TERTIARY VOLCANIC ROCKS-PHONOLITE. 
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Nei)heline.—T\\e distinctive mineral of these rocks, as of phonolites in general, 
is nepheline. It is a common constituent of every section studied and in several 
rocks makes up a large amount of the whole. It almost invariably exhibits auto- 
morphic development and occurs in the usual habit of hexagonal prisms bounded 
by basal planes. These crystals vary in size from 2 mm. down to a few thousandths 
of a millimeter. Their distribution through the rock is usually fairly uniform. 
The large crystals are naturally less abundant than the small ones. Nepheline 
appears to have cyrstallized at different times in different rocks. In some it is 
older than the feldspar and aegirine phenocrysts and was formed when the ground- 
mass was stifl fluid. In other rocks it is limited wholly to the groundmass, where it 
occurs with and between the feldspar microlites. In the majority of cases, no 
matter how small its crystals, it is older than the aegirine phenocrysts and occurs 
thus as abundant minute inclusions in the pyroxene. Occasionally the nepheline 
incloses small needles of aegirine and blue amphibole. Particularly noticeable is 
the partial or complete surrounding of nepheline crystals b} T analcite, while at times 
the two minerals are mutually allotriomorphic. 
In addition to its usual morphological and optical characters, the presence of 
nepheline is frequently indicated by the rock texture. Minute needles and inter¬ 
stitial grains of aegirine are commonly abundant in the groundmass, but seldom 
penetrate the nepheline, and the resulting numerous clear spaces, usually of rectan¬ 
gular outline, are often striking. Many of the larger nepheline crystals inclose 
numerous small flakes or scales of limonite or hematite, and this is doubtless the 
cause of their red color when viewed megascopically. 
A peculiar feature of some of the nepheline of these rocks is its behavior 
between crossed nieols. In some rocks a part of the nepheline, frequently a portion 
of a definite crystal, appears in polarized light as an aggregate of irregular elongated 
grains having indistinct boundaries, resembling almost exactly the subtrachytic 
groundmass, of which it frequently seems to form a part. This appearance is most 
noticeable in the larger individuals, but on careful observation it is found to be a 
property of much of the nepheline. Intergrowth of nepheline and feldspar is at 
once suggested, but the refractive index seems to be absolutely uniform through¬ 
out the crystal. This reason and also the freshness of the nepheline in parts so 
affected exclude the possibility of alteration products as the cause. The only 
other plausible explanation of the phenomenon is that the nepheline is optically 
anomalous. 
The common alteration products of this mineral are zeolites. They occur as 
small granular aggregates or as radially arranged fibers and cause the nepheline to 
become turbid. Their refractive index is low and they polarize more or less faintly, 
but in no case was their exact character determinable. Stilbite is probably one of 
these products. Cancrinite was not identified. In much-weathered rocks nephe¬ 
line may be completely removed. Sericite sometimes develops as pseudomorphs 
after nepheline near the walls of mineral veins, often before the feldspars show any 
sign of alteration. 
Sodalite and nosean .—Both chlorine-bearing and sulphuric anhydride bearing 
members of the sodalite group are important constituents of these phonolites. 
13001— No. 54—06-6 
