THE EARTHQUAKES AND THE DELICATE FILMS
It is by the crushing movements that made the earthquake that rocks are broken into confusions of cracks such as you often see in a thick glass window that has been broken. Then into these cracks come dissolved minerals from other rocks and harden into stone. In the marble one set of veins often runs right through another as if they had been inlaid. Then there may be other veins that cross both of these—no end of criss-crossings. The different sets of veins usually differ also in color and in grain, and even have different kinds of mineral in them. With a good hand-glass you can see this difference in texture.
WHEN THE EARTHQUAKE TAKES ITS PEN IN HAND
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.
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.