Looking again about our own country on the land that lies cradled between the eastern and the western mountains of the United States, one cannot help but regard with awe the magnificent stretches of time it must have taken for this great area to have been formed beneath the sea. This alone proves the great age of the earth. Because it was raised slowly, the strata lie there little disturbed, one over the other, "like the leaves of a great stone book," containing the history of the earth. A good view of the river strata may be obtained in the canyon of the Colorado river, for this mighty river has carved a bed through the stratified deposits to a depth of from 3000 to 6000 feet. "To quarry the heart of the earth Till, in the rocks red rise, Its age and birth, through an awful girth Of strata, should show the wonder-worth Of patience to all eyes." The elevated sea-bottom is a fertile field for the fossil hunter, affording evidence that enables the geologist to reconstruct the evolutionary development of life on our planet in its divinely ordained stages. There is often but little resemblance in the fossils found in the lower strata to the plants and animals now in existence, but traced from stratum to stratum, the connection is obvious. The "fire-formed" rocks of the Archæan Age, which support all the other strata, contain no evidence of life; neither do the first sedimentary rocks which were deposited above them. The first fossils are found in the strata overlying the first sedimentary rock. Geologists have divided the history of the earth into seven ages: 1. _The Archæan Age_, represented by the Archæan system of rocks. 2. _The Age of Invertebrates_, represented by the Cambrian, the Lower Silurian and the Upper Silurian rocks. 3. _The Age of Fishes_, represented by the Devonian rocks. 4. _The Age of Amphibians_, represented by the Carboniferous rocks. 5. _The Age of Reptiles_, represented by the Mesozoic rocks. 6. _The Age of Mammals_, represented by the Cenozoic rocks. 7. _The Age of Man_, represented by recent rocks. All of these Ages, which represent the life history of the world, left their impression on the rocks of their times, and each Age is connected with the preceding and the succeeding Age like links in a long, long chain. In regard to the distinctness and importance of the last great era, the Age of Man, Le Conte, in "Elements of Geology," says that there are two views which will ever divide geologists, depending on the two views regarding the relation of man to Nature: