630 J. H. L. VOGT 
commence until the surplus silicate mineral has solidified, so that 
the quantity of Fe,O, (and FeTiO,) in the rest of the magma has 
reached a certain amount. 
In this connection we choose as an example a rock with a surplus 
of plagioclase. This has previously been described in detail by 
myself, and treated above, viz., the porphyritic labradorite- 
norite from Flakstadden in Lofoten. In proportion to the whole 
rock 23 per cent labradorite was solidified first, and the crystalliza- 
tion of iron ore, simultaneously with the continued crystallization 
of plagioclase only commenced when the quantity of iron ore had 
reached 8.1 per cent Fe,O,+-0.9 per cent FeTiO,;. If we leave out 
of consideration the components present in small quantity (biotite, 
apatite, and ilmenite), we obtain the following figures for the 
commencement of crystallization of the magnetite: about 8.5 per 
cent magenetite; about 64.5 labradorite; about 13.5 hypersthene; 
about 13.5 diallage. This represents a point or a line on the 
individualization boundary between the labradorite and magne- 
tite in a very complicated system, magnetite:labradorite (42 Ab, 
6 Or, 52 An):hypersthene:diallage (the two last with many sepa- 
rate components). 
In hyperitic-structured gabbro and norite (with olivine gabbro 
_and norite), where at an early stage much plagioclase had solidified 
in the well-known lath-shaped individuals, the magnetite (and 
ilmenite), when present only in a quantity as small as 1-2 per cent, 
shows no sign of idiomorphic contour. 
On the contrary, the magnetite here only appears as an inter- 
vening mass chiefly between the plagioclase individuals (see Figs. 29 
and 30), indicating that much plagioclase had already solidified 
before the magnetite commenced forming. 
In this connection we refer to a treatise by S. Foslie (Kristiania) 
on ‘‘The Titanic Iron Ore Deposit at Ramséy and Its Processes of 
Differentiation,”’ where is described a hornblende gabbro, con- 
taining about 58 per cent plagioclase, 2.5 per cent magnetite, 
31 per cent hornblende, 6.5 per cent biotite, and 2 per cent quartz, 
with the magnetite formed later than the plagioclase. His photo- 
micrograph, Table I, Figure 1, accords quite well with Figures 29-30 
in this paper. 
t Geol. Survey of Norway, Aarbog for 1913. 
