A PRE-TERTIARY NEPHELINE-BEARING ROCK 161 
constituents of the rock to be plagioclase, nepheline, augite 
and hornblende. As accessory constituents, always present 
in some amount, are olivine biotite, apatite and magnetite. As 
secondary constituents are a zeolite, muscovite, kaolin, calcite and 
uralite. The rock is comparatively fresh and possesses a pan- 
idiomorphic holocrystalline structure. Secondary structures due 
to pressure are not manifest. 
Plagioclase.—The feldspar is fresh, idiomorphic and polysyn- 
thetically twinned. The twinning lamelle are indistinct and 
irregular. It is comparatively free from inclusions which, when 
present, are apatite, augite and hornblende crystals and micro- 
lites. Some of the feldspar was isolated and its specific gravity 
tested by the Thoulet solution. It proved to be 2.642 indicating 
a variety of the oligoclase-albite series. Feldspar of the second 
generation occurs in the groundmass, of which it constitutes the 
major part. It is broadly lath-shaped and does not show 
repeated twinning, but is, in many instances, twinned according 
to the Carlsbad law. Besides occurring in lath-shaped crystals 
it acts as a cementing material for the minerals of the ground- 
mass. The presence of muscovite is conspicuous as an altera- 
tion product. This alteration product, the high alkali percent- 
age of the groundmass and the optical properties, indicate that 
the feldspar of the groundmass belongs at the more acid end of 
the series, probably orthoclase in part. 
Nepheline—The groups of colorless, broadly prismatic or 
hexagonal crystals observed in the hand specimen prove to be 
nepheline. The hexagonal sections show the broad uniaxial 
cross and negative double-refraction of nepheline. The prismatic 
sections show parallel extinction, a weak double refraction, and 
two rectangular cleavages. The crystals contain as inclusions 
apatite and augite. The former is abundant throughout the 
sections in idiomorphic crystals, and is readily distinguished from 
the nepheline by its higher index of refraction. The nepheline 
is frequently quite fresh but sometimes shows alteration into a 
radiating zeolite which possesses the optical properties of natrol- 
ite. The alteration begins on the outer edge and the nepheline 
