Some Salient Points in the Science of the Earth

by Sir John William Dawson

"The tide wave that sweeps to the north-east, along the Atlantic coast of the United States, entering the funnel-like mouth of the Bay of Fundy, becomes compressed and elevated, as the sides of the bay gradually approach each other, until in the narrower parts the water runs at the rate of six or seven miles per hour, and the vertical rise of the tide amounts to sixty feet or more. In Cobequid and Chiegnecto Bays these tides, to an unaccustomed spectator, have rather the aspect of some rare convulsion of nature than of an ordinary daily phenomenon. At low tide wide flats of brown mud are seen to extend for miles, as if the sea had altogether retired from its bed; and the distant channel appears as a mere strip of muddy water. At the commencement of flood a slight ripple is seen to break over the edge of the flats. It rushes swiftly forward, and, covering the lower flats almost instantaneously, gains rapidly on the higher swells of mud, which appear as if they were being dissolved in the turbid waters. At the same time the torrent of red water enters all the channels, creeks and estuaries; surging, whirling, and foaming, and often having in its front a white, breaking wave, or 'bore,' which runs steadily forward, meeting and swallowing up the remains of the ebb still trickling down the channels. The mud flats are soon covered; and then, as the stranger sees the water gaining with noiseless and steady rapidity on the steep sides of banks and cliffs, a sense of insecurity creeps over him, as if no limit could be set to the advancing deluge. In a little time, however, he sees that the fiat, 'Hitherto shalt thou come, and no farther,' has been issued to the great bay tide: its retreat commences, and the waters rush back as rapidly as they entered.

"The rising tide sweeps away the fine material from every exposed bank and cliff, and becomes loaded with mud and extremely fine sand, which, as it stagnates at high water, it deposits in a thin layer on the surface of the flats. This layer, which may vary in thickness from a quarter of an inch to a quarter of a line, is coarser and thicker at the outer edge of the flats than nearer the shore; and hence these flats, as well as the marshes, are usually higher near the channels than at their inner edge. From the same cause,—the more rapid deposition of the coarser sediment,—the lower side of each layer is arenaceous, and sometimes dotted over with films of mica, while the upper side is fine and slimy, and when dry has a shining and polished surface. The falling tide has little effect on these deposits, and hence the gradual growth of the flats, until they reach such a height that they can be overflowed only by the high spring tides. They then become natural or salt marsh, covered with the coarse grasses and carices which grow in such places. So far the process is carried on by the hand of nature; and before the colonization of Nova Scotia, there were large tracts of this grassy alluvium to excite the wonder and delight of the first settlers on the shores of the Bay of Fundy. Man, however, carries the land-making process farther; and by diking and draining, excludes the sea water, and produces a soil capable of yielding for an indefinite period, without manure, the most valuable cultivated grains and grasses."

The mud of these great tidal flats is at the surface of a red colour, and so fine that when the tide leaves it and its surface becomes dry, it shines in the sun as if polished. It is thus capable of taking the finest impressions. When the tide is in, numerous small fish of various species occupy the ground and may leave marks of their fins and tails as they gambol or seek their food. Shell fishes, worms, and Crustaceans scramble over the same surface, or make burrows in it. As the tide recedes flocks of sandpipers and crows follow it down, and leave an infinity of footprints, and even quadrupeds like the domestic hog go far out at low water in search of food. It is said that in some parts of the Bay the hogs are so assiduous in this pursuit that they even awake and go out on the flats in the night tide, and that they have so learned to dread the dangers of the flood, that when in the darkness they hear the dull sound of the approaching bore, they squeal with fear and rush madly for the shore.

