366 GEOLOGICAL EXCURSION TO THE BOCKY MOUNTAINS. 



number are found in Montana. Its deposits were largely derived from 

 showers of volcanic dust, which, falling into the quiet waters of the 



lake, wore arranged in beds of very pure dust some 20 feet ((J m.) in 

 thickness. Above these is a very considerable thickness of beds, 

 evidently made up from the same material, which was washed into the 

 lake from the shores and surrounding country and rearranged in beds 

 of a rusty color. 



At Central Park the railroad crosses the Gallatin river, whose head- 

 branches have their source in the northwestern corner of the Yellow- 

 stone National Park. 



From Central Park to Logan alluvium and Quaternary gravels, rest- 

 ing on lake beds, are passed over. As Logan is approached very good 

 exposures of the Paleozoic and Algonkian beds are seen across the 

 Gallatin river on the right, and the road passes through a cut in the 

 former just before reaching Logan. Immediately opposite Logan, on 

 the north side of the (J alia tin river, there is a line exposure of Cambrian, 

 Devonian, and Carboniferous limestones in the binds that rise from 

 the water's edge. 



A few outcrops of the Paleozoic are noted on the left side, but the 

 road soon comes out on the lake beds after leaving the Gallatin river 

 below Logan. These beds are also well shown bordering the east side 

 of Madison River valley just before Three Forks station is reached. 

 They also show, in the distance to the south, between Three Forks and 

 Willow creek. The .Madison river is crossed just before the Three 

 Forks station is reached. The Three Porks valley, at the lower end of 

 which the < iallatin. Madison, and Jefferson rivers unite to form the 

 .Missouri river, is one of the most interesting geographical points in 

 Montana. These streams were named by Lewis and Clarke, who lirst 

 saw this valley in July, 1805, on their way to the Columbia river and 

 the Pacific coast. 



On the northwest or right side of the Jefferson river above Three 

 Forks the hills are mainly of Cambrian, Devonian, and Carboniferous, 

 the Algonkian beds forming the lower foothills farther up tin 1 , river. 



A short distance above Willow creek the road passes through a 

 canyon, almost two miles in length, which the Jefferson river has cut 

 through Carboniferous limestones. At the entrance to this canyon, on 

 the north side of the river, stands a butte of basalt. 



Prom the canyon the road comes out into the alluvial valley of the 

 Jefferson, in which lies Sappington station. On the left (south) the 

 hills or mountains are mainly of Carboniferous rocks. On the right 

 (north) the nearest small ridges are of dura, Trias, and Cretaceous. 

 Above Sappington the Jefferson river is crossed and the road enters a 

 second canyon cut in upper Carboniferous and Jura-Trias rocks, from 

 which it emerges into a narrow valley in which the rocks are of Creta- 



