THE PRIMORDIAL, OR CAMBRIAN AGE.
Between the time when _Eozoon Canadense_ flourished in the seas of the
Laurentian period, and the age which we have been in the habit of
calling Primordial, or Cambrian, a great gap evidently exists in our
knowledge of the succession of life on both of the continents,
representing a vast lapse of time, in which the beds of the Upper
Laurentian were deposited, and in which the Laurentian sediments were
altered, contorted, and upheaved, before another immense series of
beds, the Huronian, or Lower Cambrian, was formed in the bottom of the
sea. Eozoon and its companions occur in the Lower Laurentian. The
Upper Laurentian has afforded no evidence of life; and even those
conditions from which we could infer life are absent. The Lowest
Cambrian, as we shall see, presents only a few traces of living
beings. Still, the physical history of this interval must have been
most important. The wide level bottom of the Laurentian sea was broken
up and thrown into those bold ridges which were to constitute the
nuclei of the existing continents. Along the borders of these new-made
lands intense volcanic eruptions broke forth, producing great
quantities of lava and scoriæ and huge beds of conglomerate and
volcanic ash, which are characteristic features of the older Cambrian
in both hemispheres. Such conditions, undoubtedly not favourable to
life, seem to have prevailed, and extended their influence very
widely, so that the sediments of this period are among the most barren
in fossils of any in the crust of the earth. If any quiet undisturbed
spots existed in which the Lower Laurentian life could be continued
and extended in preparation for the next period, we have yet
discovered few of them. The experience of other geological periods
would, however, entitle us to look for such oases in the Lower
Cambrian desert, and to expect to find there some connecting links
between the life of the Eozoic and the very dissimilar fauna of the
Primordial.
The western hemisphere, where the Laurentian is so well represented,
is especially unproductive in fossils of the immediately succeeding
period. The only known exception is the occurrence of Eozoon and of
apparent casts of worm-burrows in rocks at Madoc in Canada, overlying
the Laurentian, and believed to be of Huronian age, and certain
obscure fossils of uncertain affinities, recently detected by Mr.
Billings, in rocks supposed to be of this age, in Newfoundland. Here,
however, the European series comes in to give us some small help.
Gümbel has described in Bavaria a great series of gneissic rocks
corresponding to the Laurentian, or at least to the lower part of it;
above these are what he calls the Hercynian mica-slate and primitive
clay-slate, in the latter of which he finds a peculiar species of
Eozoon, which he names _Eozoon Bavaricum_. In England also the
Longmynd groups of rocks in Shropshire and in Wales appears to be the
immediate successor to the Upper Laurentian; and it has afforded some
obscure "worm-burrows" or, perhaps, casts of sponges or fucoids, with
a small shell of the genus _Lingulella_, and also fragments of
crustaceans (_Palæeopyge_). The "Fucoid Sandstones" of Sweden,
believed to be of similar age, afford traces of marine plants and
burrows of worms, while the Harlech beds of Wales have afforded to Mr.
Hicks a considerable number of fossil animals, not very dissimilar
from those of the Upper Cambrian. If these rocks are really the next
in order to the Eozoic, they show a marked advance in life immediately
on the commencement of the Primordial period. In Ireland, the curious
Oldhamia, noticed below, appears to occur in rocks equally old. As we
ascend, however, into the Middle and Upper parts of the Cambrian, the
Menevian and Lingula flag-beds of Britain, and their equivalents in
Bohemia and Scandinavia, and the Acadian and Potsdam groups of
America, we find a rich and increasing abundance of animal remains,
constituting the first Primordial fauna of Barrande.
The rocks of the Primordial are principally sandy and argillaceous,
forming flags and slates, without thick limestones, and often through
great thicknesses, very destitute of organic remains, but presenting
some layers, especially in their upward extension, crowded with
fossils. These are no longer mere Protozoa, but include
representatives of all the great groups of animals which yet exist,
except the vertebrates. We shall not attempt any systematic
classification of these; but, casting our dredge and tow-net into the
Primordial sea, examine what we collect, rather in the order of
relative abundance then of classification.
