328 KEMP AND KNIGHT — LELX'ITE HILLS OF WYOMING 



valley of Killpacker creek, 25 miles or so nortli of Rock springs. The 

 valley is 5 or 6 miles wide, and from its central flat the pillar of the 

 Boars tusk arises like a monument. It is a double spire because of a 

 cleft which is due to a fault and })inch that traverses the neck in a north- 

 and-south direction (magnetic) and renders very platy the constituent 

 rock. On map 3 of the atlas of volume xii of the Haj^den Surve}^ the 

 Boars tusk is called "The Sentinel." F. M. Endlich, in volume xi, 

 page 133, speaks of it as " an isolated needle of basalt known as Rock 

 point." We adopt the name now current in the region. 



As shown in the illustration, figure 2, plate 45, the Boars tusk consists 

 of three parts. The base is a very fiat cone, all covered with fine talus, 

 lying at an angle of slope of about 17^ degrees. The diameter of the 

 base is just about four times the total altitude of the tusks. This frustum 

 has a vertical height of 100 feet. On it is set a frustum of coarser talus, 

 having a slope of 25 degrees, and continuing to the solid rock, which it 

 encircles with an irregular upper contour. Its vertical height is about 

 75 feet. It consists largely of huge fallen boulders from the tusk, with 

 v^hich are some finer materials. 



From this last frustum the tusk proper continues almost vertically 

 upward, but b}^ scrambling up the cleft between the two cusps, and 

 winding around, one could by some more or less risky work almost, if 

 not quite, ascend the lower cusp. The height of the higher cusp is 360 

 feet above the plain, as shown by solving a simple set of triangles, whose 

 altitudes could be determined by the aneroid and whose angles neces- 

 sary to solution were obtained by taking vertical angles with a Brunton 

 pocket transit. 



The material of the tusk is chiefly eruptive agglomerate and tuff, but 

 the whole mass is penetrated by the north-and south dike, whose width 

 is from 30 to 50 feet, and w^hich is sheeted b}^ the fault. The dike itself 

 contains many inclusions and much resembles a hardened tuff. The 

 boulders of the agglomerate are partly a cellular, eruptive rock and partly 

 fragments of shale and sandstone. Some of the shale is believed to have 

 been derived from the Green River beds, as it is exactly like their typical 

 representatives. Since the tusk and its frustra rest on Laramie, and there 

 are no Tertiary beds visible, it would follow that the explosive vent broke 

 through Green River strata, which must have formerly rested on the 

 Laramie at this point, and whose materials, after being blown into the 

 air, settled again into the vent and became involved with the tuffs and 

 breccia, now left in relief by the erosion of their former walls. Conse- 

 quently the outbreak was later than the Green River, and the Green 

 River beds have retreated far to the westward in the interval of time 

 since it occurred. There must have been during a part of this time a 



