Page:Popular Science Monthly Volume 50.djvu/256

242 the earth's surface, molten material may be forced through it and a volcanic eruption initiated. In brief, variations in one process may lead to the formation of dikes, sheets, laccolites, and other forms of intrusion, and to volcanic eruptions. Possibly all these results might follow from the opening of a single fissure.

In all the instances enumerated the magma may be the same, so also the source of the heat and the origin of the pressure which forces the molten rock into the earth's crust, or causes it to be discharged at the surface.

Returning to the hypothesis that steam is the mainspring of volcanic phenomena, it will be conceded, I think, by most observers that steam contained in a magma can not be called upon to account for its rise in a deeply seated fissure so as to form a dike, or, when the conditions are varied, give origin to laccolites, etc. Not only are the rocks composing such intrusions, the densest of igneous rocks, but they are without steam cavities. Besides the filling of a fissure with plastic or fluid rock, and still more strikingly in the production of other varieties of intrusions, a bulk of matter, measured in some instances by cubic miles, is forced in among the solid rocks of the earth's crust. There is thus a bodily transfer of matter, frequently for long distances, from one place to another deep within the earth's crust and against an enormous pressure. All these facts are adverse to the conception that bodies of liquid or plastic rock are moved by the expansive force of the steam contained in them. The energy expended in producing igneous intrusions is in numerous instances so far in excess of that manifest in any explosive volcanic eruption that has been recorded—not excepting Cosequina, Sumbawa, or Krakatoa, but rather combining them all and more in one—that it becomes of a different order of magnitude, and a different origin is to be suspected.

In the case of subterranean injections, it is evident that the source of the heat which renders the rocks plastic, and the source of the pressure which forces the plastic material into fissures, etc., are distinct and should be separately considered.

The heat manifest in both subterranean and surface igneous phenomena, as is well known, has been variously accounted for, but I do not wish to consider this problem at present. The consensus of opinion, however, seems to be that the heat referred to is mainly and essentially the internal heat of the earth—i. e., the residual heat of a cooling globe. It is conceded also that the matter composing the earth at a depth of a few miles below the surface is so highly heated that it would become plastic or even highly fluid if the pressure under which it exists were removed. The best conception we can frame of the general physical condition of the earth is, that it consists of a more or less spherical mass, which is highly heated and in a potentially plastic condition within, and