4. A WALK UNDER THE SEA.

This is a trip which we shall enjoy, I think, but which we shall have to take in our imagination; for though there are wagons and cars, boats and balloons, invented for carrying us on the land, on the water, and in the air, no means has yet been found by which we can travel on the ocean bed.

Do you know what it is to make a journey in imagination?

After leaving the pebbly beach and getting beyond the slope of smooth sand, we find ourselves in the midst of a delightful garden where seamosses, green, red, and purple, look gay and beautiful; and fish and other curious little animals sport with each other, and float here and there in the water.

As we descend, the water becomes colder and colder, and the beautiful gardens disappear. At last we stand in darkness in the midst of the sediment brought down by rivers. This is soft and slimy, made slimy by the decay of the many plants and animals of the sea.

We are not alone, however. There are life and work all around us. See these curious little creatures, working in and out of the mud and slime.

They must be very short-lived; for the floor of the sea is strewn with their skeletons or with lime from their broken or decayed coverings.

Upon this floor a stream of mud from the rivers is constantly pouring, which, being mixed with the lime and shells, forms a layer of fine clay on which the animals of the sea work and die. There are so many of these animals they never could be counted. Then the waters of the rivers bring here with them lime which they steal from the limy rocks over which they flow down the mountain sides on their way to the sea.

In this way the work goes on, until we find the broad sea-bed thus formed, pushing up above the sea. The waters disappear, leaving a broad plain, composed of many layers. This set of layers is a geologic formation or a part of the earth's surface.

"How high the hills are, father!" cried Fred, as he seated himself on a chair near the rail of the "Horicon," and looked up at the walls of the rugged mountains which border the beautiful lake on which they rode, with its clear, blue water and forest-covered islands.

"They are not hills, but mountains," replied his father. "Can you tell me the difference between mountains and hills?"

"Oh, yes! a mountain is a grown-up hill," said Fred.

"A mountain does look like a high hill. Let us see if we can understand how they are formed.

"You can easily see how some hills are formed, by noticing the water as it flows down a dirt road

after a rain, cutting channels and heaping up little hillocks as it flows. Hills are slowly carved in the same way by the cutting power of streams as they flow down over the earth and the rocks.

"Mountains, however, and most hills are born in the sea, and grow by a more difficult process than that we have seen in the dirt road after a rain.

"All the time the rocky beds are being made in the sea by the sediment washed down by rivers. and by the busy little creatures clothed in armors of lime, about which you have read, the earth's heat is constantly leaking out into the air.

"As it loses heat the earth shrinks and puckers these rocky layers together, forming huge wrinkles in about the same way that an apple shrivels its skin after its juice has dried up or evaporated.

"These wrinkled layers become tilted; and when they reach the surface of the water they appear somewhat like those whose pictures we see.

"Thus the building and the wrinkling of the rock goes on for centuries until these tilted layers are pushed far above the surface of the sea. They are then carved by the rivers and streams in the same manner as the hills are. Thus you see a mountain is only a large hill.

"This is the story of the mountains which shut in the beautiful lake on which we are sailing."

6. WHAT IS AN EARTHQUAKE?

You have been told that the crust of the earth wrinkles something as an apple wrinkles after the juice has been dried up or evaporated. Will you read the lesson again, beginning on page 17? Then read what is said of the uplifting or tilting of the earth's crust on page 25.

You may think that the breaking of the earth's crust is caused by earthquakes. If you take a sound apple in your hands, you will see that the skin is smooth as if tightly drawn over the surface. Now you know the skin, though not very thick, is tough and strong compared with the pulp inside of it. The soft, juicy pulp will dry up, as we say; that is, the moisture of the pulp will evaporate. This will make the pulp inside the skin smaller.

The skin, except a very small part, has not been evaporated. It is as large and tough to all appearances as it ever was; but to adjust itself to the smaller size of the apple, or to fit the apple as its pulpy part grows smaller, the skin wrinkles and becomes rough. Some of these wrinkles you will notice have sharp edges; others have broad surfaces at the top. Some are short; others are long. Some are broken by cross-wrinkles; in some places