If the reader would realize the relation between the eroded material and the surviving mountains, let him turn to the Frontispiece. A perspective view is there given of a tract ten miles square, with Mount Ellsworth in the center. It is represented as cut out from all surroundings by vertical planes which descend to the level of the ocean. The southern or nearer half of the block shows the present aspect of the country; the remote half shows the form it is supposed to have had if the uplift was completed before the erosion began, or what is the same thing, the form it would have, had there been no erosion. The difference between the two represents the total amount of the material that has been washed away since the completion of the Tertiary sediments. Partly in review, let us now sketch the HISTORY OF THE LACCOLITE. When lavas forced upward from lower-lying reservoirs reach the zone in which there is the least hydrostatic resistance to their accumulation, they cease to rise. If this zone is at the top of the earth’s crust they build volcanoes; if it is beneath, they build laccolites. Light lavas are more apt to produce volcanoes; heavy, laccolites. The porphyritic trachytes of the Plateau Province produced laccolites. The station of the laccolite being decided, the first step in its formation is the intrusion along a parting of strata, of a thin sheet of lava, which spreads until it has an area adequate, on the principle of the hydrostatic press, to the deformation of the covering strata. The spreading sheet always extends itself in the direction of least resistance, and if the resistances are equal on all sides, takes a circular form. So soon as the lava can uparch the strata it does so, and the sheet becomes a laccolite. With the continued addition of lava the laccolite grows in height and width, until finally the supply of material or the propelling force so far diminishes that the lava clogs by congelation in its conduit and the inflow stops. An irruption is then complete, and the progress of the laccolite is comparable with that of a volcano at the end of its first eruption. During the irruption and after its completion, there is an interchange of temperatures. The laccolite cools and solidifies; its walls are heated and metamorphosed. At the edges, where the surface of the laccolite is most convex, the heat is most rapidly dissipated, and its effect in metamorphism is least. A second irruption may take place either before or after the first is solidified. It may intrude above or it may intrude beneath it; and observation has not yet distinguished the one case from the other. In any case it carries forward the deformation of cover that was begun by the first, and combines with it in such way that the compound form is symmetric, and is substantially the same that would have been produced if the two irruptions were combined in one. Thus the laccolite grows by