MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 441 
According to my own extensive investigations of the question in 
hand (whereof I have only given a very short résumé) the facts 
previously set forth by Rosenbusch are confirmed, and we draw 
the conclusion that the sequence of crystallization of plagioclase and 
pyroxene depends upon the relative quantity of the two minerals, for 
the crystallization commences with the solidification of the mineral 
present in excess of a certain limit. 
I have bestowed much labor on the determination of this limit, 
trying to get it as exactly as possible, by structural investigation 
on a series of norites, and partly also of gabbros, of which we have 
numerous quantitative analyses, so that the proportion by weight 
between the plagioclase (with a determined Ab:An relation) and 
the pyroxene may be quite exactly calculated. We then find 
that the limit essentially depends on the composition of the plagio- 
clase. 
As an example may be mentioned that in a diallage-bearing 
quartz-norite with about 58 per cent SiO. (and standing on the 
boundary near quartz-hypersthene diorite), containing about 
28 per cent pyroxene (hypersthene with a little diallage), about 
60 per cent plagioclase (andesine, AbssAn,.), a trifle orthoclase, 
about 4 per cent biotite, about 1 per cent iron ore, and about 5 per 
cent quartz, the hypersthene in a great measure appears with 
idiomorphic contour against the plagioclase (andesine). In rocks 
with basic labradorite, as Ab,An., the plagioclase on the other 
hand partly shows an idiomorphic contour against the pyroxene, 
even when there is as much pyroxene as 35~-40 per cent present. 
The individualization boundary, determined by the sequence of 
crystallization, amounts approximately in the deep-seated rocks to: 
With Ab,.Any» about 45-50 per cent ae 
With AbsAns about 40-45 per cent pyroxene 
With Ab;An; about 35-40 per cent pyroxene 
With AbsAny about 25-30 per cent pyroxene 
With Ab,;,An;. probably 15-20 per cent ered 
the remainder 
plagioclase 
By pyroxene we here understand, with regard to the norites, 
hypersthene of the common composition of these rocks, viz., 
6.3-0.35 FesSi0;:0.7-0.65 MegsiO,, and, with regard to the 
gabbros, diallage with about the corresponding iron content. 
