As a general introduction to geological chronology, a short summary may be given of the different formations or groups of strata, to which certain names have been assigned to serve as convenient designations for succeeding epochs in the world’s evolution. The following table (Fig. 3, pp. 32, 33) represents the geological series in a convenient form; the most characteristic rocks of each period are indicated by the usual conventional shading, and the most important breaks or lacunae in the records are shown by gaps and uneven lines. The relative thickness of the rocks of each period is approximately shown; but the vertical extent of the oldest or Archaean rocks as shown in Fig. 3 represents what is without doubt but a fraction of their proportional thickness. This table is taken, with certain alterations, from a paper by Prof. T. McKenny Hughes in the Cambridge Philosophical Proceedings for 1879. Speaking of the graphic method of showing the geological series, the author of the paper says, “It is convenient to have a table of the known strata, and although we cannot arrange all the rocks of the world in parallel columns, and say that _ABC_ of one area are exactly synchronous with _A′B′C′_ of another, still if we take any one country and establish a grouping for it, we find so many horizons at which equivalent formations can be identified in distant places, that we generally make an approximation to HOMOTAXIS as Huxley called it. The most convenient grouping is obviously to bracket together locally continuous deposits, _i.e._ all the sediment which was formed from the time when the land went down and accumulation began, to the time when the sea bottom was raised and the work of destruction began. In the accompanying table I have given the rocks of Great Britain classified on this system, and bearing in mind that waste in one place must be represented by deposit elsewhere, I have represented the periods of degradation by intervals estimated where possible by the amount of denudation known to have taken place between the periods of deposition in the same district[49].” [Illustration: TABLE OF STRATA. RECENT ╮ GLACIAL ├ QUATERNARY PLIOCENE ╯ MIOCENE ╮ │ OLIGOCENE ├ TERTIARY │ EOCENE ╯ CRETACEOUS ╮ │ JURASSIC │ ├ MESOZOIC TRIAS │ │ PERMIAN ╯