Joaquin, and other rivers has been established; cañons 2000 feet deep have been carried through lava, gravels, and into the bed rock; and the gravels, once the bed of large rivers, now cap hills 6000 feet high. There is ample ground for the belief that these gravels are of Pliocene age, but the presence of objects of human formation invests them with a higher interest to the anthropologist than even to the geologist." I will return to this subject more fully in a later chapter, when dealing with the question of the human remains found in these Californian gravels. Those who wish to pursue the subject further will find abundant evidence in the works of Lyell, Geikie, Evans, Boyd Dawkins, and other modern geologists, and a popular summary of it in my _Modern Science and Modern Thought_. It is sufficient for my present purpose to have shown that even taking the quaternary period alone, geology shows that there is an abundant balance in the bank of Time to meet any demands that may be made upon it by any of the kindred sciences. But it is to those we must look for any chance of even an approximate measurement in years or centuries, for geology and palæontology only show immense periods, but give no certain information as to definite durations. The clue, if any, must be sought in Croll's astronomical theory of the glacial period, which I now proceed to consider. CHAPTER IX. THE GLACIAL PERIOD AND CROLL'S THEORY. Causes of Glacial Periods--Actual Conditions of existing Glacial Regions--High Land in High Latitudes--Cold alone insufficient--Large Evaporation required--Formation of Glaciers--They flow like Rivers--Icebergs--Greenland and Antarctic Circle--Geographical and Cosmic Causes--Cooling of Earth and Sun, Cold Spaces in Space, and Change in Earth's Axis, reviewed and rejected--Precession alone insufficient--Unless with High Eccentricity--Geographical Causes, Elevation of Land--Aerial and Oceanic Currents--Gulf Stream and Trade Winds--Evidence for greater Elevation of Land in America, Europe, and Asia--Depression--Warmer Tertiary Climates--Alps and Himalayas--Wallace's _Island Life_--Lyell--Croll's Theory--Sir R. Ball--Former Glacial Periods--Correspondence with Croll's Theory--Length of the different Phases--Summary--Croll's Theory a Secondary Cause--Conclusions as to Man's Antiquity.