The basis for the generalization is exceedingly broad. I have enumerated only seven groups of laccolitic mountains and ten groups of eruptive; but with few exceptions each group is composed of many individuals, each one of which is entitled to rank as a separate phenomenon. In the Uinkaret Mountains Professor Powell has distinguished no less than one hundred and eighteen eruptive cones, and in the Henry Mountains I have enumerated thirty-six individual laccolites. In one locality basic lava has one hundred and eighteen times risen to the surface by channels more or less distinct, instead of opening chambers for itself below. In the other locality porphyritic trachyte has thirty-six times built laccolites instead of rising to the surface. If our attention was restricted to these two localities we might as naturally correlate the types of structure with some accidents of locality as with types of lava; but when all the localities are taken into account it is evident that there is no common mark by which either the laccolitic or the volcanic are distinguished. THE QUESTION OF CAUSE. We are now ready to consider the question: Why is it that in some cases igneous rocks form volcanoes and in other cases laccolites? It is not necessary to broach the more difficult problem of the source of volcanic energy. We may assume that molten rock is being forced upward through the upper portion of the earth’s crust, and disregarding its source and its propelling force may restrict our inquiry to the circumstances which determine its stopping place. Let us further assume, but for a moment only, that the cohesion of the solid rocks of the crust does not impede the upward progress of the fluid rock, nor prevent it from spreading laterally at any level. The lava will then obey strictly the general law of hydrostatics, and assume the station which will give the lowest possible position to the center of gravity of the strata and lava combined. (1) If the fluid rock is less dense than the solid, it will pass through it to the surface and build a subaërial mountain. (2) If the upper portion of the solid rock is less dense than the fluid, while the lower portion is more dense, the fluid will not rise to the surface but will pass between the heavy and light solids and lift or float the latter. (3) If the crust be composed of many horizontal beds of diverse and alternating density, the fluid will select for its resting place a level so conditioned that no superior group of successive beds, including the bed immediately above it, shall have a greater mean specific gravity than its (the fluid’s) own; and that no inferior group of successive beds, including the bed immediately beneath, shall have a less mean specific gravity than its own. [Illustration: FIG. 51.—Diagram to illustrate the application of the law of Hydrostatic Equilibrium to the movements of lavas. The shaded bands represent heavy strata; the open, light. ]