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 ai'eas 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 regi- 

 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 a;girine 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 wgirine needles so abun- 

 dant in and between the feldspars, and evidently a primary constituent. 

 Brogger ^ describes undoubted cases of secondary segirine in analcime, 

 and J. Francis Williams - a3girine 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 t3-pical tracliytic structure. 



(El<eolite) — SyenUe Type. — This coarse variet}' occurs in tlie thick 

 sheets. The rock has a gray color, passing into wliite as the decompo- 

 sition of the feldspars increases, and has a tendency to porphyritic struc- 

 ture. The feldspar phenocrysts, unlike tliis 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 al)out 

 half as coarse as the "gray granite" (ela3olite syenite) of Fourche 

 Mountain, Arkansas, which it resembles. 



Character in Thin Sections. — In thin sections the structure is panidi- 

 omorphous, the angular spaces between tlie 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 regirine needles. The 

 extinction angles on basal cleavage sections (0 P) (Specimen No. 145) 

 were 2° to 4° to the trace of the second cleavage ; and on second cleav- 

 age sections (go 1* oo ) fi'c^i ^° to d],° oblique to tlie lirst cleavage. The 



1 Mincralion i1. Syciiitpofimatitganfre. p. '■]?,(). 

 - Igneous Ivocks nf Arkansas, pp. (iS ami 70. 



