Another folk-lore explanation in Japan attributes the cause of the tremblings of the earth to a subterranean monster whose head lies in the north of the island of Hondo, while his tail lies between the two principal cities. The shaking of his tail causes earthquakes. Fantastic and foolish as these explanations are, it is worthy of note that the first of the Japanese explanations shows no little observation on the part of the people, since it locates the starting-points of earthquakes as being not on the land, but on the bottom of the sea. In point of fact, nearly all the great earthquakes in Japan seem to start somewhere between the coasts of the islands on the sea-bottom that leads down to a very deep part of the Pacific known as the Tuscarora Deep. Many years ago nearly everyone believed that earthquakes were caused solely by the forces that produce volcanic eruptions; that all earthquakes, whether in the neighborhood of active volcanoes, or at great distances therefrom, were to be regarded solely as volcanic in their origin. It is now recognized that the most severe and far-reaching earthquakes have no immediate connection with volcanic explosions, but are due to the sudden slippings of the earth's strata over lines of faults; or, in other words, earthquakes are most frequently of the tectonic type. At the present time there is unfortunately much difference in opinion as to the exact cause of earthquakes. By this is not meant the immediate cause, but the ultimate cause. As to the immediate cause, practically all are agreed that quakes of volcanic origin are to be traced to the same forces that produce volcanic eruptions, while quakes of tectonic origin are due directly to the slipping of the strata along the faults. But when inquiry is instituted as to the nature of the forces that cause the volcanic eruptions, or that produce such an alteration of the strata as permits them afterwards to slip and thus jar the earth, there is much difference of opinion. As can be seen from a few quotations of well-known authorities, only two kinds of earthquakes exist; namely, volcanic earthquakes and tectonic earthquakes. Dana, for example, while acknowledging that small earthquakes may be caused by the sudden falling of large rock masses into cavities in the crust of the earth, says: "But true earthquakes come, for the most part at least, from one or the other of the following sources of disturbance. "1. Vapors suddenly produced, causing ruptures and friction. "2. Sudden movements or slips along old or new fractures.