Over great breadths of the sea bottom we find vast numbers of little
bivalve shells of the form and size of a finger-nail, fastened by
fleshy peduncles imbedded in the sand or mud; and thus anchored,
collecting their food by a pair of fringed arms from the minute
animals and plants which swarm in the surrounding waters. These are
the _Lingulæ_, from the abundance of which some of the Primordial beds
have received in England and Wales the name of Lingula flags. In
America, in like manner, in some beds near St. John, New Brunswick,
the valves of these shells are so abundant as to constitute at least
half of the material of the bed; and alike in Europe and America,
Lingula and allied forms are among the most abundant Primordial
fossils. The Lingulæ are usually reckoned to belong to the great
sub-kingdom of mollusks, which includes all the bivalve and univalve
shell-fish, and several other groups of creatures; but an able
American naturalist, Mr. Morse, has recently shown that they have many
points of resemblance to the worms; and thus, perhaps, constitute one
of those curious old-fashioned "comprehensive" types, as they have
been called, which present resemblances to groups of creatures, in
more modern times quite distinct from each other. He has also found
that the modern Lingulæ are very tenacious of life, and capable of
suiting themselves to different circumstances, a fact which, perhaps,
has some connection with their long persistence in geological time.
They are in any case members of the group of lamp-shells, creatures
specially numerous and important in the earlier geological ages.
[Illustration: Fig. 8.--LIFE IN THE PRIMORDIAL SEA.
On the bottom are seen, proceeding from left to right, _Oldhamia
antiqua_, _Lingulæ_, _Arenicolæ_, _Oldhamia radiata_, _Paradoxides_,
_Histioderma_, _Agnostus_, _Oldhamia radiata_, _Algæ_, and _Lingulæ_.
In the water are _Hymenocaris_, different species of _Trilobites_, and
_Pteropods_.]
The Lingulæ are especially interesting as examples of a type of beings
continued almost from the dawn of life until now; for their shells, as
they exist in the Primordial, are scarcely distinguishable from those
of members of the genus which still live. While other tribes of
animals have run through a great number of different forms, these
little creatures remain the same. Another interesting point is a most
curious chemical relation of the Lingula, with reference to the
material of its shell. The shells of mollusks generally, and even of
the ordinary lamp-shells, are hardened by common limestone or
carbonate of lime: the rarer substance, phosphate of lime, is in
general restricted to the formation of the bones of the higher
animals. In the case of the latter, this relation depends apparently
on the fact that the albuminous substances on which animals are
chiefly nourished require for their formation the presence of
phosphates in the plant. Hence the animal naturally obtains phosphate
of lime or bone-earth with its food, and its system is related to this
chemical fact in such wise that phosphate of lime is a most
appropriate and suitable material for its teeth and bones. Now, in the
case of the lower animals of the sea, their food, not being of the
nature of the richer land plants, but consisting mainly of minute algæ
and of animals which prey on these, furnishes, not phosphate of lime,
but carbonate. An exception to this occurs in the case of certain
animals of low grade, sponges, etc., which, feeding on minute plants
with siliceous cell-walls, assimilate the flinty matter and form a
siliceous skeleton. But this is an exception of downward tendency, in
which these animals approach to plants of low grade. The exception in
the case of Lingulæ is in the other direction. It gives to these
humble creatures the same material for their hard parts which is
usually restricted to animals of much higher rank. The purpose of this
arrangement, whether in relation to the cause of the deviation from
the ordinary rule or its utility to the animal itself, remains
unknown. It has, however, been ascertained by Dr. Hunt, who first
observed the fact in the case of the Primordial Lingulæ, that their
modern successors coincide with them, and differ from their
contemporaries among the mollusks in the same particular. This may
seem a trifling matter, but it shows in this early period the
origination of the difference still existing in the materials of which
animals construct their skeletons, and also the wonderful persistence
of the Lingulæ, through all the geological ages, in the material of
their shells. This is the more remarkable, in connection with our own
very slender acquaintance with the phenomenon, in relation either to
its efficient or final causes.
Before leaving the Lingulæ, I may mention that Mr. Morse informs me
that living specimens, when detached from their moorings, can creep
like worms, leaving long furrows on the sand, and that they can also
construct sand-tubes wherein to shelter themselves. This shows that
some of the abundant "worm burrows" of the Primordial may have been
the work of these curious little shell-fishes, as well as, perhaps,
some of the markings which have been described under the name of
_Eophyton_, and have been supposed, I think incorrectly, to be remains
of land plants.
In addition to Lingula we may obtain, though rarely, lamp-shells of
another type, that of the Orthids, These have the valves hinged along
a straight line, in the middle of which is a notch for the peduncle,
and the valves are often marked with ribs or striae. The Orthids were
content with limestone for their shells, and apparently lived in the
same circumstances with the Lingulæ; and in the period succeeding the
Primordial they became far more abundant. Yet they perished at an
early stage of the world's progress, and have no representatives in
the modern seas.
