These are, so to speak, the autographs of earthquakes--the records earthquakes themselves make on an instrument called the "seismograph," using a stylus, as the ancients did, as you will see by looking up "seismograph" in the dictionary or encyclopædia. After an earthquake starts it seems to stop for breath or for want of the right word--just like people; for you notice portions of the lines are almost straight. These were made when the earthquake was comparatively quiet. Then, when it got excited again--as in the second record from the top--the stylus fairly jumped up and down; and there where the waves are long and close together the shocks were particularly severe and followed each other rapidly. ] II. How Vulcan Drove his Autograph into the Rocks But there is another kind of handwriting on the walls that was made with such a vigorous stroke that it also made the earth shake. Of course we might expect Vulcan to write a rather vigorous hand--Vulcan, forger of thunderbolts for Jove. The ancients thought volcanoes belonged to the kingdom of Vulcan, so in scientific language everything connected with volcanic action comes under the head of "Vulcanism." These queer letters we are talking about are called "dikes." They are made of lava that was driven into cracks in the rocks and afterward cooled into rock that is as hard as iron. Lava is often largely made of iron. [Illustration: MR. VULCAN'S FAMOUS CASTLE ON THE HUDSON This is a part of Mr. Vulcan's famous castle on the Hudson known as the Palisades. Here the lava rock has formed into columns which make the mass look all the more like some old castle of the Middle Ages. The "windows" are where the softer spots in the rock have decayed away. This castle--come to think of it--really belongs to mediæval architecture, for it was built in the Middle Ages of earth's long history. ] [Illustration: THIS IS THE HAND OF VULCAN, TOO] Were you ever down by the seashore in a storm? If so you remember how the ground under your feet shook when a great wave rushed into some narrow passage or crevice in the rocks, and was tossed high in the air in spray. Then just imagine molten lava, which is many times heavier than water, driven into a crack in a rock with the force of a cannon-ball. That's how it happened. That's how those dark strokes in the rock with their heavy shading were made. This was done in the depths of the earth; not on the surface where you see these rocks now. They used to have piles of other rocks above them, but these in course of time have been weathered away. This is known, not only from the marks of the wearing but from the fact that these dikes, as well as the rock into which they have been driven, are crystallized, wholly or in part. Such crystallizing, as we know, takes place away down in the earth.