48 REPOET UNITED STATES GEOLOGICAL SURVEY. 



this tile valley is wide and smoothly terraced, the terraces for the most 

 part being underlaid by thin sheets of basalt, which may be seen out- 

 cropping on both sides of the river. 



The belt of very compact granite is evidently the most easterly ex- 

 posure of these rocks in this valley, as they are overlaid by limestones 

 which appear on the trail a short distance above the last exposure of 

 granite. On the north side of the river the line of contact between the 

 granite may be traced by a depression which follows the strike of the 

 latter. Further back toward Slough Creek the limestones appear in 

 force, and on the broad plateau to the northwest of Slough Creek seem 

 to lie in massive strata nearly horizontally upon the granites. Lime- 

 stones again appear near the trail on Amethyst Creek, and at the mouth 

 of Soda Butte Creek we have 500 or GOO feet of limestones ; from these 

 I obtained Carboniferous fossils. These strata are everywhere appar- 

 ently horizontal and rest unconformally upon the granites. 



The paleozoic formations have very limited exposures in the deep val- 

 leys. They underlie the massive formations of the so-called volcanic 

 Tertiary age, which in this region reach their maximum development. 

 These strata rest upon the unevenly-eroded surfaces of the paleozoic 

 and granite rocks, and form a great part of the mountain ranges that 

 inclose the valley. They are horizontal, and apparently conformable 

 thoughout the entire thickness of 5,000 feet. The greater part of this 

 immense group of strata is filled with the silicified remains of ancient 

 forests. 



The section given in the accompanying plate occurs in the north face 

 of Amethyst Mountain, opposite the valley of Soda Butte Creek, and in- 

 cludes upwards of 2,000 feet of strata. The bed of the river is at an eleva- 

 tion of 6,700 feet above the sea, and the summit of Amethyst Mouutain 

 9,400. On the north side of the valley, near the mouth of Soda Butte 

 Creek, there are between 300 and 400 feet of Carboniferous strata exposed 

 along the base of the mountain slope. On the south side occasional ledges 

 of limestone appear above the detrital deposits. The valley is here less 

 than a mile in width, and is covered with thin sheets of basalt. 



The north face of Amethyst Mountain does not present as abrupt a 

 profile as that given in the section, the middle part only being sa pre- 

 cipitous. At the base and top there are comparatively gentle slopes ; 

 nevertheless the actual stratigraphical conditions are truthfully repre- 

 sented. 



As we ride up the trail that meanders the smooth river bottom, we 

 have but to turn our attention to the cliffs on the right hand to discover 

 a multitude of the bleached trunks of the ancient forests. In the steeper 

 middle portion of the mountain face, rows of upright trunks stand out 

 on the ledges like the columns of a ruined temple. On the more gentle 

 slopes farther down, but where it is still too steep tosupport vegetation, 

 save a few pines, the petrified trunks fairly cover the surface, and were 

 at first supposed by us to be the shattered remains of a recent forest. 



In ascending one of the steep spurs that project from the main wall, 

 the strata were found to consist, toward the base, of shales and fine- 

 grained sandstones. Higher up conglomerates occur, and still higher 

 coarse conglomerates and breccias prevail. Interbedded with the mas- 

 sive, irregular beds of the latter rocks are always thin layers of sand- 

 stones and shales. The sandstones are fine-grained, thinly bedded, and 

 contain more or less tufaceous material. Their prevailing color is green- 

 ish and greenish gray. They are apparently composed chiefly of material 

 derived directly or indirectly from volcanic sources. In no case are 



