According to the observations of Dr. Hayden, the mountain is formed of alternate beds of sandstone, limestone, and quartzites, elevated to a nearly vertical position by those internal forces which acted in ages past to lift the mountain ranges to their present heights. Standing at the base and looking up the sides of the mountain, the geologist could not but be filled with wonder at the convulsions which threw such immense masses of rocks into their present position. Ridge after ridge extends down the steep sides of the mountain like lofty walls, the intervening softer portions having been washed away, leaving the harder layers projecting far above. In one place the rocks incline in every possible direction, and are crushed together in the utmost confusion. Between the walls at one point is a band of bright brick-red clay, which has been mistaken for cinnabar, and hence the name Cinnabar Mountain. The most conspicuous ridge is composed of basalt, which must have been poured out on the surface when all the rocks were in a horizontal position. For reasons best known to himself, one of the first explorers of this region gave these parallel ridges the title of "Devil's Slide." "The suggestion was unfortunate," writes the historian of the Expedition, "as, with more reason perhaps, but with no better taste, we frequently had occasion to appropriate other portions of the person of his Satanic Majesty, or of his dominion, in signification of the varied marvels we met with. Some little excuse may be found for this in the fact that the old mountaineers and trappers who preceded us had been peculiarly lavish in the use of the infernal vocabulary. Every river and glen and mountain had suggested to their imaginations some fancied resemblance to portions of a region which their pious grandmothers had warned them to avoid. It is common for them, when speaking of this region, to designate portions of its physical features, as "Firehole Prairie,"--the "Devil's Den,"--"Hell Roaring River," etc.--and these names, from a remarkable fitness of things, are not likely to be speedily superseded by others less impressive." These "impressive" titles stand in curious contrast with the fanciful names bestowed in this region by Capts. Lewis and Clarke,--Wisdom River, Philosophy River, Philanthropy Creek, and the like.