ICHNOLOGY (Gr. science of footprints) is the name given to that section of paleontol ogy which treats of the impressions made on mud or sand, now indurated into rock, by the animals of the period to which the rocks belong, or by meteoric or other transitory physical forces. The actual remains of the hard portions of time animals them selves are the materials on which chiefly rest our knowledge of the former inhabitants of the globe; but of many animals we know nothing more than the more or less distinct impressions made by them as they moved over the surface of a muddy shore. And in some beds, not only is the evidence of the shore-wave preserved in the ripple-mark, and the influence of the sun's heat exhibited in the superficial cracks, but frequently the passing hail-storm, or the sudden and heavy thunder-shower, has left its impress upon them, and this so perfectly, that it is not difficult to determine, from the form of the cup-like depression, whether or not the rain was accompanied by a breeze, for, by observing the amount of difference in the sides of the cup, and the position of the highest side, the direction of the gale, and its velocity may be approximately deter mined. Though the force or body forming the impression has been removed imme diately after it has made the pressure, yet in these prints the evidences of animal life and of the activity of physical forces have conic down to us from the remotest periods. • The impressions occur almost invariably on rocks that have been deposited as mud; only in a few cases have they been noticed in sandstone. Sometimes the argillaceous deposit is a thin layer between two sandstone beds; it is then difficult to obtain a clear surface in the shale; but the details are carefully preserved in relief in the natural cast on the under surface of the superimposed sandstone. In this manner the footprints are preserved at Stourton in Cheshire.
The necessary conditions for the preservation of footprints seem to be either of the following. The silt-bed may have formed an extensive flat shore, uncovered by the tide at each ebbing. Whatever impressions were made on this plastic surface would be baked and hardened by the influence of the sun, if it remained for a sufficient time uncovered by the water; and when the tide again flowed, the hardened mud;resisting its influence, would receive another film oesediment, which would specially deposit itself in the depressions, and thus secure the permanence of the impressions. These
influences would operate more powerfully on portions of the shore which were under water only at spring-tides. The numerous wading birds are preserved in this manner at the present day, on the plastic mud which covers the flat shore of the bay of Fundy, where the tide rises, it is said, as much as 70 feet. Both Gould and Lyell have given detailed accounts of the process as it goes on there. The other method is one independent of the sun's influence, where, on an ordinary muddy shore dur ing the recession of the tide, the depresssions are filled up by blown sand, and the tide, on its return, flows over a level surface, on which it deposits a fresh layer of silt.
The study of ichnology carries us back to the remotest known period of animal life on the globe. The deposit from which has been obtained the fragment of time oldest known trilobite (pahieopyge), contains the borings of certain worms, and impressions of rain-drops. In strata of the same period, but a little later, series of regularly recurring groups of markings are considered by Mr. Salter as having been produced by the sharp claws of crustacea in walking; while other sets lie refers, with considerable show of probability, to the strokes of the bifurcate tail of an unknown crustacean as it swam through shallow water. From the American representatives of the same rocks (Potsdam sandstones), prof. Owen lies described a number of impressions made apparently by different animals, to which lie has given the generic name of protichnites. The slabs show that the animals made at each step 14, 10, or more impressions, They were most probably crustacea, furnished with three or four pairs of bifurcating limbs, like the modern king-crab. Similar impressions have been observed in the lower Silurian rocks of Eskdale in Scotland, and have been named P. Scoticus. The tracks of numerous annelids occur also in these rocks. They exhibit the impressions of the creatures as they moved along, or sometimes through, time soft mud, and they frequently terminate in a distinct impression of the form of the worm itself, produced perhaps by the dead body, although no trace of the body itself is preserved.