217 
(not very dark) and for a very pale greenish’. The biotite is 
often mingled with small grains of iron-ore and irregular ag- 
glomerations of titanite-like grains. 
Chemical composition. — The chemical composition is 
given in No. 22 of the above table. Upon a comparison with 
the analysis of the Narsak essexite (p. 203) it will be seen that 
the rock must be derived from an essexitic magma and may 
therefore according to the system of Rosensusch be termed а 
trachydolerite. The main difference from the trachydolerite of 
Nunasarnausak above described (p. 210) is in the alkalies which 
are higher in the Tasek rock. In reality this later rock ap- 
proaches the tephrites in chemical composition and it is prob- 
able that in fresh condition it has been nepheline-bearing. 
For comparison the analysis is given of a trachydoleritic lava 
from Meru which bears a close resemblance to the rock here 
described. 
ILIMAUSAK PORPHYRY. 
The Ilimausak porphyry has a dense violet-black ground- 
mass with rather small and sparse felspar-phenocrysts of a 
shape that gives square or short-tabular sections. It occurs in 
sheets of considerable thickness, probably effusive, in the 
highest parts of the Ilimausak mountains. The sheets are in- 
tensely altered by contact-metamorphism and owing to the ex- 
tremely fine grain the microscopic examination gives rather 
unsatisfactory results. 
The specimens to be described were taken at a height of 
' The brown, strongly pleochroic mica so frequent in contact-meta- 
morphosed rocks is wanting in the Tasek-porphyrite but is otherwise 
very extensively spread in the igneous rocks of the Ilimausak region. 
In a diabase from the summit (1370 meters) of Mount Steenstrup the 
brown mica is partly converted into a more faintly coloured green mica 
of the same appearance as that of the Tasek-porphyrite. It is therefore 
probable that also the green mica of this rock is an alteration-product 
of brown mica. 
