B3A. INA TE. VOGI 
Arthur Holmes’ has used another method in order to determine 
the quantitative proportions of quartz and feldspar in graphic 
granite from dikes of granite-pegmatite (in Mozambique), viz., 
Rosiwal’s planimetric method. In this manner, from different 
localities, he found quartz amounting to 27.9, 27.1, 26.3, 25.6, 
25.3, and 24.2, average 26.1 per cent (calculated in percentage by 
weight) and rest, 73.9 per cent of microcline with the ordinary 
perthitic admixture of albite-oblioclase. Consequently we have 
the following determinations of the quantity of quartz in micro- 
cline graphic granite: 
Calculated from the quantitative analyses: 28.5 (too high ?), 25.5, 25.4, 25.3, 
24.6, 23.5 (?), 21.9 (too low?), 21.0 (?), and 20.3 (too low ?). 
By the Rosiwal method: 27.9, 27.1, 26.3, 25.6, 25.3 and 24.2 per cent 
of quartz. 
The majority of these determinations are subject to great sources 
of error, which may amount to several per cent. If we take this 
into consideration, I think I am justified in drawing the conclusions 
that the proportions of quartz and feldspar in microcline graphic 
granite from dikes of pegmatite are subject only to small variations 
or are practically constant, and that we may fix the proportion 
pretty closely at: 
26 per cent quartz:74 per cent microcline.? 
The graphic granite in granite-pegmatite dikes crystallized at 
a relatively late stage, viz., after the essential part of the mica and, 
most frequently, also a part of the feldspar had solidified. Only a 
trifle mica was left at the time for the forming of the graphic 
granite. But in addition to this, besides the components of the 
feldspars and the quartz, there was surely some H.O present, 
possibly partly connected with SiO, in forming a separate com- 
ponent (as H,SiO, [?]). The graphic granite will thus have 
crystallized from a solution, which consisted predominantly of the 
components of feldspar and quartz, but also of a little mica and a 
small quantity of a component, as H.O and H,SiO, (?). The 
graphic granite has in this manner crystallized at a eutectic boundary 
t Quart. Jour. Geol. Soc. London, Vol. LXXIV (2919), p. 77- 
2In “Silikatschmelzlés.,” IL (1904), I gave the proportion 25.75:74.25, which 
is practically the same. 
