"We have thus three stages of crystallization here : — (1) separa- 

 tion of a alone, (2) then of a and b simultaneously, and 

 (3) lastly of all three components simultaneously in 

 the ternary eutectic mixture. 



The complications which may be caused by supersaturation or 

 undercooling need not be treated of here. Regarding these points 

 I would refer to the treatment of them in my papers in Tscherm. 

 Min. Petrogr. Mitth. vols, xxiv, xxv, & xxvii (1905-1908). 



The ground-mass, after the crystallizing alone of a on the 

 curve P to Q, represents the composition of the mutual solution at 

 the point Q, that is at a point of the eutectic curve JE a _ b — E a . b _ c . 



In magmas the laws of the sequence of crystallization are 

 generally much more complicated than in the simple case described 

 here, which contains only three mutually independent components. 

 Magmas contain generally quite a number of components, among 

 which only some are mutually independent. Almost without 

 exception, some components stand in mixed-crystal relation to each 

 other. Then it must also be borne in mind that, as a rule, several 

 components have an ion in common. In the porphyritic labradorite- 

 norite here described, we thus find, as may be seen from the 

 enumeration on p. 90, at least eighteen different components, 

 among which some, however, occur only in small quantity. 



As I have explained in earlier memoirs, we may generally 

 presume that the components occurring in solidified rocks were 

 present also in the original solution. 1 To these must of course be 

 added components in which water, carbon-dioxide, etc., played a 

 part, although undoubtedly they were present as a rule only in 

 comparatively small quantity in basic magmas. The percentage 

 has been so small, that the influence of these components on the 

 crystalline succession may be left wholly or nearly out of con- 

 sideration. Consequently I shall here take no notice of these 

 compounds of water, carbon-dioxide, etc. 



The plagioclastic components, anorthite and albite, besides a little 

 orthoclase, may be looked upon as a unity when compared with the 

 other components. 



The hypersthene-components Mg 2 Si o 6 , Pe o Si 2 6 , etc. may like- 

 wise be classed together as against the magnetite and plagioclase ; 



1 See principally 'Die Silikatschmelzlosungen ' I & II, in Yidensk. Selsk. 

 Skrifter, Kristiania, 1903 & 1904; and the above-mentioned paper in Tscherm. 

 Min. Petrogr. Mitth. vol. xxiv (1905). The same theoretical view has been 

 lately maintained by Mr. Alfred Harker, ' Igneous Rock-Magmas as Solutions ' 

 (Science Progress, Oct. 1907), with reference to my previous works. 



