There are four main methods of determining prehistoric {60} time.[3] One is called the (1) _geologic method_, which is based upon the fact that, in a slowly cooling earth and the action of water and frost, cold and heat, storm and glacier and volcanic eruption, the rocks on the earth are of different ages. If they had never been disturbed from where they were first laid down, it would be very easy to reckon time by geological processes. If you had a stone column twenty feet high built by a machine in ten hours' time, and granting that it worked uniformly, it would be easy to see just at what hour of the period a layer of stone four feet from the bottom, or ten feet from the top, was laid. If, however, in the building of the wall, it should have toppled over several times and had to be rebuilt, it would require considerable study to see just at what hour a certain stone was put in the wall. Studying the geology of the earth in a large way, it is easy to determine what strata of the earth are oldest, and this may be verified by a consideration of the process in which these rocks were being made. Chemistry and physics are thus brought to the aid of geology. It is easy to determine whether a rock has been fused by a fire or whether it has been constructed by the slow action of water and pressure of other rocks. If to-day we should find in an old river bed which had been left high and dry on a little mesa or plateau above the present river bottom, layers of earth that had been put down by water, and we could find how much of each layer was made in a single year, it would be easy to estimate the number of years it took to make the whole deposit. Also if we could find in the lowest layer certain relics of the human race, we could know that the race lived at that time. If we should find relics later on of a different nature, we should be able to estimate the progress of civilization. The second method is of (2) _paleontology_, which is developed along with geology. In this we have both the vertebrate and invertebrate paleontology, which are divisions of the science which treats of ancient forms of animal and vegetable life. There are many other divisions of paleontology, some {61} devoting themselves entirely to animal life and others to vegetable, as, for instance, paleobotany. As plants and animals have gradually developed from lower to higher forms and the earth has been built gradually by formations at different periods of existence, by a comparison of the former development with the latter, that is, comparison with the earth, or inorganic, development to the life, or organic, development, we are enabled to get a comparative view of duration. Thus, if in a layer of earth, geological time is established and there should be found bones of an animal, the bones of a man, and fossilized forms of ancient plants, it would be easy to determine their relative ages.