THE BUILDING Ur OF A VOLCANO. It has al ready been remarked that probably much the greater number of volcanoes have made their own masses through the simple accumulation in time of their ejected products: in other words, the heaping up about the point or points of ac tivity of lava, cinders, or ashes, most generally, perhaps, of all three of these products. Inas much as these are thrown out with it Pertain amount of alternation or regularity, the moun tain itself acquires a regular internal structure, which, in section, would appear in alternating layers (strata) of deposition. Some volcanoes, whose eruptive energies are of a greater kind, and are not expressed in steam eruptions, are built up almost entirely of lava, and their outer dress, depending., upon the viscosity or liquidity of the outflows, is either steep (like the ?name Ions of the island of Bourbon, and some of the pays of the Auvergne region of France) or very gentle, descending in a gradient of from 15° to 5° or less, as in some of the Hawaiian volcanoes. Other volcanoes are constructed 'almost wholly of cinders or scoria, hut the composite cone is the one of most general occurrence.
It was at one time held that volcanoes, in stead of in the main accumulations of ejected material, were massive upheavals or swellings of the earth's surface, caused by the pent-up force within acting upon the depressing surface. The more accurate study of volcanic
phenomena has not given proof of the existence of 'craters' or 'cones of elevation,' as these as sumed structures have been called. At the same time it is by no means improbable that in the initial formation of some (or even many) vol canoes a preparatory step has been the upheaving of the deeper strata through magmatic intru sions, a condition that is fairly well indicated by the uplifts which are recognized in or asso ciated with the structures known as laceolitcs intruded lava-masses which have forcibly domed up the strata lying over the areas of upward passage of escaping lava. The Henry Mountains of southeastern Utah, the Euganean Hills (or portions of them) in Northern Italy, are familiar examples of this type of structure. The prodi gious force of this uprising, lava is measured by the thousands of feet of thickness of sedimentary strata which it has bodily uplifted.