The sedimentary rocks are not altered beyond the region of trachyte intrusion. The mere flexure of the strata was not accompanied by a perceptible change of constitution. In the zone of sheets there is little change except along the surfaces of the contact. For a few feet, or perhaps only a few inches, there is a discoloration (usually a decolorization) and a slight induration, without notable alteration of minerals. But in the region of reticulated dikes none of the sedimentaries are unchanged; crystals are developed, colors are modified, and hardness is increased, so that the physical properties of familiar strata no longer serve for their identification. Still there is no crumpling. The trachyte masses and the altered rocks in contact with them are so much more durable than the unaltered strata about them that they have been left by the erosion in protuberances. The outcrop of every dike and sheet is a crag or a ridge, and the mountain itself survives the general degradation of the country only in virtue of its firmer rock masses. Nevertheless, the mountain, because it was higher than its surroundings, has been exposed to more rapid erosion, and has been deprived of a greater depth of strata. From the base of the arch there have been worn 3,500 feet of Cretaceous, and from 500 to 1,500 feet of the Jura-Trias series, which is here about 3,000 feet thick. From the summit of the arch more than 2,500 feet of the Jura-Trias have been removed. The strata exposed high up on the mountain being older than those at the base, and the dip being everywhere directed away from the center, it is evident that the mountain is surrounded by concentric outcrops of beds which lift their escarpments toward it. It is usually the case, where the strata which incline against the flank of a mountain are eroded, that the softer are excavated the more rapidly, while the harder are left standing in _ridges_; and an alternation of beds suitable for the formation of a ridge occurs here. One of the upturned beds is the massive Vermilion Cliff sandstone, and beneath it are the shales of the Shinarump Group. By the yielding of the shales the sandstone is left prominent, and it circles the base of the mountain in a monoclinal ridge. But the ridge is of a peculiar character, and has really no title to the name except in the homology of its structure with that of the typical monoclinal ridge. It lacks the continuity which is implied by the word “ridge”. The drainage of Mount Ellsworth is from the center of the dome outward. A half dozen drainage lines originate in the high crests and pass outward through the zone of upturned strata. Lower down their interspaces are divided by others, and when they reach the circling escarpment of the Vermilion sandstone their number is fifteen. Each of these cuts the ridge to its base, and the effect of the whole is to reduce it to a row of sandstone points circling about the mountain.