346 L. V. Pirsson — Phonolitic Rocks from Black Hills. 



These minerals lie in a groundmass of trachytic structure 

 consisting mainly of singly twinned feldspars between which 

 lie patches of a colorless mineral of very low single and 

 double refraction which is assumed to be nephelite. Also 

 shreds and fibers of segirine and the hornblende just men- 

 tioned are present. The rock is considerably altered and for 

 that reason no analysis is given. It effervesces considerably in 

 acid and gelatinizes readily. 



A rock from this region has been supplied to European 

 dealers by the specimen dealers in Deadwood. The exact 

 locality is unknown — the labels reading simply " Black Hills," 

 but it is probably from the northern part of the region. It is 

 sold under the name of "Tinguaite." The hand-specimen 

 shows a splintery fracture and is of bright green color with a 

 grayish cast and the rock strongly resembles some of the vari- 

 eties of the acmite trachyte from the Crazy Mts. already men- 

 tioned. The grain is dense and an occasional long columnar 

 augite is the only phenocryst to be seen. 



In thin section it is seen to consist chiefly of a singly 

 twinned feldspar arranged in trachytic structure with an occa- 

 sional patch of nephelite. It is everywhere penetrated by a 

 fine interminable mesh of segirine rods and needles which 

 sink to trichites in dimensions. While the feldspars are arranged 

 in flow structure these segirine needles are scattered without 

 orientation, pass through and through the smallest feldspars 

 and penetrate to some distance the larger ones. They par- 

 ticularly abound in the interspaces and with low powers give 

 the section a peculiar mossy appearance. To them is due the 

 green color of the rock. They are evidently a final product 

 of consolidation, forming after the moving mass which had 

 developed some augite phenocrysts and to a considerable ex- 

 tent the microlites of feldspar of its groundmass, had come to 

 rest. The augite phenocrysts are of segirine-augite with a 

 deep green mantle of segirine. The rock appears quite fresh. 

 It gelatinizes in acid. As the locality is unknown, no analysis 

 is given. 



From Caswell's report and from what has been shown in the 

 foregoing it is evident that in the Black Hills region there is 

 an extremely interesting series of high alkali-rocks, a fuller 

 knowledge of whose mode of occurrence and petrographical 

 and chemical relations would aid materially in solving for the 

 region that most interesting problem in petrology which Judd 

 has so aptly characterized in the term " petrographical prov- 

 inces." 



Laboratory of Mineralogy and Petrography, 

 Sheffield Scientific School, New Haven, Dec", 1893. 



