280 
The chemical composition of this rock is given in No. 29 
in the table above. For comparison two analyses of typical 
hedrumites from southern Norway are cited in the same table. 
It deserves notice that the hedrumite is chemically closely 
related to the pulaskite of Ilimausak (vide analysis No. 3, 
p. 124). 
Tinguaite-porphyry of Tavdlorutit. 
Tavdlorutit (about 800 meters above sea level) is the west- 
ernmost top of the mountain ridge which borders the southern 
side of the great nepheline-syenite area of Igdlerfigsalik. The 
country rock is a diorite at this place, but at a short distance 
east of Tavdlorutit the ridge consists of a red biotite-granite. 
At altitudes between 800 and 1000 meters this granite is cut by 
a number of tinguaite-porphyry dykes which contain crystals of 
lavenite, a mineral which has not been found at other places in 
Greenland. The width of the several dykes are from half a meter 
to six meters, and their direction is from N. №. Е. to 5. 5. W. 
The tinguaite-porphyry has a fine-grained or dense ground 
mass dotted with phenocrysts of felspar, nepheline, lävenite, 
sometimes also black mica, and a few grains of iron ore. The 
colour of the ground mass is greenish-gray or less frequently 
dark reddish-gray. When examined under the microscope the 
ground mass shows a typical tinguaitic structure: it is made 
up of ægirine in rather short needle-shaped crystals which are 
imbedded in a hypidiomorphic aggregate of felspar and nephe- 
line with some sodalite and fluorite. The felspar of the ground 
mass tends to assume a lath-shaped form, and consists of a 
more or less turbid microperthite; it is intermingled with abun- 
dant nepheline, partly converted into muscovite. The felspar 
phenocrysts are up to à millimeters long and of a tabular 
habit; they are soda-orthoclase. The nepheline phenocrysts are 
about 1 millimeter in diameter; most of them have been altered 
to muscovite. 
