sand kilometres distant, in thirteen minutes; that of October 27, 1894, in Santiago, Chili, in Rome, eleven thousand five hundred kilometres distant, in seventeen minutes, and in Charkow, Russia, two thousand kilometres from Rome, between one and two minutes later. It reached Tokio at the same time, after a transit of seventeen thousand four hundred kilometres.
Still another task of modern seismology is the investigation of earthquakes at sea, or seismic movements of the bottom of the ocean, and the manner in which they are propagated through the water, of which a very fine cartographic representation has been published by Dr. C. Rudolph, of Strasburg.
The question of the origin of earthquakes stands in constant connection with this external development of seismology. It is significant and remarkable that the answers to it, though they may be given differently from different scientific points of view, are always consistent in one fact, that earthquakes are a phenomenon of the whole earth. Some of the investigators seek to explain them, aside from those that occur in volcanic regions, as a part of the great changes in the earth's crust which have taken place during the last geological epoch, and are still, perhaps, taking place; others find their seat and cause in the unstable condition of the interior of the earth, beneath its solid and red-hot envelope. The former explanation, the older and heretofore the prevalent one, is called the tectonic theory, because it is based, leaving out volcanic earthquakes, on the structure of the earth's crust; the second, which is gaining ground, and requires no separate explanation for volcanic earthquakes, may be called, reviving an expression used by L. Fr. Naumann, of Leipsic, the Plutonic theory, because it goes down into the unexplored depths of the earth. If seismic manifestations depend upon the action of the whole earth, a single explanatory principle, as is always the case with great natural phenomena, is not sufficient, and tectonic as well as Plutonic earthquakes must be recognized, and the reverse.
The tectonic theory is of geological origin, and properly supplanted the older Plutonic theory of Humboldt, which was only an unverified supposition. As a whole it was first worked out by Otto Volger in 1858, after various similar hypotheses had been set forth by other investigators. He was confirmed by the independent researches of Rudolf Hoernes, Edouard Suess, and most of the German, French, and English seismologists.
Their theory supposes that there are large hollow spaces in the crust of the earth, into which immense falls of material take place, and that these are the cause of a part of the earthquakes; that the crust of the earth is often and variously disturbed in consequence of the