If we examine it minutely, we shall find that the tidal deposit is laminated. The tidal water is red and muddy, and holds in suspension sediment of various degrees of coarseness. This, undergoes a certain process of levigation. In the first run of the flood the coarser material falls to the bottom. As its force diminishes the finer material is deposited, and at full tide, when the current has ceased, the finest of all settles, forming a delicate coat of the purest and most tenacious clay. Thus, if a block of the material is taken up and allowed to dry, it tends to separate into thin laminæ, each of which represents a tide, and is somewhat sandy below, and passes into the finest moulding clay above. The tracks and impressions preserved are naturally made on the last or finest deposit, and filled in with the coarser or more sandy of the next tide. But this may take place in different ways. Impressions made under water at flood tide, or on the surface left bare by the ebb, may in favourable localities be sufficiently tenacious or firm to resist the abrading action of the flood, and may thus be covered and preserved by the next layer, and in this way they may be seen on splitting up a block of the dried mud. But in shallow places and near the shore, where the deposit has time to consolidate and become dried by the sun and air before the next tide, much better impressions are preserved; and lastly, on those parts of the shore which are reached only by the spring tides, the mud of the highest tide of course may have several days to harden before the next tide reaches it, and in this case it becomes cracked by an infinity of shrinkage cracks, which, when it is next covered with the tide, are filled with new sediment. In this way is produced in great perfection that combination of footprints, or even of impressions of rain, with casts of cracks, which is so often seen in the older rocks. Where on the sides of channels or near the shore the mud has a considerable slope, another and very curious effect results. As the tide ebbs the water drains off the surface, or oozing out of the wet sand and mud, forms at the top of the bank minute grooves often no larger than fine threads. These coalesce and form small channels, and these, again, larger ones, till at low tide the whole sloping surface is seen to be covered with a smooth and beautiful tracery resembling the rivers on a map, or the impressions of the trunks and branches of trees, or the fronds of gigantic seaweeds. These "rill marks," as they have been called, are found in great abundance in the coal formation and triassic sandstones and shales, and I am sorry to say, have often been named and described as Fucoids, and illustrated by sumptuous plates. Sometimes these impressions are so fine as to resemble the venation of leaves, sometimes so large as to simulate trees, and I have even seen them complicated with shrinkage cracks, the edges of which were minutely crenulated by little rills running into them from the surface.

It is further to be noticed that all these markings and impressions on tidal shores may, when covered by succeeding deposits, appear either in intaglio or relief. On the upper surface they are of course sunken, but on the lower surface of the bed deposited on them they are in relief. It often happens also that these casts in relief are the best preserved. This arises from the fact that the original moulds or impressions are usually made in clay, whereas the filling material is sandy, and the latter, infiltrated with calcareous or siliceous matter, may become a hard sandstone, while the clay may remain a comparatively soft shale. This tendency of casts rather than of moulds to be preserved sometimes produces puzzling effects. A cylindrical or branching trail thus often assumes the appearance of a stem, and any pits or marginal impressions assume the form of projections or leaves, and thus a trail of a worm or Gastropod or a rill mark may easily simulate a plant. It is to be observed, however, that these prominent casts are on the under side of the beds, that their material is continuous with that of the beds to which they belong, and that they are destitute of any carbonaceous matter. There are, however, cases where markings may be in relief, even on the upper surfaces of beds. The following are illustrations of this. Just as a man walking in newly fallen snow compresses it under his feet, and if the snow be afterwards drifted away or melted away by the sun, the compressed part resists longest, and may appear as a raised footmark, so tracks made on soft material may consolidate it so that if the soft mud be afterwards washed away the tracks may remain projecting. Again, worms eject earthy matter from their burrows, forming mounds, patches or raised ridges of various forms on the surface, and some animals burrow immediately under the surface, pushing up the mud over them into a ridge, while others pile up over their bodies pellets of clay, forming an archway or tunnel as they go. Zeiller has shown that the mole cricket forms curious roofed trails of this kind, and it seems certain that Crustaceans and marine worms of different kinds execute similar works, and that their roofed burrows, either entire or fallen in, produce curious imitations of branches of plants.

The great and multiform army of the sea worms is indeed the most prolific source of markings on sea-formed rocks. Sometimes they cover very large surfaces of these, or penetrate the beds as perforations, with tortuous furrows, or holes perfectly simple, or marked with little striæ made by bristles or minute feet, sometimes with a fringe of little footmarks a each side, sometimes with transverse furrows indicating the joints of the animal's body. Multitudes of these markings have been described and named either as plants or as worm-tracks. Again, these creatures execute subterranean burrows, sometimes vertical, sometimes tortuous. These are often mere cylindrical holes afterwards filled with sand, but sometimes they have been lined with a membranous tube, or with the rejectamenta of the food of the animals, or with little fragments of organic matter cemented together. Sometimes they open on the surface as simple apertures, but again they may be surrounded with heaps of castings, sometimes spiral in form, or with dumps of sand produced in their excavation, and which may assume various forms, according to circumstances. Sometimes the aperture is double, so that they seem to be in pairs. Sometimes, for the convenience of the animal, the aperture is widened into the form of a funnel, and sometimes the creature, by extending its body and drawing it in, surrounds its burrow with a series of radiating tracks simulating the form of a starfish or sea anemone, or of the diverging branches of a plant.

