As before said, at the present day we are little in advance of the ancients in actual knowledge of what is going on so far beneath our feet. We speak of forces where they spoke of fettered giants, but can only form theories where they formed myths. Is the earth’s centre made up of liquid fire? Does its rock crust resemble the thick ice crust on the Arctic Seas, or is the earth, as later scientists believe, solid to the core? Is it heated so fiercely, miles below our feet, that at every release of pressure the solid rock bursts into molten lava? Is the steam from the contact of underground rivers and deep-lying fires the origin of the terrible rending powers of the volcano’s depths? Truly we can answer none of these questions with assurance, and can only guess and conjecture from the few facts open to us what lies concealed far beneath.
In the history of earthquakes nothing is more remarkable than the extreme fewness of those recorded before the beginning of the Christian era, in comparison with those that have been registered since that time. It is to be borne in mind, however, that before the birth of Christ only a small portion of the globe was inhabited by those likely to make a record of natural events. The vast apparent increase in the number of earthquakes in recent times is owing to a greater knowledge of the earth’s surface and to the spread of civilization over lands once inhabited by savages. The same is to be said of volcanic eruptions, which also have apparently increased greatly since the beginning of the Christian era. There may possibly have been a natural increase in these phenomena, but this is hardly probable, the change being more likely due to the increase in the number of observers.
The structure of a volcano is very different from that of other mountains, really consisting of layers of lava and volcanic ashes, alternating with each other and all sloping away from the center. These elevations, in fact, are formed in a different manner from ordinary mountains. The latter have been uplifted by the influence of pressure in the interior of the earth, but the volcano is an immediate result of the explosive force of which we have spoken, the mountain being gradually built up by the lava and other materials which it has flung up from below. In this way mountains of immense height and remarkable regularity have been formed. Mount Orizabo, near the City of Mexico, for instance, is a remarkably regular cone, undoubtedly formed in this way, and the same may be said of Mount Mayon, on the Island of Luzon.
In many cases the irregularity of the volcano is due to subsequent action of its forces, which may blow the mountain itself to pieces. In the case of Krakatoa, in the East Indies, for instance, the whole mountain was rent into fragments, which were flung as dust miles high into the air. The main point we wish to indicate is that volcanoes are never formed by ordinary elevating forces and that they differ in this way from all other mountains. On the contrary, they have been piled up like rubbish heaps, resembling the small mountains of coal dust near the mouths of anthracite mines.