_Internal Structure._—Of the laccolites which are best exhibited in section, there are a number which appear to be built up of distinct layers. The Peale exhibits three layers with uneven partings of shale. The Sentinel shows two without visible interval. The Howell shows two. The Pennell has a banded appearance but was not closely examined. The Marvine shows at a distance a faint banding, which near by eludes the eye. No division nor horizontal structure was seen in the Hillers, Jukes, Scrope, or Steward laccolites, but observation was not sufficiently thorough to satisfy me of its absence. It is probable that all the larger laccolites are composite, having been built up by the accession of a number of distinct intrusions. There is little or no prismatic structure in the trachytes. It is sometimes simulated by a vertical cleavage induced in sheets and laccolites which are undergoing disintegration by sapping, but I did not observe the peculiar prismatic cleavage which is produced by rapid cooling in dikes, sheets, and _coulées_ of basalt. The two structures are not often discriminated, but they are really quite distinct, and their peculiar characters are easily recognized. The cleavage planes produced by cooling are as a rule perpendicular to the cooling surface, and the systems of prisms which are based on the opposite walls of a dike or sheet, do not correspond with each other, and do not run across, but meet midway in a confused manner. The cleavage planes which are produced by the shearing force when a massive bed of trap or other rock is undermined or sapped and yields under its own weight, extend from base to top, and are perpendicular to the plane of the horizon instead of the plane of the bed.[2] Footnote 2: See page 172 of the “Exploration of the Colorado River”, by J. W. Powell. _Vertical Distribution._—The range of altitudes at which laccolites have been formed is not less than 4,500 feet, and neither the upper nor the lower limit is known. The highest that are known—those which were intruded at the highest geological horizon—are near the base of the Blue Gate shale (Middle Cretaceous), but it is quite possible that higher ones have been obliterated by erosion. The lowest that is known was intruded in the Shinarump shale, but it is known of the invisible Ellsworth laccolite that upper Carboniferous strata lie above it, so that its horizon of intrusion must be still lower. The plexus of dikes and sheets on the Ellsworth arch indicates that the laccolite is not deeply buried; but in the series of arches there are nine which show no trachyte, and we have no data from which to infer their depth.