based largely on what is seen when one looks down into the throat of a volcano in a state of mild activity; let us supplement such a view by endeavoring to form a conception of the conditions that exist far below the surface.
In many instances volcanoes are known to be situated on lines of fracture in the earth's crust. In all volcanoes it is evident that there is a passageway or conduit, leading from an intensely heated region within the earth, to the surface. These conduits must be several thousand feet in depth. Indeed, it is not unreasonable to assume that they may have a depth of several miles or possibly tens of miles. What one sees, therefore, in looking into a crater of an active volcano is the summit of a column of molten rock, the bottom of which is tens of thousands of feet below.
Judd has compared the mild activity of Stromboli to the boiling of mush in a tall vessel, the heat being applied at the bottom. Steam is generated in the mush, and, rising through it in bubbles, elevates the surface. When the bubbles of steam burst, portions of the viscid material are blown into the air. Such an analogy is certainly sustained by what is seen at the summit of a column of molten lava when we look into the crater of a volcano.
In seeking for information concerning the conditions that exist far below the surface, when a volcano is giving off steam at the top, we may obtain a few facts to guide us by studying the ruins of extinct volcanoes and the nature of igneous intrusions as laid bare by erosion.
Volcanic necks tell something of the conditions that exist within a volcanic mountain. In more deeply eroded volcanic districts one finds dikes and intruded sheets. These are connected, in reference to mode of origin, with Plutonic plugs, laccolites, and what I have termed subtuberant mountains. These various forms taken by intruded rocks and surface extrusions, as I have attempted to show in the article just referred to, belong in a single genetically connected series. A break in the earth's crust which reaches a region of great heat may be injected with plastic rock and form a dike; if the fracture terminates above in a region of horizontally stratified beds, the magma rising through it may expand widely, at the same time lifting a broad cover to a comparatively small height—that is, form an intruded sheet; or be more restricted in its expansion, according to its degree of fusion, depth below the surface, and possibly other causes, and raise a cover of less diameter to a greater height—that is, form a laccolite; or, if a great volume of plastic material is intruded, give origin to a subtuberant mountain. Should the fissure reach
- On the Nature of Igneous Intrusions. In the Journal of Geology, Chicago, vol. iv, 1896, pp. 177-194.