TERTIARY VOLCANIC ROCKS-PHONOLITE. 63 
The analcite is always clear, frequently shows distinct cleavages at right angles, 
possesses the rough surface clue to very low refractive power, and is practically 
isotropic, but frequently shows irregular patterns of the faintest polarization. 
JEgirine and segirine-augite .—These pyroxenie minerals are the only dark 
silicates of importance in the plionolites. zEgirine is the more abundant and is 
frequently present alone. It is purest and best developed where inclosed in analcite 
and there appears as narrow prismatic laths that are flatly tabular parallel to the 
orthopinacoid and terminated by low dome faces. Very small prism and clino- 
pinacoidal faces may occasionally be seen in cross sections. The mineral occurs 
in varying amount in different rocks. It ranges in size from prismatic phenocrysts 
several millimeters long down to the most minute needles and irregular grains in 
the interstices of the groundmass. A common mode of occurrence is "as a grain 
elongated parallel to the prism, full of included nepbeline and feldspar crystals, and 
with an outer zone which reaches out among the adjacent mineral grains as an 
oriented cement. It is frequently the case that the growth beyond the compara¬ 
tively solid core is mainly on the ends. Sheaf-like bundles of curving and forking 
gegirine needles occur in some rocks, usually in the denser varieties, and the forms 
assumed are sometimes very delicate.”® iEgirine laths are frequently seen wrapped 
around crystals of nepheline. Grains of magnetite are often the cores of segirine indi¬ 
viduals. Numerous irregular, disconnected grains, having similar optical orienta- 
iton over a considerable distance, occur in lines or planes through the rock and along 
both sides of analcite veinlets. The optical properties are as usual for the species. 
The angle lt:c is about 5°. The plechroism is intense: a = blue green, h = green, 
jC = greenish yellow to bright yellow, and a > b > c. Double refraction is high. 
The ends of some of the purer prisms sometimes show the brilliant yellows of 
acmite, and at times that variety occurs alone. 
The combination of this soda-iron molecule in an augitic pyroxene is rather 
common. The resulting segirine-augite occurs most frequently as solid cores to a 
surrounding mantle of segirine. From the center outward the properties grade 
progressively from those of pale green, almost isochroic augite with extinction angle 
a:c of over 40°, to those of pure segirine. The segirine habit appears to dominate 
the crystals, although clinopinacoids are more developed than in segirine. . 
The principal recognizable product of the alteration of these minerals is limonite. 
Carbonates sometimes result from the segirine-augite. 
Blue amphibole .—A blue mineral was observed by Cross in some of the phono- 
lites and considered by him to be probably a soda-lime amphibole. Material 
recently collected shows the mineral with better development than in the specimens 
that he studied. It occurs mainly in stout prisms, usually much smaller than the 
maximum size of 0.1 mm. Cross sections have characteristic amphibole outline, 
bounded by prisms and prominent clinopinacoid faces, and occasionally narrow 
orthopinacoids. A section approximately at right angles to c gave a prism angle 
of 123° 40'. Prismatic cleavage is present as usual, but is less perfect than in com¬ 
moner varieties and often can not be seen in the prism zone. In many cases 
the crystals have definite terminal planes consisting of the base, which makes an 
a Cross, W., op. cit., p. 28. 
t 
