230 BULLETIN OF THE 
There is a variable amount of interstitial matter between the feld- 
spars of the groundmass which fills the triangular spaces left by their 
divergence, or appears as irregular areas in sections parallel to the flow. 
This substance is colorless, has a low index of refraction, is sometimes 
fresh and glassy, sometimes clouded by a fibrous substance, showing 
rarely traces of a rectangular cleavage, and containing scattering egi- 
rine needles. In some cases it polarizes so strongly as to be evidently 
a tabular feldspar section (or a third generation of feldspar), but gen- 
erally polarizes feebly, or is completely isotropic, and then gelatinizes 
with acid. The feebly polarizing part is probably nepheline, and the 
isotropic clear areas analcime, derived by alteration from the-nepheline. 
The egirine needles occurring as inclusions in the nepheline or analcime 
can hardly be regarded as secondary, since they are identical in size 
and parallel or network arrangement with the egirine needles so abun- 
dant in and between the feldspars, and evidently a primary constituent. 
Brogger! describes undoubted cases of secondary egirine in analcime, 
and J. Francis Williams? egirine needles in the analcime of the “gray 
granite” of Arkansas as secondary, similar in occurrence to those of the 
Montana rocks. ‘The interstitial element varies greatly in quantity, and 
may become so considerable as to give the rock a phonolitic character. 
All the specimens have the typical trachytic structure. 
(Hleolite) — Syenite Type. — This coarse variety occurs in the thick 
sheets. The rock has a gray color, passing into white as the decompo- 
sition of the feldspars increases, and has a tendency to porphyritic struc- 
ture. The feldspar phenocrysts, unlike this mineral in the groundmass, 
are in part fresh and glassy, —a fact which assists the optical determi- 
nation, —and have a fine striation on the basal cleavage. The minerals 
have an indistinct parallel arrangement, due to flow. The rock is about 
half as coarse as the “gray granite” (eleolite syenite) of Fourche 
Mountain, Arkansas, which it resembles. 
Character in Thin Sections. —In thin sections the structure is panidi- 
omorphous, the angular spaces between the feldspars being occupied 
by nepheline. The large feldspars are glassy clear, having the peculiar- 
ities of twinning previously described. The outer zones are sometimes 
opaque, owing to decomposition, and filled with zgirine needles. The 
extinction angles on basal cleavage sections (O P) (Specimen No. 145) 
were 2° to 4° to the trace of the second cleavage ; and on second cleay- 
age sections (g9 P g ) from 7° to 94° oblique to the first cleavage. The 
1 Mineralien d. Syenitpegmatitginge, p. 330. 
2 Igneous Rocks of Arkansas, pp. 68 and 79. 
