18 | ARCTIC BASALT PLATEAU 
grown together with the dark brownish rock, that it is impossible 
to separate them from the groundmass; the transition between rock 
and zeolites themselves by intergrowth is completely gradual and 
in places projecting only as indistinct spots instead of unsharp — 
phenocrysts in a normal porphyritic rock; there is no part of the 
rock without these spots. 
The true amygdale minerals are represented by icositetrahedrons oi 
analcite, measuring until 3 cm in diameter; the milky white crystals 
are sharp faced and seldom pink cloudy dulled. Optically they are 
anomalous, with biaxial sectors of low birefracting power alternating 
with isotropic patches and stripes. — 
On radiating, slender shaped prismatic crystals forming imperfect 
sphaerocrystals, the following optical properties were determined: 
elongation positive, plane of axes lying in the zone of prisma, 
y— acute bissectrix, thus „=; 
on flat shaped splinters, quite normal to £, in immersion liquid was 
found « = 1.18 0.001 
y = 1.489 + 0.001. 
On thin section by measuring with Babinets compensator: 
y—a = 0.01:[74], thickness 0.037 mm. 
7—B = 0.ooslo4] 3 Oro 
On free splinter with Babinets compensator: 
y—o = 0.008[35] bea i sitar? mae 
thus B= 1.481 
and, @— «= 0.003; 
from the refractive indexes calculated: 
2Vy = 63° [18] 
Dio 283. 
All these constants agree well with natrolite. — 
Another group of radiating coarse crystals with bright cleavage- 
surfaces and pearly lustre on them is intimately bound to the 
natrolite radiatings forming their immediate ulterior growth; the 
individuals are flat shaped, nearly leaved according to the best 
