The Henry Mountains are similar among themselves in constitution. They all exhibit dome-like uplifts; they all contain intrusive rocks; and their intrusive rocks are all of one lithologic type. They are moreover quite by themselves; the surrounding country is dissimilar in structure, and there is no gradation nor mingling of character. Thus similar and thus isolated it is natural to regard the mountains as closely related in origin, to refer their trachytes to a common source, and to look for homology in all their parts. It was the search for such homology which led to the hypothesis that the laccolite is the dominant element of their structure. It is now time to examine this hypothesis, the truth of which has been assumed in the preceding pages, and see how far it accords with the facts of observation. The facts to be correlated are the following: 1st. There are seven laccolites which lie so far above the local plane of erosion that they are specially exposed to denuding agents. They have no enveloping strata, and their only associated sheets lie in the strata under them. Their original forms have been impaired or destroyed by erosion. They are the Scrope, the Jukes, the Sentinel and its three companions, and the A laccolites. 2d. There are two laccolites so nearly bared that their forms are unmistakable, but which are still partially covered by arching strata, and which have associated sheets and dikes. They are the Marvine and the Steward laccolites. 3d. There are five supposed laccolites situated where the erosion planes are inclined, which run under the slopes and are covered at one side or end, and at the other project so far above them as to have lost something by erosion. These are accompanied by overarching strata and by sheets and dikes. They are the Peale, the Howell, the Shoulder, the D, and the E. 4th. There are seven or eight supposed laccolites, of which only a small part is in each case visible, but which are outlined in form by domes of overarching strata. Their bases are not exposed. Associated with them are dikes and sheets. They are the Hillers, the Pennell, the Geikie, the Newberry, the Bowl Creek, the C, the H, and perhaps the F. 5th. There are five domes of strata accompanied by dikes and (with one exception) by sheets, but showing no laccolite. They are the Ellsworth, the Greater Holmes, the Crescent, the Jerry, and the B arches. 6th. There are nine or more domes of strata with no visible accompaniment of trachyte. They are the Lesser Holmes, the Pulpit, the G, the Dana, and the Maze arches, and those of the foundation of Mount Pennell and of the west base of Mount Ellen.