The figures appearing in the third column, between the Ages and Periods, indicate the millions of years that have elapsed up to present time. They denote the age of the rocks at the beginning of each period. The age of a plant or animal which lived in Eocene time would be, according to this scale, somewhere between 35 million and 60 million years. In practice it is usually possible to determine whether a fossil was embedded in the rocks during an early or late portion of the period, and thus its age may be established within a shorter range, but it is impossible to be exact, even in terms of millions of years, with regard to anything as far back in prehistory as the Eocene period. The period in which we are living today is known as Recent. It began at the close of the Ice Age or Pleistocene period about ten thousand years ago and represents so little of earth history since the beginning of life that a chart many times the length of this page would be required to show the rest of the periods in proportion. The Cambrian period is an early chapter in which the story of prehistoric life suddenly becomes clear and richly varied. It is, however, much farther from the beginning of the record than it is from the present, and the Pre-Cambrian eras would require a great deal more space in order to show their relative lengths. The Archeozoic and Proterozoic eras have to some extent been divided into periods, but the great antiquity of the rocks has obscured much of their history, and divisions established for one locality have been of little service elsewhere. Consequently, the period names are in less general use and the common practice is to refer to all this great stretch of time as Pre-Cambrian. In the last column, at the right of the chart, some of the historical features are indicated. This column should be read from bottom to top in order to get the proper development of the story, and at best this sketchy outline of events can be no more than suggestive of the progress and decline through which the earth’s inhabitants have passed. Rocks of every period except probably the Silurian are known to have been deposited somewhere in the Colorado area, although in most cases the record for each period is far from complete. Formations are too numerous and too varied locally to be shown on a chart of this type. THE GEOLOGICAL SECTION