Yol. 65.] WITH PORPHYRITIC LABRADORITE-CRYSTALS. 95 



indeed, probably tbe whole of the pyroxenic components — that is, 

 not only the hypersthene, but also the diallage-components — -may 

 essentially be classed together as a unity, as against the other 

 minerals. 



Magnetite is an independent component, as against plagioclase 

 and pyroxene. 



The components occurring in quite small quantity — mica, apatite, 

 spinel, and iron-pyrites, besides ilmenite — have, in all probability, 

 because of their small percentage, had but little, if any, influence 

 on the sequence of crystallization of the plagioclase, magnetite, and 

 pyroxene ; or, because they have an ion in common with several 

 other components, at all events only an insignificant influence. 1 

 As regards the study of the processes of crystallization of the three 

 chief minerals just mentioned, they may thus be left out of 

 consideration. 



After subtracting the mica, apatite, spinel, pyrites (besides 

 ilmenite), and re-calculating the sum-total of the chief minerals 

 to 100 per cent., we get the following mixture in the original 

 solution : 



72 p 8 per cent, of plagioclase-components. 



6 - n per cent, of magnetite-components (Fe 3 OJ. 

 207 per cent, of pyroxene-components. 



Regarding the crystallization of plagioclase and magnetite, these- 

 three components — or, let us say, groups of components — may be 

 looked upon essentially as independent one of the other, although 

 several have an ion in common (Ca in CaAL,Si.,0 3 and CaMgSi.,O c , 

 CaFeSi 2 ? , etc. ; Fe in Fe.Fe 3 4 and Fe 2 Si 2 6 , etc.). 



According to the foregoing observations we find in the labradorite- 

 norite here described the following three stages of crystallization : 

 (1) first plagioclase alone, (2) then plagioclase and magnetite simul- 

 taneously, and (3) plagioclase, magnetite, and pyroxenic mineral 

 (or minerals) simultaneously. If, then, we confine ourselves to the 

 chief minerals, and if we provisionally look upon the two jyyroxenic 

 minerals as one unit, we find here the same three stages of crystal- 

 lization as in the ordinary ternary systems consisting of independent 

 components. 



Consequently, to the processes of crystallization in this magma 

 may be applied the physico-chemical laws which have been men- 

 tioned above (pp. 93-94 and figs. 2 a, & 2 b) as applying to a ternary 

 system in general. The reservation must, however, be made that 

 the composition of the plagioclase changes somewhat during its 

 separation, and that our 'pyroxenic mineral' is not a single unit.. 

 The following diagram (fig. 3, p. 96) though not quite exhaustive, 

 may nevertheless illustrate the most important processes of crystal- 

 lization with which we are here concerned. 



In this diagram PI, M, and Hp represent respectively pure plagio- 

 clase, magnetite, and hypersthene (or pyroxenic mineral) ; E P i_ M , 



1 On the significance of a common ion in the order of crystallization I would 

 refer to certain passages of my memoirs already cited. 



