Sir James Hall has shown, by experiment, that earthy substances, reduced to a state of fusion, become more highly crystalline as they are allowed to cool more slowly, and are subjected to greater pressure. It is difficult to conceive of these conditions existing in a higher degree than they do in the cooling masses of lava below the stratified rocks. These lavas must therefore take the highly crystalline form which the granitic rocks are found to have. All the igneous rocks have therefore existed as subterranean lavas. The volcanic rocks have become vitreous, the granitic are crystalline, and the trappean are intermediate in structure, coinciding with the circumstances of pressure and rate of cooling under which they have severally been formed. 5. _The Elevation of Mountains_ is another result of volcanic action. The height of mountains depends, in part, upon general elevation. Yet there is a different action, upon which the existence of the mountain, as such, depends. Whenever igneous action becomes intense under any portion of the earth's surface, and the elastic force greater than the repressive, the solid crust will be broken and raised up, and along this line of fracture the lava will rise above its general level elsewhere. This lava, thus lifted out of the general mass, in time solidifies, and forms the nucleus of a mountain. At successive periods the elevating force is renewed, and adds somewhat to the mountain mass before supplied. In this way the mountain is ultimately formed. So far as observations have been made, the elevation of mountains seems not to be gradual, but spasmodic; and yet the elevating force probably accumulates constantly and uniformly. The repressing force consists of the weight of the strata above, which may be regarded as constant, and their strength, which is variable. When the elevating force becomes greater than both the repressing forces, the crust is fractured. The strength of the strata then becomes nothing, and the repressing force is the weight alone. The elastic mass below at once expands, and the requisite space is furnished by the uplifting of the strata along the line of fracture. As the ridge of lava which fills this additional space cools, it recloses, in part, the original fracture, and the repressing force again consists of the two elements,--weight and strength. There will therefore be no further elevation till the elevating force is again superior to these two forces. Thus the elevating force, though it may accumulate at a uniform rate, will manifest itself only at considerable intervals.