[Footnote DZ: See Island Life, chapters viii and ix.] CHAPTER X. THE DATE OF THE GLACIAL PERIOD. In approaching the subject of glacial chronology, we are compelled to recognise at the outset the approximate character of all our calculations. Still, we shall find that there are pretty well-defined limits of time beyond which it is not reasonable to place the date of the close of the Glacial period; and, where exact figures cannot be determined, it may yet be of great interest and importance to know something near the limits within which our speculations must range. For many years past Mr. Croll's astronomical theory as to the cause of the Glacial period has been considered in certain circles as so nearly established that it has been adopted by them as a chronological table in which to insert a series of supposed successive Glacial epochs which are thought to have characterised not merely the Quaternary epoch but all preceding geological eras. What we have already said, however, respecting the weakness of Mr. Croll's theory is probably sufficient to discredit it as a chronological apparatus. We will therefore turn immediately to the more tangible evidences bearing upon the subject. The data directly relating to the length of time which separates the present from the Glacial period are mainly connected with two classes of facts: 1. The amount of erosion which has been accomplished by the river systems since the Glacial period; and 2. The amount of sedimentation which has taken place in lakes and kettle-holes. We will consider first the evidence from erosion. [Illustration: Fig. 103.--Diagram of eccentricity and precession: Abscissa represents time and ordinates, degrees of eccentricity and also of cold. The dark and light shades show the warmer and colder winters, and therefore indicate each 10,500 years, the whole representing a period of 300,000 years.]