35 
A thin section of another type of rock, of similar appearance except 
for its much finer texture, shows a holocrystalline mass of plagioclase 
grains, less than one millimetre in diameter, with a very minor amount of 
orthoclase. The plagioclase, by index of refraction and extinction angle, 
is proved to be nearly pure albite. The only other important constituent 
is a blue amphibole occurring as shreds and radiating aggregates that 
cut across the feldspar crystals. It is clearly a late mineral, produced 
after the crystallization of the feldspar. 
The amphibole is biaxial and negative, has negative elongation, and 
Y=b. Its pleochroic scheme is: X=blue; Y= violet; Z=pale yellow. 
The dispersion, y < v, is strong. The optic angle is large. The optic 
plane is parallel to the elongation. The indices are: 
a = 1-677 ± 0-003; = 1-687 ± 0-005; y= 1-692 ± 0 003. 
Typical amphibole cleavage is well developed on basal sections. 
The extinction angle XAC is essentially 0°. On account of the small 
size and scattered occurrence of the mineral grains, it is impossible to 
separate any for a chemical analysis. 
The amphibole falls in the hastingsite-riebeckite-arfvedsonite group 
of amphiboles, but does not check optically with any of them. In pleo- 
chroism it most closely approaches arfvedsonite, but its index is decidedly 
lower. What is probably a similar mineral is mentioned by Ussing 1 in 
connexion with the peculiar soda granite that has formed around sand- 
stone inclusions in augite syenite. His mineral, however, has an extinc- 
tion angle of about 10 degrees and occurs as a marginal zone around a 
katophorite-like hornblende. A similar occurrence in the syenite porphyry 
will be described later. 
Other varieties of the late acidic dykes are distinctly porphyritic, with 
phenocrysts of orthoclase, microcline, and albite, the latter in many eases 
showing zoning. Fine needles and larger prisms of aegirine (negative 
elongation and extinction Z A C in the neighbourhood of 90 degrees) are 
common. The blue amphibole described above is also quite commonly 
present. Magnetite, mostly in euhedral grains, apatite, and titanite are 
common accessories, and kaolinitic alteration products in many cases 
cloud the feldspar. Epidote occurs in a few cases, resulting from altera- 
tion of the pyroxenes. 
In one extremely fine-grained dyke a large crystal of pyroxene was 
observed. The interior is faintly green and exhibits an extinction angle 
of 40 degrees, thus checking up as augite. Surrounding this is a layer of 
green, pleochroic aegirine-augite (extinction angle of 66 degrees). Outside 
the aegirine-augite zone, near the edge of the crystal, are abundant small 
grains of magnetite or ilmenite and the pyroxene is practically entirely 
altered to an indistinct, colourless material. The iron oxide evidently is 
a decomposition product of the original pyroxene. The zone of aegirine- 
augite may be taken to indicate an increase in the amount of soda in the 
magma following the crystallization of the augite nucleus. 
In one or two thin sections a considerable amount of muscovite, in 
large plates, occurs. Some green biotite was noticed in the more acid 
varieties that occur as small dykes. Chlorite has in many cases formed at 
the expense of biotite, rarely from muscovite. A clear, red, iron oxide 
lies in some cases along the cleavage cracks in the muscovite; it is also a 
common associate of apatite. 
'Ussing, N. V.: Geology of Julianehaab, Greenland, 1911, pp. 54 and 117. 
