While we were talking about the earth-crust, I daresay you were wishing to know, as I did, how thick it is—how far down the layers of rocks go, and what lies underneath the lowest layer of all.
These are questions which cannot be answered; for no one has ever been able to search so far into the hidden parts of the earth as to tell us what lies beneath those fire-rocks, which are the lowest known, although they are sometimes found upon the tops of mountains, cast up by a mighty heaving of the crust, such as happens when there is an earthquake, or what is called the "eruption" of a volcano.
But what power could be strong enough to heave up solid rocks, and to make the firm ground upon which we tread, and upon which the houses are built, waver to and fro like the restless sea, so that the strongest buildings begin to totter and fall, and the bravest men run for their lives?
It is the mighty power of steam—caused by the great heat far down below—which, when it does come to any part of the earth's surface, makes itself known in very terrible ways.
We do not often hear of earthquakes near home; but in some of the most beautiful parts of the world they are so common that the houses are built only one storey high, and of wood, not stone, because low houses are less likely to fall, and wooden ones are easily built up again, if overthrown. I think you have heard of the boiling springs in Iceland, which burst through the ground, shaking it and making it tremble; just as the steam shakes the lid of the teakettle; and rising almost to the clouds, with a noise like fireworks; and perhaps you may have seen the hot springs at Bath, from which a cloud of steam rises almost in the heart of the beautiful old city, and which are believed to come from a depth of nearly a mile.
Such is the force of this steam that even the bed of the sea has been heaved up by it into a burning mountain, from which great stones are cast high into the air; while down its sides flow melted rocks and metals, forming the lava which, when seen at night, looks like a stream of liquid fire, but quickly cools into a river of mud. All these strange things tell us terrible tales of the great heat which is somewhere in the heart of the earth, and help us to understand the verse which tells us all we really know about it: "As for the earth, out of it cometh bread: and under it is turned up as it were fire."
New Zealand is a country where there are many hot springs, and several mountains which were once volcanoes, but were supposed to have died out. One of these, Mount Tarawera, was situated in what was called the Hot Lake district, because there were not only boiling springs, but pools of hot water there. The Hot Lakes valley was not only a lovely green spot, but it was noted for the wonderful Pink and White Terraces, which were so beautiful as to be one of the sights which people from all countries came to see.
Imagine, if you can, basins of white and pink marble rising one above another, filled with water of the deepest blue, by a warm stream which kept flowing over them in a constant cascade. You would have enjoyed a bath there, I am sure, and would have been interested to see the country-people cooking their food in some of the neighbouring springs where the water came from so great a depth that it was always boiling.
But this lovely place was full of hidden dangers; for miles around these lakes the ground was hot and crumbling, and in many places so thin that if you did not tread very carefully, you might find yourself sinking into hot mud.
It was in June, which you know is winter-time in New Zealand, in the year 1885, that the people of Wairoa, a beautiful place where some missionaries had settled that they might teach the Maoris, were awakened at midnight by a heavy shock of earthquake, accompanied by a fearful roar, which made them rush out of their houses in terror. The sight which greeted them was grand but awful. Ernest has a picture of it in his room; but I suppose it would not be possible for any picture to give an idea of what the poor frightened people saw. Mount Tarawera had been asleep for a hundred and twenty years, so that it was supposed to have burnt itself out, and to be no longer dangerous. But it was awake now: the fearful roar which had aroused the sleepers was caused by its having suddenly burst into flame; and it continued to throw high into the sky fire and mud and stones, while the inhabitants of the peaceful little village saved what they could carry, and then fled away in their night-dresses.