In many parts of the Primordial ocean the muddy bottom swarmed with
crustaceans, relatives of our shrimps and lobsters, but of a form
which differs so much from these modern shell-fishes that the
question of their affinities has long been an unsettled one with
zoologists. Hundreds of species are known, some almost microscopic in
size, others a foot in length. All are provided with a broad flat
horseshoe-shaped head-plate, which, judging from its form and a
comparison with the modern king-crabs or horseshoe-crabs, must have
been intended as a sort of mud-plough to enable them to excavate
burrows or hide themselves in the slimy ooze of the ocean bed. On the
sides of this buckler are placed the prominent eyes, furnished with
many separate lenses, on precisely the same plan with those of modern
crustaceans and insects, and testifying, as Buckland long ago pointed
out, to the identity of the action of light in the ancient and the
modern seas. The body was composed of numerous segments, each divided
transversely into three lobes, whence they have received the name of
_Trilobites_, and the whole articulated, so that the creature could
roll itself into a ball, like the modern slaters or wood-lice, which
are not very distant relatives of these old crustaceans.[F] The limbs
of Trilobites were long unknown, and it was even doubted whether they
had any; but recent discoveries have shown that they had a series of
flat limbs useful both for swimming and creeping. The Trilobites,
under many specific and generic forms, range from the Primordial to
the Carboniferous rocks, but are altogether wanting in the more recent
formations and in the modern seas. The Trilobites lived on muddy
bottoms, and their remains are extremely abundant in shaly and slaty
beds, though found also in limestone and sandstone. In the latter they
have left most curious traces of their presence in the trails which
they have produced. Some of the most ancient sandstones have their
surfaces covered with rows of punctured impressions (_Protichnites_,
first footprints), others have strange series of transverse grooves
with longitudinal ones at the side (_Climactichnites_, ladder
footprints); others are oval burrows, marked with transverse lines and
a ridge along the middle (_Rusichnites_, wrinkle footprints). All of
these so nearly resemble the trails and tracks of modern king-crabs
that there can be little doubt as to their origin. Many curious
striated grooves and bifid marks, found on the surfaces of Primordial
beds, and which have been described as plants, are probably only the
marks of the oral organs or feet of these and similar creatures, which
passed their lives in grubbing for food in the soft, slimy ooze,
though they could, no doubt, like the modern king-crabs, swim when
necessary. Some still more shrimp-like creatures, Hymenocaris, which
are found with them, certainly had this power.
[F] Woodward has recently suggested affinities of Trilobites with the
Isopods or equal-footed crustaceans, on the evidence of a remarkable
specimen with remains of feet described by Billings.
A lower type of annulose or ringed animal then that of the Trilobites,
is that of the worms. These creatures cannot be preserved in a fossil
state, except in the case of those which inhabit calcareous tubes:
but the marks which their jointed bodies and numerous side-bristles
leave on the sand and mud may, when buried under succeeding sediments,
remain; and extensive surfaces of very old rocks are marked in this
way, either with cylindrical burrows or curious trails with side
scratches looking like pinnate leaves. These constitute the genus
_Crusiana_, while others of more ordinary form belong to the genus
_Arenicolites_, so named from the common Arenicola, or lobworm, whose
burrows they are supposed to resemble. Markings referable to seaweed
also occur in the Primordial rocks, and also some grotesque and almost
inexplicable organisms known as _Oldhamia_, which have been chiefly
found in the Primordial of Ireland. One of the most common forms
consists of a series of apparently jointed threads disposed in
fan-like clusters on a central stem (_Oldhamia antiqua_). Another has
a wider and simpler fan-like arrangement of filaments. These have been
claimed by botanists as algæ, and have been regarded by zoologists as
minute Zoophytes, while some more sceptical have supposed that they
may be mere inorganic wrinklings of the beds. This last view does not,
however, seem tenable. They are, perhaps, the predecessors of the
curious _Graptolites_, which we shall have to represent in the
Silurian.
Singularly enough, Foraminifera, the characteristic fossils of the
Laurentian, have been little recognised in the Primordial, nor are
there any limestones known so massive as those of the former series.
There are, however, a number of remarkable organisms, which have
usually been described as sponges, but are more probably partly of the
nature of sponges and partly of that of Foraminifera. Of this kind are
some of the singular conical fossils described by Billings as
_Archæocyathus_, and found in the Primordial limestone of Labrador.
They are hollow within, with radiating pores and plates, calcareous in
some, and in others with siliceous spicules like those of modern
sponges. Some of them are several inches in diameter, and they must
have grown rooted in muddy bottoms, in the manner of some of the
deep-sea sponges of modern times. One species at least of these
creatures was a true Foraminifer, allied, though somewhat distantly,
to Eozoon. In some parts of the Primordial sandstones, curious
funnel-shaped casts in sand occur, sometimes marked with spiral lines.
