=Minor causes of earthquakes.= Since any concussion within the crust sets up an earth jar, there are several minor causes of earthquakes, such as volcanic explosions and even the collapse of the roofs of caves. The earthquakes which attend the eruption of volcanoes are local, even in the case of the most violent volcanic paroxysms known. When the top of a volcano has been blown to fragments, the accompanying earth shock has sometimes not been felt more than twenty-five miles away. =Depth of focus.= The focus of the Charleston earthquake, estimated at about twelve miles below the surface, was exceptionally deep. Volcanic earthquakes are particularly shallow, and probably no earthquakes known have started at a greater depth than fifteen or twenty miles. This distance is so slight compared with the earth's radius that we may say that earthquakes are but skin-deep. Should you expect the velocity of an earthquake to be greater in a peneplain or in a river delta? After an earthquake, piles on which buildings rested were found driven into the ground, and chimneys crushed at base. From what direction did the shock come? Chimneys standing on the south walls of houses toppled over on the roof. Should you infer that the shock in this case came from the north or south? How should you expect a shock from the east to affect pictures hanging on the east and the west walls of a room? how the pictures hanging on the north and the south walls? In parts of the country, as in southwestern Wisconsin, slender erosion pillars, or "monuments," are common. What inference could you draw as to the occurrence in such regions of severe earthquakes in the recent past? CHAPTER XI VOLCANOES Connected with movements of the earth's crust which take place so slowly that they can be inferred only from their effects is one of the most rapid and impressive of all geological processes,--the extrusion of molten rock from beneath the surface of the earth, giving rise to all the various phenomena of volcanoes. In a volcano, molten rock from a region deep below, which we may call its reservoir, ascends through a pipe or fissure to the surface. The materials erupted may be spread over vast areas, or, as is commonly the case, may accumulate about the opening, forming a conical pile known as the volcanic cone. It is to this cone that popular usage refers the word _volcano_; but the cone is simply a conspicuous part of the volcanic mechanism whose still more important parts, the reservoir and the pipe, are hidden from view. Volcanic eruptions are of two types,--_effusive_ eruptions, in which molten rock wells up from below and flows forth in streams of _lava_ (a comprehensive term applied to all kinds of rock emitted from volcanoes in a molten state), and _explosive_ eruptions, in which the rock is blown out in fragments great and small by the expansive force of steam. Eruptions of the Effusive Type