Briefly, the condition of things is believed to be as follows: The entire interior is filled with rock hot enough to melt at the level of the sea, but under too great pressure to melt. If this be granted, as it is by perhaps the greatest number of men who are competent to judge, the phenomena of earthquakes can be readily explained, as can, indeed, the phenomena of those great movements whereby great changes of level take place in different parts of the earth. Now let us see how volcanoes can be explained on the assumption that the interior of the earth is hot enough to melt, but remains solid only because there is no room for the heated mass to expand in. Such a heated interior as we have imagined, must be constantly losing its heat and, therefore, shrinking. Every now and then this shrinkage must produce great fissures or cracks in the solid crust of the earth. Now should such cracks or fissures extend downwards to the heated interior, there must result a decrease in the pressure. The rocks would, therefore, begin to expand and would be forced by the great pressure to rise slowly in such cracks or fissures. The further they rise the greater the relief of pressure, until they at last assume a molten condition in which they are forced out through the craters of volcanoes as molten rocks or lava. But it is not only volcanoes that seem to indicate a highly heated plastic condition as existing in the earth's interior. As geologists well know, there are to be found in the various strata of the earth places where great fissures have been made at various times during the geological past. These fissures vary in width from a few inches to many hundreds of feet, and are frequently scores of miles in length. Lava either flows out of them, and covers adjoining sections of the country, or simply rises in them and, afterwards cooling, forms dikes. In many instances, however, the lava is forced in between more or less horizontal layers and in some cases has caused these layers to assume the shape of what geologists know as _subtruderant mountains_. Some of the eastern ranges of the Rocky Mountains have been formed in this manner. We can, therefore, picture to ourselves the following as the manner of formation of an ordinary volcano. A fissure is first formed in the solid crust of the earth, extending downwards to the regions of great heat. There is thus produced a relief of pressure, so that at this point the highly heated rocks begin to be slowly forced up through the fissure. As they rise higher and higher they become less solid and finally expand into fused masses that can flow out of the crater or opening in the earth's surface. In this way a volcano is started.