In the following arrangement the strata are enumerated as if lying in regular sequence, one beneath the other; but in nature such an unbroken series has never been observed. A few groups only occur in a serial order, and these but rarely in their original position. The beds are for the most part disrupted, and lie in various angles of inclination; sometimes they are completely retroverted, the newer strata underlying those upon which they were originally deposited. The order of succession has been ascertained by careful observation of the relative superposition of the respective members of the series in different countries; and from an immense number of facts collected by able observers in every part of the globe. This synopsis presents a chronological arrangement of the rocks according to the present state of geological knowledge, but it must not be supposed that these rigid distinctions, these hard lines, which are necessary to facilitate the acquisition of a general idea of the phenomena attempted to be explained, exist in nature. By whatever names we designate geological periods, there appear to be no clearly defined boundaries between them in reference to the whole earth: such well marked lines may be seen in particular localities, but daily experience teaches us that there is a blending, and a gradual and insensible passage, from the lowest to the highest sedimentary strata, particularly in respect of fossil remains. The terms employed to designate formations can only be considered as expressing the predominance of certain characters, to be used provisionally, as a convenient mode of classifying and generalizing the facts collected, whilst that knowledge is accumulating which in after times will reveal the nature and order of succession of the principal events in the earth's physical history.[11] [11] "Wonders of Geology," p. 892. Dr. Buckland's "Bridgewater Treatise" (Vol. II. _frontispiece_) contains a comprehensive Diagram of the rocks and strata of which the crust of the earth is composed; it was drawn by the late Mr. Thomas Webster. CHAPTER II. SYNOPSIS OF THE BRITISH STRATA. "Hard lines are admissible in Science, whose object is not to imitate Nature, but to interpret her works."--Greenough. The classification of the stratified rocks is based on three principal characters; namely, 1, the mineral structure; 2, the order of superposition; and 3, the nature of the organic remains; the following synopsis has been drawn up in accordance with these principles.[12] [12] See "Wonders of Geology," vol. i. pp. 200-207, for a Synoptical Table of the principal rocks. CHRONOLOGICAL ARRANGEMENT OF THE BRITISH FORMATIONS. COMMENCING WITH THE UPPERMOST OR NEWEST DEPOSITS. Modern or Human Epoch. Alluvial Deposits: remains of Man and existing species of mammalia. Post-Pliocene.