A PRE-TERTIARY NEPHELINE-BEARING ROCK 163 
both by the brightly polarizing specks and by the efferves- 
cence of the powdered rock. Apatite is particularly abundant 
in fine crystals and may be included in any of the other con- 
stituents. 
Olivine.— Olivine occurs characteristically in small crystals 
and rounded grains of the first generation. The high index of 
refraction, high double-refraction, parallel extinction, and dis- 
persion of the still unaltered grains are all characteristic of 
olivine. It is, however, an inconspicuous and infrequent constit- 
uent of the rock, and is much altered to serpentine, hematite and 
magnetite. 
In the order of their abundance the primary constituents 
should be named as follows: feldspar, nepheline, augite, horn- 
blende and olivine. Both the feldspar and nepheline occur in 
greater abundance than any one of the ferromagnesian constit- 
uents and together constitute perhaps two-thirds of the rock 
mass. The structure is not so characteristically that of either a 
dyke or surface rock as to make it possible to determine the 
exact position occupied by the rock when cooling. 
The character of the crystallization indicates the com- 
paratively slow cooling of a ‘‘hypabyssische”’ rock. This crystal- 
lization may have taken place along the edge of a deep-seated 
magma, throughout an intrusive magma or in the central part of 
a surface flow. 
A determination of the exact species of this rock from the 
study of a single bowlder must be more or less inaccurate and 
hence undesirable. Since a generic term only, can be affixed to 
the rock, the choice lies between the nepheline-syenite-porphyry 
and the theralite-porphyry groups. The highly alkaline char- 
acter of the feldspars which constitute so large a proportion 
of the rockmass relates the rock perhaps more closely to the 
nepheline-syenite-porphyry group than to the theralite-por- 
phyries. Like the nepheline-syenite-porphyries described by 
Brogger, it contains accessory olivine,* and comparatively 
*W.C. BROGGER, Die Mineralien der Syenit-pegmatit-gange der siidnorwegischen 
Augit-und Nephelinsyenit. Groth’s Zeitsch. fiir Krys. und Nien., Vol. XVL, pp. 32, 39. 
