I
I believe my attention was first directed to the markings made by animals on the surfaces of rocks, when travelling with the late Sir Charles Lyell in Nova Scotia, in 1842. He noticed with the greatest interest the trails of worms, insects, and various other creatures, and the footprints of birds on the surface of the soft red tidal mud of the Bay of Fundy, and subsequently published his notes on the various markings in these deposits in his "Travels in North America," and in a paper presented to the Geological Society of London. I well remember how, in walking along the edge of the muddy shore, he stopped to watch the efforts of a grasshopper that had leaped into the soft ooze, and was painfully making a most complicated trail in his effort to escape. Sir Charles remarked that if it had been so fortunate as to make these strange and complicated tracks on some old formation now hardened into stone and buried in the earth, it might have given occasion to much learned discussion.
At a later period I found myself perplexed in the study of fossil plants by the evident errors of many palæobotanists unacquainted with modern markings on shores, in referring all kinds of mere markings to the vegetable kingdom, and especially to the group of fucoids or seaweeds, which had become a refuge for destitute objects not referable to other kinds of fossils. It thus became necessary to collect and study these objects, as they existed in rocks of different ages, and to compare them with the examples afforded by the modern beach; and perhaps no locality could have afforded better opportunities for this than the immense tidal flats of the finest mud left bare by the great tides of the Bay of Fundy in Nova Scotia. At a more recent period still, the subject has come into great prominence in Europe, and if we are to gauge its importance by the magnitude of the costly illustrated works devoted to it by Delgado, Saporta, Nathorst, and others, and the multitude of scattered papers in scientific periodicals, we should regard it as one of the most salient points in Geology.[149]
[149] Journal of London Geological Society, vol. vii. p. 239.
It may be well further to introduce the subject by a few extracts from Lyell's work above referred to.
"The sediment with which the waters are charged is extremely fine, being derived from the destruction of cliffs of red sandstone and shale, belonging chiefly to the coal measures. On the borders of even the smallest estuaries communicating with a bay, in which the tides rise sixty feet and upwards, large areas are laid dry for nearly a fortnight between the spring and neap tides, and the mud is then baked in summer by a hot sun, so that it becomes solidified and traversed by cracks caused by shrinkage. Portions of the hardened mud may then be taken up and removed without injury. On examining the edges of each slab we observe numerous layers, formed by successive tides, usually very thin, sometimes only one-tenth of an inch thick, of unequal thickness, however, because, according to Dr. Webster, the night tides rising a foot higher than the day tides throw down more sediment. When a shower of rain falls, the highest portion of the mud-covered flat is usually too hard to receive any impressions; while that recently uncovered by the tide, near the water's edge, is too soft. Between these areas a zone occurs almost as smooth and even as a looking-glass, on which every drop forms a cavity of circular or oval form; and if the shower be transient, these pits retain their shape permanently, being dried by the sun, and being then too firm to be effaced by the action of the succeeding tide, which deposits upon them a new layer of mud. Hence we find, on splitting open a slab an inch or more thick, on the upper surface of which the marks of recent rain occur, that an inferior layer, deposited perhaps ten or fourteen tides previously, exhibits on its under surface perfect casts of rain prints which stand out in relief, the moulds of the same being seen in the layer below."
After mentioning that a continued shower of rain obliterates the more regular impressions, and produces merely a blistered or uneven surface, and describing minutely the characteristics of true rain marks in their most perfect state, Sir Charles adds:—
"On some of the specimens the winding tubular tracks of worms are seen, which have been bored just beneath the surface. Sometimes the worms have dived beneath the surface, and then re-appeared. Occasionally the same mud is traversed by the footprints of birds (Tringa minuta), and of musk-rats, minks, dogs, sheep and cats. The leaves also of the elm, maple and oak trees have been scattered by the winds over the soft mud, and having been buried under the deposits of succeeding tides, are found on dividing the layers. When the leaves themselves are removed, very faithful impressions, not only of their outline, but of their minutest veins, are left imprinted on the clay."
This is a minor illustration of that application of recent causes to explain ancient effects of which the great English geologist was the apostle and advocate, and which he so admirably practised in his own work. It is also an illustration of the fact that things the most perishable and evanescent may, when buried in the crust of the earth, become its most durable monuments. Footprints in the sand of the tidal shore are in the ordinary course of events certain to be obliterated by the next tide; but when carefully filled up by gently deposited new material, and hardened into stone, there is no limit to their duration.
Let us inquire how this may take place, and the tidal flats of the Bay of Fundy and Basin of Minas may supply us with the information desired. In the upper parts of the Bay of Fundy and its estuaries the rise and fall of tide, as is well known, are excessive. I quote the following description of the appearance they present from a work of earlier date:—