Mount Hillers is more rugged in its character, and although compact in its general form, is carved in deep gorges and massive spurs. Its rugosity is contrasted by the smoothness of its pedestal, which to the south and west and north is a sloping plain merging with the surrounding plateau. Mount Ellsworth (8,000 feet) and Mount Holmes (7,750 feet) stand close together, but at a little distance from the others. The pass which separates them from Mount Hillers has an altitude of 5,250 feet. They are single peaks, peculiarly rugged in their forms, and unwatered by springs. They stand almost upon the brink of the Colorado, which here flows through a cañon 1,500 feet in depth. THE ROCK SERIES. The sedimentary rocks which occur in the Henry Mountains and their immediate vicinity, range from the summit of the Cretaceous to the summit of the Carboniferous. It is probable that they were covered at one time by some thousands of feet of Tertiary strata, but from the immediate banks of the Colorado these have been entirely eroded, and their nearest vestiges lie thirty miles to the westward, where they have been protected by the lava-beds of the Aquarius Plateau. _Cretaceous._—The Cretaceous strata do not reach to the Colorado River, but they extend to the Henry Mountains, and are well displayed upon the flanks. They include four principal sandstones, with intervening shales, in the following (descending) order: 1. The Ma-suk′ Sandstone, yellow, heavy-bedded 500 feet. 2. The Ma-suk′ Shale, gray, argillaceous, and toward the top slightly arenaceous 500 feet. 3. The Blue Gate Sandstone, yellow and heavy-bedded 500 feet. 4. The Blue Gate Shale, blue-black and argillaceous, weathering to a fine gray clay (_Inoceramus deformis_ and _I. problematicus_) 1,000 feet. 5. The Tu-nunk′ Sandstone, yellow and heavy-bedded 100 feet. 6. The Tu-nunk′ Shale, blue-black and argillaceous, weathering to a fine gray clay (_Inoceramus problematicus_ and _Baculites anceps_) 400 feet. 7. The Henry’s Fork Group, consisting of— _a._ Friable yellow sandstone with numerous fossils (_Ostrea prudentia_, _Gryphea Pitcheri_, _Exogyra læviuscula_, _Exogyra ponderosa_, _Plicatula hydrotheca_, _Camptonectes platessa_, and _Callista Deweyi_) 10 feet. _b._ Arenaceous shales, purple, green, and white, with local beds of conglomerate 190 feet. _c._ Coarse sandstone and conglomerate, with many white grains and pebbles, interleaved with local beds of purple and red shale, and containing immense silicified tree-trunks 300 feet. —————