"When the first layer has reached a depth of a few thousand feet the rocks which contain the water of crystallization are subjected to a terrific heat. This heat generates steam, which is held in a state of frightful tension in its rocky prison. "It is at these moments that volcanic eruptions occur. They result from wrinkling in the outer crust of the earth's surface--wrinklings caused by the constant shrinking of the earth itself and by the contraction of the outer surface as it settles on the plastic center underneath. Fissures are caused by these foldings, and as these fissures reach down into the earth the pressure is removed from the rocks and the compressed steam in them and it explodes with tremendous force. "The rocks containing the water are blown into dust, which sometimes is carried so high as to escape the power of the earth's attraction and float by itself through space. After the explosions have occurred lava pours forth. This is merely melted rock which overflows like water from a boiling kettle. But the explosion always precedes the flow, and one will notice that there is always an outpouring of dust before the lava comes." Professor W. J. McGee, of the Smithsonian Institution at Washington, says: "It may be that a violent earthquake tremor came after the volcanic eruption, but it does not necessarily follow that the two travel together. Oftentimes we hear of earth tremors with no apparent accompaniment. This was true of the Charleston earthquake in 1886. Earthquakes are caused by mysterious disturbances in the interior of the earth. The most commonly accepted belief is that massive rock beds away down in the earth, at a depth of twelve miles or more, become disturbed from one cause or another, with the result that the disturbance is felt on the earth's surface, sometimes severely, sometimes faintly. "Probably the most violent earthquake in history occurred about ten years ago at Krakatoa. The explosion could be heard for more than one thousand miles, and the earth's tremors were felt for thousands of miles. The air was filled with particles of earth for months afterward. The air-waves following the explosion are believed to have passed two and one-half times around the globe. The face of the land and sea in the vicinity of the eruption was completely changed." Dr. E. Otis Hovey, professor in the Museum of Natural History, New York, offers the following explanation of the Martinique disaster: