and that it threw off, one by one, the planets of the solar system. Our earth thus separated from the parent sun probably much more than a hundred million years ago.
The geologist tells us that in process of time the cooling earth gradually raised over its molten interior a thin crust of fire-fused rocks. Then the steam in the atmosphere began to condense and, falling upon this earth crust, formed the first rivers, lakes, and seas. The dust and rock particles in the water accumulated in layers, or strata, which hardened into the stratified rocks. They reach to a depth of perhaps twenty-five miles below the surface and contain fossil remains of plants and animals. The fossils show that life began in lowly forms on the earth, and that all existing life has evolved from these earlier, lowlier forms.
Most of geological time since the origin of the earth is divided into three great epochs. The first or Primary epoch saw the appearance of plants, such as seaweeds, mosses, ferns, and finally of huge-stemmed trees, whose abundant vegetation formed our coal measures. It saw also the appearance of animals, beginning with simple invertebrate creatures which lived in the water and passing to fishes and amphibians. The Secondary epoch was especially the age of enormous reptiles, whose skeletons are shown in museums. During this time bird-like animals developed and became true birds as they grew wings and modified their reptilian scales into feathers. In the third or Tertiary epoch there appeared for the first time a variety and abundance of mammals. Such is the record of the rocks for untold millions of years before the first traces of man.
The Tertiary epoch was characterized by a semi-tropical climate, even in the Arctic region. Toward the close of the Tertiary profound climatic changes began to occur in northern latitudes, producing what is called the Ice Age. An immense ice cap formed in the lands encircling the North Pole and gradually moved southward. North America to the valleys of the Ohio and the Missouri and Europe to the Rhine and the Thames were covered by an
