The earth is supposed to be a small, condensed portion of the gaseous material which astronomers tell us at one time pervaded all space. The heat given off when the gas was condensing has been largely converted into mechanical energy which makes the earth revolve once in twenty-four hours and sends it flying through space. As soon as the earth decreased to about its present size and became cool enough for water to be condensed on its surface, it began to write its own history. Its entire surface may have at one time been covered to a uniform depth by water. If such was ever the case it did not remain so long. The interior of the earth was very hot and the crust cooled very irregularly and portions of it rose above the surface of the water. Since then there have been two antagonizing forces at work. The heat has caused the earth's surface to become irregular and the water has made a strong effort, which has been partially successful, to reduce all irregularities to the same level. We do not know how long these forces nearly balanced each other, but sooner or later dry land appeared in many places on the earth's surface. This was for a long period of time washed by heavy rains while the shores for some distance seaward were worn away by action of the tidal waves. Much of the land area then sank below sea level, and became covered with sand, gravel and the like. The portion which remained above the level is called the Archaean. Later a general elevation of the land area brought above sea level much of this land and gravel, forming around the Archaean an increased land era, which we call Silurian. The time when the sand and gravel was deposited forming this land is known as the Silurian age. Following this came the Devonian age. After this in the following order came the following geological ages: Carboniferous, Triassic, Jurassic, Cretaceous, Tertiary, Quaternary and then the present or Recent age, the one in which we now live. Each of these ages is characterized by the peculiar animals which then predominated, and these animals are known only from their remains imbedded in the rocks as fossils.