520 DESCRIPTIVE GEOLOGY 



bounding mountain-ridges, are considerable developments of white hori- 

 zontal Tertiary beds, which have been referred to the Humboldt Pliocene. 

 These find their greatest development in Holmes' Creek Valley, just beyond 

 the limits of the map, whose waters drain into the Snake River. On the 

 low ridge which forms the divide between this valley and Thousand Spring 

 Valley, the upper bed is a drab- white, earthy, impure limestone, full of den- 

 dritic markings, and containing irregular-shaped cavities apparently left by 

 no longer recognizable shells. As developed in Holmes' Creek Valley, 

 the Tertiary beds seem to be largely made up of fine volcanic and pum- 

 iceous material. In this region, at least, their deposition seems to have 

 been connected with a period of volcanic ejection of sand and ash. A 

 peculiar castellated cliff on the east side of the valley, called Citadel Cliff, 

 shows an exposure of a hundred feet or more of these beds, which are reg- 

 ularly stratified and for the most part made up of fine, white, friable sand, 

 containing many small transparent glass particles, with a development of 

 beds of fine breccia material, in which are grains of pumice; the most notice- 

 able bed has a thickness of about 5 feet of a reddish-brown, nearly com- 

 pact, glassy rock, breaking with a conchoidal fracture, which is composed 

 of crystals of feldspar and quartz in a groundmass of red and black vol- 

 canic ash, consisting of thin splinters of obsidian partly fused together; 

 under the microscope, the quartz is seen to have large glass-inclusions. The 

 upper bed consists of a thickness of about 10 feet of soft, gravelly, cream- 

 colored conglomerate. Within the beds is also a thin seam, a few inches 

 thick, of a white, cherty material, banded with green, which breaks easily 

 with a conchoidal fracture. Its hardness is less than that of quarts, and it 

 probably contains a considerable mixture of felsitic material. A small coni- 

 cal hill not far from Citadel Clifi" was found to be capped by a black, glassy 

 obsidian. 



The hills between Holmes' Creek Valley and Thousand Spring Valley, 

 whose southern point extends into the region of the map, are made up of a 

 body of limestone overlaid by qiiartzites dipping to the south and west, 

 which have been referred respectively to the Lower Carboniferous and 

 Weber Quartzite group. 



Fountain Head Hills. — To the west of Thousand Spiking Valley, the 



