[Illustration: THE KING OF THE NORTHLANDS] GEOLOGY _Finds tongues in trees, books in the running brooks, Sermons in stones, and good in everything._ --_Shakespeare, As You Like It._ The Structure and History of the Earth There is nothing eternal about the earth except eternal change, some one has said. It requires only a little looking about us to see that this is true. The earth is not as it was in the past. Every shower of rain changes or modifies its surface. And many other and some very great changes have occurred during the past few millions of years. During one age, the coal was formed of plants that grew luxuriantly on the earth's surface. At one period in the development of the earth there were many kinds of invertebrate animals, but no animals with backbones. Later, the vertebrates appeared. At one time the whole Mississippi Valley was under the water of the sea. ("The Story of Our Continent," by N. S. Shaler. Ginn & Co.). These statements suggest just a few of the things that have been going on in the history of the earth. By the study of Geology we can learn much more about it, and we should supplement our study of books with the more important actual observation of conditions out-of-doors. To those living in that part of North America, which is shaded in the map on page 451, the easiest and most natural approach to the subject of the structure and history of the earth is by studying the effects of the continental glacier which formerly moved down over this region. Tracks of the Glacier When we see the foot-prints of an animal in the mud or in the snow, we are sure that an animal has passed that way at some previous time. Those who live in Canada or northern United States (See map page 451) can be just as sure that a great glacier or ice-sheet formerly moved down over northern North America, by the tracks it has left. Although it is estimated by geologists that between 10,000 and 40,000 years have elapsed since the Great Ice Age, these tracks or evidences can still be seen by any one who lives in this region or who can visit it. The principal ones are: (1) Boulders or Lost Rocks which were brought down by this glacier; (2) The Glacial Drift or Boulder Clay which covers nearly all of the glaciated region; (3) Scratches on the bed-rock which show the direction the glacier moved. Notice in the field the size and shape of the glacial boulders, where they are found, evidence of the place where the glacier melted off (terminal moraine). Do these boulders increase or decrease in size as we go south over the glaciated area? Can you discover any place where they can be traced back in their native ledge? Present-day glaciers, like the Muir Glacier in Alaska, can be seen transporting boulders and drift just as this great prehistoric ice-sheet must have done.