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RAINPRINTS, SUN-CRACKS, FOOTPRINTS,
IN rainprints, sun-cracks, ripple-marks, footprints, miscellaneous tracks, and burrows, we are furnished with a set of fossil evidences of rare and curious interest. What more marvellous than that the shower which fell millions of ages ago should have registered in the earth's crust not only its intensity, but the direction of the wind by which it was driven ; what more wonderful than that the wandering water-bird of bygone epochs should have left its footprints in the same crust when every other evidence of its existence has been destroyed! These “physical and physiological impressions," as they have been termed, are indeed surpassingly strange, and often startle the young geologist more than the discovery of the veritable substance of a fossil organism. He is prepared, in some measure, for shells, bones, and other organic remains, but traces like these are beyond his expectation, and when discovered, he views them not only with wonder, but with a certain amount of doubt and incredulity. Let us take a passing glance at their nature, their modes of occurrence, and the leading facts which they so instructively reveal.
And first, of the rainprint, which can be traced on rocks of all formations, from the sandy slabs of the Silurian up to the desiccated muds of the summer just gone by. As on these muds the heavy thunder-shower left its deeper indent and elevated margin, the drifting blast its slanting impress, and the gentle rain its less distinct and close-set patter, so all of these in their varying intensities have been registered in the rocky crust-proving, if aught can prove, that in every epoch of the world's history winds have blown and rains bave beaten even as they still blow and beat around us. On the Old Red Sandstones of Forfarshire (Ferryden) we have seen rainprints of the deepest intensity ; on the Millstone Grit of Derbyshire (Buxton and Whaley Road) we have traced the slanting spatter of the wind-driven shower; and on the Carboniferous flags of Linlithgowshire (Kirkton of Bathgate) have discovered the closer patter of the gentler rainfall, and this crossed and recrossed by the winding track of an annelid or of a mollusc, just as the rainprints on muds of the present day are frequently crossed by the trail of the earth-worm. In all of these instances there could be no doubt as to the producing agent; and to render the evidence still more certain, 'we constructed in 1858 a mud-tray, in which every form of rainfall was temporarily registered, and in each case the resemblance was perfect. Indeed, if we had dried these rain prints of 1858, and covered them over with a coating of mud-silt or sand-silt, and allowed both to harden, the mould and its cast, on splitting, would have been in almost every example undistinguishable from those of the palæozoic flagstones.
And in the same way the sun-crack or desiccation-crack has registered in the rocky crust its evidence of drying winds and evaporating sunshine. These sun-cracks occur, too, in all formations, and for the most part on the surfaces of fine-grained argillaceous flagstones. Some of the finest examples we have witnessed were taken from the yellow
sandstones of Dura Den (now or lately in Cupar Museum), and from a quarry of Lower Carboniferous age at Grange, near Edinburgh ; but equally startling specimens are of frequent occurrence in the New Red Sandstones of Cheshire and Dumfriesshire. Let the observer note some shallow pool or clay-pit when the drought of summer has evaporated the water, and he will find that the clayey mud begins to shrink and crack in a most fantastic fashion, and occasionally with something approaching to geometrical regularity. Let him suppose this fissured mud to be thoroughly dried and hardened, and then overlaid by a new deposit of sandy silt or other sediment; and further, that both are consolidated into rock, he would find, on splitting them asunder, the mould and cast preserved in every line to the minutest reticulation. And this is precisely what the geologist discovers in these old-world sun-cracks—some reticulated like the venation of a gigantic leaf, others tessellated like a work of art, and some again so curiously arranged that they have been mistaken by the quarrymen for the tracery of Gothic windows, and noted in local newspapers as antiquarian marvels. These desiccation-cracks are often, indeed, of very curious interest, and, as might be anticipated, the overlying bed, or that retaining the cast, is that which presents the most entire and satisfactory evidence-the cast being always less or more raised or in relief, whereas the mould usually appears as a hackly and imperfect depression.
Belonging to the same category as the rainprint and sun-crack are those water-marks or ripple-marks, whose tiny undulations diversify the surface of many sandstones in all the stratified formations. As the existing shoresands are frequently rippled by the receding tide, or as the lighter sands of terrestrial tracts are thrown into similar wavelets by the passing winds, so have these old sandstones been rippled and hardened and preserved in the integrity of their undulated surfaces for millions of ages. Nor is it merely the ripple-mark that has been preserved, but frequently over this ripple the trail of the annelid or of the mollusc may be traced as clearly as it can be followed over the sands that have been deserted by the latest tide. The finest of this description we have ever witnessed occurred on the Old Red flagstones of Ardoch Mill, in Perthshire, and there the ripple extended over every foot of exposed surface, and was marked throughout by the winding trails of some annelid, or of some gasteropod mollusc. In examining these ripple-marks, the question may sometimes be raised, Are they water-ripples or wind-ripples, or how can the one be distinguished from the other? It is true there is a close similarity between all kinds of ripple, whether produced by tides, by gentle movements of shallow water, or by gentle currents of wind; but seeing that these ripples are so frequently traversed by winding trails, and knowing, moreover, that they must have had sufficient consistency to retain the form of these tracks, the likelihood is that the greater portion we witness on sandstone surfaces have been formed on the shores of open seas and tidal estuaries.
But wonderful as these mere physical impressions are, they fall short of the interest which attaches to the physiological—the footprints, the tracks, and burrows of creatures that flourished during these far-distant epochs. As the sea-bird and reptile leave their footprints on the silty shores of existing seas and estuaries, so the old wader and amphibious reptile left their foot-tracks on the shores of palæozoic and mesozoic waters—here traversed thickly and indistinctly in crowds, and there singly and distinctly, footfall after footfall, in continuous directions ; here in a single bipedal row, and there in a double quadrupedal series. There can be nothing more beautiful and convincing than these ichnites or fossil footprints (Gr. ichnon, a footstep), whether they be those of birds, of reptiles, or of lowlier creatures. Though no bone, nor tooth, nor scute of the creatures may have been detected, yet there the impress of the foot is conclusive of its presence, and often indicative of its kind and proportions. Though ichnites have been discovered in all formations from the Silurian upwards—it is chiefly those in the New Red Sandstone (Hildburghausen in Saxony, Stourton and Taporley in Cheshire, Corncockle in Dumfriesshire, Cummingstone in Moray, and Connecticut in North America) that have received the attention of palæontologists. On some of these New Red flagstones the footprints are evidently those of birds (ornithichnites) with the impress of every toe, and the corrugations of the skin entire; while on others (or intermingled with those of birds) the implants are those of reptiles (sauroidichnites)—sometimes light and slender as those of a lizard, and sometimes heavier and more decided than those of the largest crocodile. And here, in the New Red Sandstone, where no bird-bone has yet been discovered, these footprints become the only and sufficient testimony to the palæontologist that, on the shores of these old tidal waters, birds of various orders wandered in quest of food—now searching slowly with tortuous step, and now coursing rapidly with lengthened stride and heavier footfall. To the student who may not have the opportunity of examining these curious impressions in the field, we could not recommend a source of a few hours' more pleasant study than the 'Ichnology of New England,' by Professor Hitchcock ; the 'Ichnology of Annandale,' by Sir William Jardine ; or the “Fossil Foot-marks in the Red Sandstones of Pottsville, Pennsylvania,' by Professor Lea of Philadelphia.