213 
metrical sections of such twins the extinction-angles were 
measured. The values obtained indicate that felspar is a 
labradorite of about the composition Ab,An,. The extinction- 
angle in some cases slightly decreases from the middle towards 
the margin of the crystals. These statements refer to the 
brownish felspar which constitutes the main component of the 
rock. The outer colourless shell which covers many of the 
brownish labradorite-crystals, in sections perpendicular to (010) 
shows a fine twin-lamellation and very small extinction-angles. 
It has an index of refraction somewhat lower than that of the 
labradorite but higher than that of Canada balsam. The outer 
shell consequently consists of oligoclase. 
Besides the crystals and pseudomorphs mentioned, numer- 
ous formless areas, consisting of aggregates of serpentine, 
muscovite, actinolite, and sometimes epidote and calcite, are 
seen in the slices. 
Chemical composition. — For the chemical analysis speci- 
mens were taken in a spot at a height of 560 meters on the 
north side of Nunasarnausak, near the lower boundary of the 
sheet. The results of the analysis are given in No. 21 of the 
table above. The composition is evidently of an essexitic type 
and very similar to that of the essexite of Narsak given in A 
of the same table. According to the system of Воземвозсн the 
rock may be characterized as a trachydolerite related to ne- 
pheline-basanite. In the table the analyses of some other 
related rocks are quoted for comparison. It is of special 
interest to note that rocks approaching these in composition 
occur in the petrographical province of the Christiania district. 
PORPHYRITE (TRACHYDOLERITE) OF TASEK. 
At the southwestern end of Lake Tasek and about 4 kilo- 
meters northeast of Narsak a mountain rises from the flat 
moraine-covered tableland to a height of 750 meters above the 
level of the sea. This mountain consists of a porphyrite here 
