Lastly, we sometimes have impressions or footprints representing the locomotion of fossil animals, rather than the fossils themselves. In this way some extinct creatures are known to us only by their footsteps on sand or clay, once soft, but now hardened into stone; and in the case of some of the lower animals the trails thus made are often not easily interpreted (Figs. 12, 12_a_). It has been found that even sea-weeds drifted by the tide make impressions of this kind, which, when they occur in old rocks, are very mysterious. Even rain-drops are capable of being permanently impressed on rocks, and constitute a kind of fossils. Besides these we have many kinds of imitative markings which simulate fossils, as those of concretions or nodules, which are often very fantastic in shape, those of dendritic crystallisation giving moss-like forms, and the complicated tracery produced on muddy shores by the little rills of water which follow the receding tide (Fig. 13). Such things are often mistaken by the ignorant for fossil remains, but are easily distinguished by a practised eye. [Illustration: FIG. 12_a_.—Footprints of modern _Limulus_, or king-crab, in the sand, which enable us to interpret those in Fig. 12.] The reader who has followed these, perhaps somewhat dry, details, will be rewarded for his patience by having some conception of the conditions in which we find fossil remains, and of the evidence by which we can refer these to different periods in the history of the earth. [Illustration: FIG. 13.—Current markings on shale, resembling a fossil plant. Reduced from a photograph (Dawson’s _Acadian Geology_).] Carrying this knowledge with us, and at the same time glancing at the table of successive formations prefixed to this chapter, we shall be prepared, without any additional geological study, to understand the statements to be made in the following chapters, and to appreciate the actual nature of the succession of life in so far as it is at present known. [Illustration: MAGNIFIED AND RESTORED SECTION OF A PORTION OF EOZOON CANADENSE. The shaded portions show the animal matter of the Chambers, Tubuli, Canals, and Pseudopodia; the unshaded portions the calcareous skeleton.] CHAPTER II. THE BEGINNING OF LIFE ON THE EARTH.