L. V. Pirsson — Phonolitic Rooks from Black Hills. 345 



to be paralleled with those occurring in various parts of Eu- 

 rope and especially in southern Norway. For phonolites they 

 are rather high in silica ; thus an analysis of " Nordmarkite " 

 or quartz syenite from Auerod as quoted by Brogger* has a 

 composition very similar but a greater amount of the ferro- 

 magnesian oxides in forming meta-silicates has left a little 

 more silica than was necessary to turn the alkalies and alumina 

 into feldspar and this excess appears as a minute amount of 

 interstitial quartz ; while in the present case as the amount of 

 bivalent metals is very small there has not been sufficient 

 silica to turn all the alkalies and alumina into feldspar and 

 nephelite has thus been forced to form. 



Another rock very similar to this has been furnished the 

 writer by Prof. C. E. Beecher, who collected it at Deadwood 

 in the summer of 1890. In regard to its occurrence Prof. 

 Beecher says "it occurs as a vertical dike about fifty feet wide 

 cutting through the schists and Paleozoic series in the moun- 

 tain just south of Deadwood. The specimen was collected in 

 the little ravine above the cemetery near the base of the 

 mountain and where the dike cuts the Potsdam sandstone." 

 The rock has a dense felsitic looking groundmass in which lie 

 reddish feldspar phenocrysts, very thin tabular on 5(010) and 

 often l cm across. They are not very numerous. Phenocrysts 

 of a glittering black hornblende occur freely sprinkled through 

 the rock. They are generally quite small, and needle-like but 

 are sometimes stouter and attain a length of several m.m. 



In thin section are seen in addition apatite, titanite, pyroxene, 

 nephelite, calcite and zeolitic material. The feldspar is mostly 

 sanidine in Carlsbad twins. A smaller amount of a striated 

 feldspar occurs. The hornblende is very idiomorphic, often 

 twinned on «(100) and has the following pleochroism : a light 

 brownish yellow, 6 olive brown, c olive green, with absorp- 

 tion c = B>o. The angle c A c is about 12° and the double 

 refraction is weak. A crystal extracted from the rock was 

 found to have the forms ra(llO), a(100), 5(010) and r(011) and 

 to be twinned on a(100). The prism m(110) is largely devel- 

 oped, the pinacoids being very narrow. On the reflecting 

 goniometer m^m (110^110) was measured and found 55° 45'. 

 Before the blowpipe the mineral fuses easily to a black shining 

 magnetic globule coloring the flame strongly and persistently 

 yellow — thus indicating the soda-iron molecule to be largely 

 present. From these facts it would seem to approach most 

 nearly to barkevikite, differing somewhat in pleochroism. 

 There is also a gray green augite present in the rock in much 

 smaller amount, which is generally greatly decomposed — 

 limonite and calcite having formed at its expense. 



*Min. der Syen. peg. gange. Zeit. fur Kryst., vol. xvi, p. 51, 1889. 



