TERTIARY VOLCANIC ROCKS-PHONOLITE. 
59 
MINER A LOGICAL CHARACTER. 
The characteristic mineral constituents of these phonolites are alkali feldspar, 
nepheline, sodalite, nosean, analcite, aegirine, and aegirine-augite. As accessory 
minerals occur titanite, apatite, magnetite, and occasionally small amounts of a 
blue amphibole, biotite, and zircon. Olivine occurs very sparingly in one rock. 
A few minerals are present in grains so small as to be indeterminable 
Feldspar .—The feldspars are limited to the potash-soda varieties. In fresh 
rocks the phenocrysts are clear and glassy. They are usually tabular parallel to 
well-developed clinopinacoidal faces. Other crystallographic boundaries are in 
general only poorly defined, though occasionally smooth faces parallel to the base 
and, still less commonly, good orthopinacoids are seen. Intersecting cleavages are 
nearly always apparent, the basal being a little more prominent than the clinopin¬ 
acoidal. Carlsbad twinning is prevalent and in one rock mass the Baveno law also 
is exemplified. No indication of polysynthetic twinning nor of microperthitic 
intergrowth was observed. The feldspars of the groundmass present two definite 
habits, with probably all gradations between. In some rocks they are typically 
microlitic, being developed in rude laths parallel to the edge 010:001. Cross sections 
are nearly equidimensional, but as in the phenocrysts the clinopinacoid is more per¬ 
fect than the base. Carlsbad twinning is almost universal and a transverse parting, 
probably parallel to an orthodome, is often observed. In other rocks, and at times 
even in parts of the same section with the microlitic feldspars, the groundmass is a 
granular aggregate of feldspar grains which utterly lack definite crystallographic faces. 
Both the porphyritic and the groundmass feldspars are, so far as observed, 
monoclinic. Although Cross mentions the presence of anorthoclase, no indication 
of triclinic character could be detected. Sections from the orthodiagonal zone 
exhibit cleavages at right angles, with extinction parallel. In sections from this 
zone which are cut perpendicular to a bisectrix, it is ascertained that the axial plane 
is in the usual position—at right angles to the clinopinacoid—that the ortho-axis 
is the bisectrix of least elasticity, and that the feldspar is optically negative. Sec¬ 
tions parallel to the clinopinacoid show the perpendicular emergence of c, and a 
maximum extinction angle relative to the trace of the basal cleavage of about +9°. 
The maximum double refraction is always low and the refractive index is notice¬ 
ably lower than that of Canada balsam. These feldspars then appear to be entireh' 
homogeneous and monosymmetric. The relatively high extinction angle in clino¬ 
pinacoidal sections may be attributed to normal isomorphous replacement of pot¬ 
ash by soda. The feldspars of these rocks may therefore be called soda-orthoclase.° 
The most abundant product of l^drometamorphism of the phonolite feldspars 
is kaolin, probably accompanied by a little finely divided silica. The groundmass 
feldspars are frequently less kaolinized near individuals of segirine than elsewhere. 
In certain rocks, however, some of the phenocrysts have been partially changed 
into a colorless isotropic material, of lower index than the feldspar, which is prob¬ 
ably analcite. This alteration is not seen in very much decomposed rocks. The 
analcite almost always occurs in patches within the feldspar crystal, and the process 
by which it is formed is not revealed. Close to mineral veins the feldspars are 
partially changed to sericite. 
aCf. Pirsson, L. V., Am. Jour. Sci., 3d ser., vol. 47, 1894, p. 342. 
