185 
AUGITE-SYENITE. 
The augite-syenite of the Ilimausak region is an alkali- 
syenite related to the Larvik-type of the Christiania district’. 
It appears as a zone, only a few hundred meters wide, along 
the southwestern, southern, and southeastern margins of the 
igneous complex. 
In general appearance the rock shows only very small local 
.variations. It is a coarse- or medium-grained, somewhat wea- 
thered rock of grayish-white colour in which the dark-coloured 
minerals form conspicuous spots sometimes attaining a diameter 
of 1 centimeter. There are no traces of a parallel arrangement 
of the constituent minerals. The main component, the white 
felspar, does not show the lenticular form characteristic of the 
felspar of the Larvik-type but occurs in irregular grains without 
any definite shape or elongation; in some cases, however, it 
shows a strong tendency towards a thick tabular development. 
The common variety of the rock occurs irregularly intermingled 
with the type containing the tabular felspars and connected with 
it by transitions. 
Microscopic characters. — Under the microscope the mi- 
neral composition proves somewhat variable. In the variety 
which is here considered as typical and is found represented 
by specimens from all the chief localities, the main components 
are as follows: felspar (soda-orthoclase), augite of pale violet- 
brown colour, and olivine. In smaller amount the rock con- 
tains: nepheline, green hornblende, brown hornblende (barke- 
vikite), biotite, iron-ore, and apatite. In other varieties of the 
rock some of these minerals may be absent while others may 
1 W. С. BRôGGER, Die silurischen Etagen 2 und 3. Kristiania 1882, 
p. 256, aud Zeitschrift für Krystallographie XVI, 1890, p. 30. — There 
is also a noteworthy agreement between the Ilimausak augite-syenite 
and the ‘larvikite, type Il’ from Madagascar (see A. Lacroix, Les roches 
alcalines, 1902, p. 103). 
