The problem before us is this. Granting that it is proved that active volcanoes are always in close proximity to the ocean, are we to explain the fact by supposing that the agency of sea-water is necessary to volcanic outbursts, or by regarding the position of the coast-lines as to some extent determined by the distribution of volcanic action upon the surface of the globe? The first supposition is the one which perhaps most readily suggests itself, but the latter, as we shall hereafter show, is one in favour of which not a few weighty arguments may be advanced. Another problem which suggests itself in connection with the distribution of volcanoes is the following. Are the great depressed tracts which form the bottom of the oceans, like the elevated tracts which constitute the continents, equally free from exhibitions of volcanic energy? When we remember the fact that the area of the ocean beds is two and three-quarter times as great as that of the continents, it will be seen how important this question of the existence of volcanoes at the bottom of the ocean really is. The fact that recent deep-sea soundings have shown the deepest parts of the ocean to be everywhere covered with volcanic _débris_ is by no means conclusive upon this question; for, as we have seen, the ejections of sub-aerial volcanoes are by the wind and waves distributed over every part of the earth's surface. [Sidenote: SUBMARINE ERUPTIONS.] Submarine volcanic outbursts have occurred in many parts of the globe, but it may well be doubted whether any such outburst has ever commenced at the bottom of a deep ocean, and has succeeded in building up a volcanic cone reaching to the surface. Most, if not all, of the recorded submarine outbursts have occurred in the midst of volcanic districts, and the volcanic cones have been built up in water of no great depth. Indeed, when it is remembered that the pressure of each 1,000 fathoms of water is equivalent to a weight of more than one ton on every square inch of the ocean-bottom, it is difficult to imagine the ordinary explosive action of volcanic vents taking place at abysmal depths. If, however, fissures were opened in the beds of the ocean, quiet outwellings of lava might possibly occur.