Creatures of higher grade, provided with jointed limbs, naturally make their actions known in more complicated ways. Some years ago I had the pleasure of spending a few weeks at the favourite sea-side resort of Orchard Beach on the New England coast, and there made my first acquaintance with that very ancient and curious creature the Limulus, or Horse-shoe Crab, or King-crab, as it is sometimes called. Orchard Beach is, I presume, near its northern range on our coast, and the specimens seen were not very large in size, though by no means rare, and not infrequently cast on shore in storms. But the best facilities for studying their habits were found in a marsh at no great distance from the hotel, where there were numerous channels, ditches and little ponds filled with sea water at high tide. In these were multitudes of young Limuli, varying from an inch to three or four inches in breadth, and though many were dead or merely cast shells, it was easy to take young specimens with a landing net. A number of these were secured, and I made it my business for some time to study their habits and mode of life, and especially the tracks which they made in sand or mud.

The King-crab, viewed from above, consists of three parts. The anterior shield or carapace is semi-circular in form, with two spines or projecting points at the angles, raised in the middle and sloping down to a smooth or moderately sharp edge in front. The eyes are set like windows in this shield. Two large ones at the sides, which are compound eyes consisting of numerous ocelli or little eyes, and two microscopic ones in front, at the base of a little spine, which are simple. The second or abdominal part is also in one piece, somewhat quadrate in form, with ridges and serratures at the sides armed with spines, and which may be said to simulate the separate joints into which the abdomen of an ordinary Crustacean is divided. The third part is a long tail spine, triangular in cross section, sharply pointed, and so jointed to the posterior end of the abdomen that it can be freely moved in any direction as a bayonet-like weapon of defence. When unable to escape from an enemy it is the habit of the creature to double itself up by bending the abdomen against the carapace, and erecting the sharp spine. Thus, with fixed bayonet it awaits attack, like the kneeling soldier in front of a square.

Below this upper shield, which is thin and papery in the young, somewhat horny in the adult, are the numerous limbs of the creature, with which we are at present most concerned. Under the carapace are several pairs of jointed limbs differing in size and form. The two anterior are small and peculiarly formed claws, used apparently in manipulating the food. The four next are larger in size, and are walking feet, each furnished with two sharp points which form a pincer for holding. The last pair is much larger and stronger than any of the others, and armed not only with a pair of pincers, but with four blunt nail-like points. Under the abdomen are flat swimming feet, as they have been called, each composed of a broad plate notched and divided in the middle. When at rest these lie flat on each other, but they can be flapped back and forth at the will of the animal.

Let us now see what use the creature can make of these numerous and varied pedal appendages, and for distinctness' sake we shall call the anterior set thoracic and the posterior abdominal. When placed in shallow water on fine sand it walked slowly forward, and its tracks then consisted of a number of punctures on the sand in two lines. If, however, the water was very shallow or the sand very soft or inclined upward the two edges of the carapace touched the bottom, making a slight furrow at each side; and if the tail was trailed on the bottom, this made a third or central furrow. When climbing a slope, or when placed at the edge of the water, it adopted another mode of locomotion, pushing with great force with its two posterior limbs, and thus moving forward by jerks. It then made four deep marks with the toes of each hind limb, and more or less interrupted marks with the edges of the carapace and the tail. In these circumstances the marks were almost exactly like those of some forms of the Protichnites of the Potsdam sandstone. When in sufficiently deep-water and desirous to escape, it flapped its abdominal feet, and then swam or glided close to the bottom. In this case, when moving near the soft bottom, it produced a series of transverse ridges and furrows like small ripple marks, with a slight ridge in the middle, and sometimes, when the edges of the carapace touched the bottom, with lateral furrows. In this way the animals were able to swim with some ease and rapidity, and on one occasion I observed an individual, confined in a tub of water, raise itself from the bottom and swim around the tub at the surface in search of a way of escape. Lastly, the young Limuli were fond of hiding themselves by burrowing in the sand. They did this by pushing the anterior rounded end of the carapace under the sand, and then vigorously shovelling out the material from below with their feet, so that they gradually sank under the surface, and the sand flowed in upon them till they were entirely covered. If carefully removed from the hollow they had made, this was found to be ovoid or hoof shaped in form and bilobed, not unlike the curious hollows (Rusophycus Grenvillensis of Billings) which I have supposed to be burrows of Trilobites.