It is to be noted also, that Prestwich pronounces part of this high level or southern drift to be older than the Westleton pebble drift which forms part of the Upper Pliocene series in Suffolk and Norfolk, and which the Professor has traced over many of our southern counties. If this conclusion is correct, it solves the problem of tertiary man by showing numerous palæolithic implements in a stage older than an undoubted Pliocene bed. The implements found in these high-level southern drifts are all of a very rude type, and the discovery is confirmed by similar implements having been found at corresponding elevations on the chalk downs of Hertfordshire and on the South Downs.[10]
I will mention only one other instance, which shows that the New World confirms the conclusion as to the antiquity of the quaternary age. The auriferous gravels of California consist of an enormous mass of débris washed down by pre-glacial or early glacial rivers from the western slopes of the great coast range. During their deposition they became interstratified with lavas and tuffs from eruptions of volcanoes long since extinct, and finally covered by an immense flow of basalts, which formed a gently inclined plane from the Sierra Nevada to the Pacific. This plane was attacked by the denudations of the existing river-courses, and cut down into a series of flat-topped hills, divided by steep cañons and by the valleys of the present great rivers. In one case, that of the Colombia river, this denudation has been carried down to a depth of over 2000 feet, and the river flows between precipitous cliffs of this height. The present gold-mining is carried on mainly by shafts and tunnels driven through superficial gravels and sheets of basalts and tuffs, to the gravels of the pre-glacial rivers, which are brought down in great masses by hydraulic jets. In a great number of these cases stone implements of undoubted human origin have been found at great depths under several successive sheets of basalts, tuffs, and gravels. Mr. Skertchly, an eminent English geologist, who recently visited the district, says of these gravels, "Whatever may be their absolute age from a geological standpoint, their immense antiquity historically is beyond question. The present great river system of the Sacramento, 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.
I turn from the effects to the causes of that great glacial period which has been described in the last chapter. This line of investigation is peculiarly interesting in the search after human origins, for it affords the only chance of reducing the vague periods of immense duration shown by geology, to something like a definite chronology of years and centuries. If astronomical causes, the dates of which admit of mathematical calculation, can be shown to have been, if not the sole or principal, yet one of the causes which must have influenced the phenomena of the glacial epoch, we may assume these dates for the occurrence of the human remains which accompany these phenomena. Otherwise we must fall back on immense antiquity, which may mean anything from 50,000 or 100,000, to 500,000 or 1,000,000 years, since the first authentic evidence for palæolithic man.
The first step towards an investigation of the cause of glacial periods, is to consider what are the conditions of the actual ones which are now prevailing. We have one such period in Greenland, another in the Antarctic region, a third in high mountain chains like those of Alaska, and of the Swiss and New Zealand Alps. In all these cases we find certain common conditions. High land in high latitudes, rising in great masses above the snow-line or temperature which condenses water in the solid form; and winds which are charged with great quantities of watery vapour raised by evaporation, to be so condensed.