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The form of typical volcanic mountain is that of a cone, with a circular basin or depression, called a crater, at its summit. In the center of the crater is the mouth of a perpendicular shaft or chimney, which emits clouds of hot vapor and gases; and in periods of greater activity, ejects ashes, fragments of heated rock, and streams of fiery lava.

Volcanic ashes, when examined under a microscope, are found to be simply pulverized lava, frequently in minute crystals, and bear no resemblance to ashes in the ordinary sense of the term.

The lava stream, when flowing white hot from the crater, is not unlike a jet of melted iron escaping from a furnace, and moves at first with considerable rapidity. It soon cools on the surface, and becomes covered with a hard, black, porous crust, while the interior remains melted and continues to flow. If the stream is thick, the lava may be found still warm after ten or even twenty years.

The amount of matter ejected by volcanoes is very great. The whole island of Hawaii, the largest of the Hawaiian Islands, seems to be only an accumulation of lava thrown out by its four craters. All high oceanic islands are of the same character. Iceland, with an area of forty thousand square miles, is a vast table-land from three thousand to five thousand feet in elevation, composed of volcanic rock similar to the lavas still ejected by its numerous volcanoes.