W. Cross — Igneous Hocks in Wyoming. 119 



discussion of the genesis of this deposit of nitre is at present 

 useless, it may well be pointed out that ammonia gas is a com- 

 mon exhalation product of volcanoes in their fumarolic stage, 

 and that ammonium chloride, salammoniac, is deposited in 

 clefts, fissures, or tubular cavities of lavas at Vesuvius, ^Etna, 

 Solfatara, Hecla, and other volcanoes. The lavas of the Leucite 

 Hills contained fluorine, chlorine, and sulphurous compounds, 

 as will be shown by the rock analyses, and it is certainly a 

 noteworthy coincidence, if nothing more, that one of the best 

 known occurrences of salammoniac is in the leucitic lavas of 

 Vesuvius, rich in potash. 



As regards the occurrence of North Table Eutte, there is no 

 special reason to assume a volcanic conduit at this point, yet 

 this occurrence would seem to suggest such a channel at no 

 great distance. 



Other minerals were not seen in this cavity, the nitre being 

 deposited directly on the rocks. The presence of soda-nitre 

 at the Boar's Tusk, described below, renders this occurrence 

 all the more interesting. Although the nature of the nitre 

 was not definitely recognized at the time of its discovery," the 

 peculiar astringent taste was noted then and the true character 

 as a nitrate was speedily established. Specimens of this nitre 

 may be seen in the National Museum. 



The Boar's Tusk. — Northwest of the Leucite Hills, in the 

 valley of Killpacker Creek, about 20 miles north of Rock 

 Springs, there is an interesting volcanic plug known as the 

 Boar's Tusk, a column of leucite rock rising about 300 feet 

 above the valley. Debris and the Eocene sandstones pierced 

 by the plug form a cone reaching to nearly half the height on 

 all sides. An outcrop of horizontal sandstone is to be seen 

 near the base of the cone. Seen from the east or west the 

 Tusk is broader and much less regular than in the view from 

 the south. 



The mass of the column is partly a compact breccia and 

 partly massive rock. On the southern and western sides is a 

 breccia made up of the same rock type which in massive form 

 reaches to the top of the column on the east and north, and is 

 apparently separated from the breccia by fissures. The breccia 

 is coarse or fine-grained, containing fragments several feet in 

 diameter. The principal component is always the leucite rock, 

 but mingled with it are numerous fragments of sandstones, 

 clays baked very hard, oolitic limestone, shell limestone, and a 

 coarse-grained feldspathic rock. 



In several places where the breccia was open and cavernous 

 a scanty white coating was observed on protected rock faces. 

 Unfortunately the character of this substance was not sus- 

 pected, and only a single small specimen was with difficulty 



