238 DESCEIPTIVB GEOLOGY. 



to suggest that this or a similar flow might once have extended in that 

 direction. 



Pilot Butte is a curious little conical, castle-like mound, rising about 400 

 feet above the surface of the plateau country, in the angle between Bitter Creek 

 and Green River, to the north of the railroad. It is a rudely circular mass, 

 scarcely 1,000 yards in diameter, ha^ang abrupt faces on all sides, and com- 

 posed of a rather singular volcanic rock, unlike any other found within the 

 limits of the survey. It is evident that the soft Green River Tertiaries, which 

 once surrounded and covered it, must have been eroded away in a similar 

 manner to those around Fortification Rampart. The main mass is a dark-gray, 

 rather porous rock, having at first glance somewhat the appearance of a basalt. 

 Its fracture is, however, less conchoidal, and the pores have not the rounded 

 vesicular shape characteristic of those of basalt. To the unaided eye, it pre- 

 sents an almost homogeneous mass, without distinguishable crystals. On 

 the upper surface of the Butte, the rock is of a light-green color, showing 

 plentiful little reddish specks, and its pores largely filled with white cal- 

 cite, while the weathered surfaces are of drab-white color, and of a rough 

 fleece-like texture, evidently altered at the time of outflow by contact with 

 the enclosing Tertiary beds. 



By the aid of the microscope, it is found that the feldspars, of which 

 the mass is largely composed, are mostly sanidins ; but among the smaller 

 crystals are some which have triclinic striation. Besides the feldspars, it 

 contains a little mica, whose decomposition has produced the reddish specks, 

 but no hornblende ; a more important and quite unusual constituent is found 

 in small, exceedingly well-defined, colorless, six-sided crystals of augite, 

 whose larger anglie is 133°. The rock has also a glassy base, containing, 

 besides opacite, numerous microlites, and may, therefore, be classed under 

 the head of the augitic trachytes, though it presents a rather unusual occur- 

 rence among these comparatively rare rocks. 



Bridger Basin. — In this basin, the Tertiary beds of the Bridger 

 group are comparatively undisturbed, while those of the lower groups show 

 none of the flexures and local dislocations, which have been noticed in the 

 eastern basin, except along the immediate base of the mountains in the valley 

 of Henry's Fork. The beds of the former group slope off" gently to the 



