328 fee SVOGE 
contain a great quantity, 40 per cent and still more, of foreign 
components, as quartz and Fe-Mg silicates. Petrographic experi- 
ence proves that in such rocks we can detect no difference in the 
horizontal distance of the two curves, and this is in best accordance 
with the general law cited on page 326. 
Or:Ab or Or:Ab+An.—Because of the extreme viscosity which 
characterizes melted KAISi,Os and NaAISi,Os (without or with 
only a small quantity of CaAlLSi,Os), the synthetic study of the 
system Or:Ab or Ab+An is connected with exceptionally great 
difficulties. We may therefore here use the analytic method, cf. 
my earlier publication in Tscherm. Mitt., Vol. XXIV (1905). 
When Or is predominant, orthoclase first crystallizes, and when 
Ab-+An is predominant, plagioclase first. The boundary (“indi- 
vidualization-boundary’’) is decided by the following method of 
investigation: 
In a number of rocks, where the proportion Or:Ab:An in the 
entire rock was determined on the basis of the rock analysis, we 
find crystallized as No. I: plagioclase at 32, 32, 32.5, 32-5, 33-5, 
3A, 36, 36, 37-5, 39-5, 40, and 4x Or:Rest Abita,; orihoclaye 
(or microcline) at 42, 43, 43-5, 43-5, 46, 47, 47, 50; 50, 50-5, 52, 
and 52.5 Or:Rest Ab+an.' Ab+an here represents Ab with a 
small but variable quantity of An, consequently albite, oligoclase, 
and andesine. 
Between the two “‘individualization-fields”’ lies the boundary 
for orthoclase: albite, oligoclase, or andesine at about 0.4 Or:0.6 
Ab or Ab4an, perhaps nearest 0.42 Or:o0.58 Abian, applying 
to magmas at high pressure and with predominant Or+Ab+An 
with some mixture of other components. This boundary must be 
interpreted as an eutectic boundary curve. ; 
When orthoclase (or microcline) crystallizes at high temperatures 
from granitic magmas, which besides predominant Or also contain 
a good deal of Ab+an, there enters in the orthoclase up to about 
30 per cent (or 28 per cent) of Ab+an, which is partially separated 
by the cooling of the solid solution as albite or albite-oligoclase- 
«Two uncertain determinations with 38 and 36 Or are here left out of 
consideration. 
