If we suppose, on this evidence, that the centre of the globe is intensely heated, and that it gets gradually cooler towards the surface, there is what at once will account for the hot springs, the increasing heat in descending mines, the constant action of some volcanoes, the occasional action of others, the streams of lava, and the nature of all substances that are thrown out.
It is very natural to imagine that every now and then the action of the great heated mass inside, would crack open the crust, and the sea wearing away its bottom in some places as it does, may have something to do in assisting this. Thus, new volcanoes would be formed, and earthquakes would happen where they never happened before; and over these immense cracks there would be a volcanic band.
Then, after a time, these cracks might be partially, or wholly, filled up by the matter thrown into them, and the volcanoes on the surface above may become quiet for a time.
CHAPTER IX.
PARLEY TELLS SOMETHING ABOUT THE HISTORY OF MOUNT VESUVIUS.
I am going to relate to you the biography of a Volcano, and I hope you will find it amusing.
Before the time of the Roman Emperor Trajan, Mount Vesuvius had been quiet for a great many centuries. It is known that it had had many eruptions before then, because there are some great lava streams close by it, on which some of the most ancient Italian cities are built. In particular, Herculaneum, which was said to have been built by the Demi-god Hercules, and must therefore have been of the remotest antiquity, was built chiefly of blocks of lava, and founded upon a vast stream of the same substance.