The facts which we have stated, with others of a similar kind, have led geologists to look with grave feelings of doubt upon the old hypothesis which regarded the increase of temperature found in making excavations into the earth's crust as a proof that we are approaching a great incandescent nucleus. They have thus been led to enquire whether there are any conceivable sources of high temperatures at moderate depths--temperatures which would be quite competent to produce locally all the phenomena of volcanic action. There are not wanting other facts which seem to point to the same conclusion: namely, that volcanic action is not due to the existence of a universal reservoir of incandescent material occupying the central portion of our globe, but to the local development of high temperatures at moderate depths from the surface. [Sidenote: DEPTHS AT WHICH EARTHQUAKES ORIGINATE.] The close connection between the phenomena of volcanoes and earthquakes cannot be doubted. It is true that some of those vibrations or tremors of the earth's crust, to which we apply the name of earthquakes, occur in areas which are not now the seat of volcanic action; and it is equally true that the stratified rock-masses of our globe, far away from any volcanic centres, exhibit proofs of violent movement and fracture, in the production of which, concussions giving rise to earthquake vibrations, could scarcely fail to have occurred. But it is none the less certain that earthquakes as a rule take place in those areas which are the seats of volcanic action, and that great earthquake-shocks precede and accompany volcanic outbursts. Sometimes, too, it has been noticed that the manifestation of activity at a volcanic centre is marked by the sudden decline of the earthquake-tremors of the district around, as though a safety-valve had been opened at that part of the earth's surface. Mr. Mallet has shown that by the careful study of the effects produced at the surface by earthquake-vibrations, we may determine with considerable accuracy the point at which the shock or concussion occurred which gave rise to the vibration. Now it is a most remarkable fact that such calculations have led to the conclusion that, so far as is at present known, earthquake shocks never originate at greater depths than thirty miles from the surface, and that in some cases the focus from which the waves of elastic compression producing an earthquake proceed is only at the depth of seven or eight miles. As we have already seen, there can be no doubt that in the great majority of instances the forces originating earthquake-vibrations and volcanic outbursts are the same, and independent lines of reasoning have conducted us to the conclusion that these forces operate at very moderate distances from the earth's surface.