_MARKINGS, FOOTPRINTS AND FUCOIDS._
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:--
"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.
I thus found that the common King-crab could produce a considerable
variety of tracks and burrows comparable with those which have been
named Protichnites, Climactichnites, Bilobites, Cruziana, Rusichnites,
etc.; and that the kind of markings depended partly on the differences
of gait in the animal, and partly on the circumstances in which it was
placed; so that different kinds of tracks do not always prove diversity
in the animals producing them.
The interest of this investigation as applied to Limulus is increased
by the fact that this creature is the near ally of Trilobites,
Eurypterids and other Crustaceans which were abundant in the earlier
geological ages, and whose footprints are probably among the most
common we find on the rocks.
[Illustration: Rusichnites Grenvillensis, Billings a "Bilobite."
Probably the Cast of a Crustacean burrow.]
Lastly, on this part of the subject, it is to be observed that many
other marine animals, both crustaceans and worms, make impressions
resembling in general character those of Limulus. In addition to those
already mentioned, Nathorst and Bureau have shown that various kinds
of shrimps and lobster-like Crustaceans, when swimming rapidly by
successive strokes of the tail, make double furrows with transverse
ridges resembling those of Bilobites, and there are even some mollusks
which by the undulations of the foot or the hook-like action of its
anterior part, can make similar trails. A question arises here as to
the value of such things as fossils. This depends on the fact that
many creatures have left their marks on the rocks when still soft on
the sea bottom, of which we have no other indications, and it also
depends on our ability to understand the import of these unconscious
hieroglyphics. They will certainly be of little use to us so long as
we persist in regarding them as vegetable forms, and until we have
very carefully studied all kinds of modern markings.[150] Nor does it
seem of much use to assign to them specific names. The same trail
often changes from one so-called species, or even genus, to another in
tracing it along, and the same animal may in different circumstances
make very different kinds of tracks. There will eventually, perhaps,
arise some general kind of nomenclature for these markings under a
separate sub-science of Ichnology or the doctrine of Footprints.
[150] Geologists are greatly indebted to Dr. Nathorst of Stockholm for
his painstaking researches of this kind.
I have said nothing of true Algæ or seaweeds, of which there are many
fossil species known to us by their forms, and also by the carbonaceous
or pyritous matter, or discharge of colour from the matrix, which
furnishes evidence of the presence of organic material; nor of the
marks and trails left by seaweeds and land plants drifting in currents,
some of which are very curious and fantastic; nor of those singular
trails referred to the arms of cuttle-fishes and the fins of fishes,
or to sea jellies and starfishes. These might form materials for a
treatise. My object here is merely to indicate the mode of dealing with
such things, and the kind of information to be derived from them.
When we come to the consideration of actual footprints of vertebrate
animals having limbs, the information we can obtain is of a far more
definite character. This has already been referred to in treating of
the first Air-breathers in a previous chapter. One very curious example
we may close with. It is that of the celebrated "bird tracks" of the
sandstone quarries in the Trias of Connecticut and Massachusetts.
These tracks, of immense size, as much as eighteen inches in length,
and so arranged as to indicate the stride of a long-legged biped, were
naturally referred to gigantic birds, allied to modern waders. But
when it was found that some of them showed a central furrow indicating
a long tail trailing behind, this conclusion was shaken, and when in
tracing them along, places were found where the animal had sat down
on its haunches and the end of its tail, and had brought down to the
ground a pair of small fore feet with four or five fingers, it was
discovered that we had to deal with biped reptiles; and when the
tracks were correlated with the bones of the extinct reptiles known
as Dinosaurs, we found ourselves in the presence of a group of the
most strange and portentous reptilian forms that the earth has ever
known. Marsh has been enabled, by nearly perfect skeletons of some
allied reptilian bipeds found in the West, to reproduce them in their
exact forms and proportions, so that we can realize in imagination
their aspect, their gait, and their gigantic proportions. Examples of
this putting together of footprints and osseous remains of vertebrate
animals are not rare in the history of geology, and show us how the
monsters of the ancient world, equally with their human successors,
could leave "footprints on the sands of time."
The Dinosaurs which have left their footprints on the sandstones of
Connecticut and Massachusetts are, however, greatly more numerous
than those known to us by osseous remains. Thus footprints have the
further use of filling up the imperfections of our geological record,
or at least of pointing out gaps which but for them we might not have
suspected. The remarkable inferences of Matthew already referred to,
respecting cuttle-fishes in the Cambrian period, constitute a case in
point. Footprints of Batrachians in the Carboniferous rocks were known
before their bones. The strange hand-like tracks in the Trias were
known before we knew the Labyrinthodon that produced them. We are still
ignorant of the animals whose tracks in the old Potsdam sandstones we
name Protichnites.
References:--On Rusichnites (a form of Bilobite), _Canadian
Naturalist_, 1864. On Footprints of Limulus compared with
Protichnites, etc. _Ibid._ On Footprints and Impressions of Aquatic
Animals and Imitative Markings, _Amer. Journal of Science_, 1873.
On Burrows and Tracks of Invertebrate Animals, _Quarterly Journal
of Geological Society_, 1890. On Footprints of Carboniferous
Batrachians. "Acadian Geology," "Air-breathers of the Coal Period,"
etc.
_PRE-DETERMINATION IN NATURE._
DEDICATED TO THE MEMORY OF
ELKANAH BILLINGS,
First Palæontologist of
the Geological Survey of Canada,
who laid the Foundations of our Knowledge
of the Invertebrate Fossils of Canada.
Fixity of Laws and Properties of Energy and Matter--Permanence of
Continents and Oceans--The Permanent and the Changeable--Permanence of
Animal and Vegetable Forms and Structures--Principles of Construction
in the Parts of Trilobites--In the Skeletons of Sponges--In Early
Vertebrates--In Plants Laws of Fixity and Diversity
[Illustration: Restoration of Protospongia Tetranema. Quebec group;
Siluro-Cambrian, Little Metis (p. 335).]