MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 343 
however, especially for MgO, depend somewhat upon the Mg- 
bearing mineral component in question (biotite, hypersthene, etc.). 
2. As previously mentioned and as illustrated by analyses 
Nos. 14-19 and 20-22, plagioclase crystallizes when there is a 
surplus of Ab++An; orthoclase, on the contrary, when there is a 
surplus of Or in the original solution. The boundary lies, as 
previously explained, at about 0.4 Or:0.6 Ab+An. In the 
plagioclase which crystallized first, relatively much An appears, 
consequently relatively much CaO (and ALO,) (cf. the analyses 
Nos. 140-19)). In consequence, the remaining magma shows a 
decreasing percentage of CaO, and this in the “granite eutectic” 
falls to 0.25, 0.5, or 1 per cent CaO, or, with predominating 
plagioclase in the eutectic, not quite so low. 
If we leave magmas with only a trifle of Na,O (Ab), or of 
K.O (Or), out of consideration—where the crystallizing orthoclase 
absorbs practically all of the Ab or the crystallizing albite or albite- 
oligoclase the Or—the contents of Or (or K,O) in the magma 
remnant increases by the crystallization of plagioclase (cf. the 
analyses Nos. 14c-19¢ and 24c, 27c),and the contents of Ab (or Na,O) 
in the magma remnant increases by the crystallization of ortho- 
clase (cf. No. 21c). But we especially emphasize that this relative 
imcrease has a limited course, and that the limit of about 0.4 Or:0.6 
Ab or Ab+An ts not exceeded, or only perhaps now and then some- 
what exceeded because of supersaturation. In this matter we 
especially refer to the analyses of the glass basis or groundmass 
of porphyritic rocks. In judging these analyses, however, we 
must take into consideration that the glass, as is shown, for example, 
by the water content, is always or nearly always somewhat decom- 
posed, whereby especially a little alkali will be extracted. We 
further refer to the intervening material between basic concretions 
or orbicules in granites. 
As examples these intervening masses from casually chosen 
granites show: 
In Nos. 28 and 209, where orthoclase crystallized first, res- 
pectively 0.45 Or:0.55 Abyan and 0.46 Or:0.54 Ab+an. 
In No. 23, where both orthoclase and plagioclase seem to have 
crystallized at an early stage: 0.37 Or:0.63 AD+an. 