The name _Histioderma_ has been given to some of these, and they have
been regarded as mouths of worm-burrows. Others of larger size have
been compared to inverted stumps of trees. If they were produced by
worms, some of these must have been of gigantic size, but Billings has
recently suggested that they may be casts of sponges that lived like
some modern species imbedded in the sand. In accordance with this view
I have represented these curious objects in the engraving, On the
whole, the life of these oldest Palæozoic rocks is not very abundant;
but there are probably representatives of three of the great
subdivisions of animals or, as some would reckon them, of four the
Protozoa, the Radiata (Coelenterata), the Mollusca, and the Annulosa.
And it is most interesting thus to find in these very old rocks the
modern subdivisions of animals already represented, and these by types
some of them nearly allied to existing inhabitants of the seas I have
endeavoured in the engraving to represent some of the leading forms of
marine life in this ancient period.
Perhaps one of the most interesting discoveries in these rocks is that
of rain-marks and shrinkage-cracks, in some of the very oldest
beds--those of the Longmynd in Shropshire. On the modern muddy beach
any ordinary observer is familiar with the cracks produced by the
action of the sun and air on the dried surfaces left by the tides.
Such cracks, covered by the waters of a succeeding tide, may be buried
in newer silt, and once preserved in this way are imperishable. In
like manner, the pits left by passing showers of rain on the mud
recently left bare by the tide may, when the mud has dried, become
sufficiently firm to be preserved. In this way we have rain-marks of
various geological ages; but the oldest known are those of the
Longmynd, where they are associated both with ripple-marks and
shrinkage-cracks. We thus have evidence of the action of tides, of
sun, and of rain, in these ancient periods just as in the present day.
Were there no land animals to prowl along the low tidal flats in
search of food? Were there no herbs or trees to drink in the rains and
flourish in the sunshine? If there were, no bone or footprint on the
shore, or drifted leaf or branch, has yet revealed their existence to
the eyes of geologists The beds of the Primordial age exist in
England, in Bohemia, in Sweden and Norway, and also in North America.
They appear to have been deposited along the shores of the old
Laurentian continent, and probably some of them indicate very deep
water. The Primordial rocks are in many parts of the world altered and
hardened. They have often assumed a slaty structure, and their
bedding, and the fossils which they contain, are both affected by
this. The usual view entertained as to what is called slaty structure
is, that it depends on pressure, acting on more or less compressible
material in some direction usually different from that of the bedding.
Such pressure has the effect of arranging all the flat particles as
scales of mica, etc. in planes parallel to the compressing surface.
Hence, if much material of this kind is present in the sediment, the
whole rock assumes a fissile character causing it to split readily
into thin plates. That such yielding to pressure has actually taken
place is seen very distinctly in microscopic sections of some slaty
rocks, which often show not only a laminated structure, but an actual
crumpling on a small scale, causing them to assume almost the aspect
of woody fibre. Such rocks often remind a casual observer of decaying
trunks of trees, and sections of them under the microscope show the
most minute and delicate crumpling. It is also proved by the condition
of the fossils the beds contain. These are often distorted, so that
some of them are lengthened and others shortened, and if specimens
were selected with, that view, it would be quite easy to suppose that
those lengthened by distortion are of different species from those
distorted so as to be shortened. Slaty cleavage and distortion are
not, however, confined to Primordial rocks, but occur in altered
sediments of various ages.
The Primordial sediments must have at one time been very widely
distributed, and must have filled up many of the inequalities produced
by the rending and contortion of the Laurentian beds. Their thicker
and more massive portions are, however, necessarily along the borders
of the Laurentian continents, and as they in their turn were raised up
into land, they became exposed to the denuding action first of the
sea, and afterwards of the rain and rivers, and were so extensively
wasted away that only in a few regions do large areas of them remain
visible. That of Bohemia has afforded to Barrande a great number of
most interesting fossils. The rocks of St. David's in Wales, those of
Shropshire in England, and those of Wicklow in Ireland are also of
great interest; and next to these in importance are, perhaps, the
Huronian and Acadian groups of North America, in which continent--as
for example in Nova Scotia and in some parts of New England--there are
extensive areas of old metamorphic rocks whose age has not been
determined by fossils, but which may belong to this period.
The question of division lines of formations is one much agitated in
the case of the Cambrian rocks. Whether certain beds are to be called
Cambrian or Silurian has been a point greatly controverted; and the
terms Primordial and Primordial Silurian have been used as means to
avoid the raising of this difficulty. Many of our division lines in
geology are arbitrary and conventional, and this may be the case with
that between the Primordial and Silurian, the one age graduating into
the other. There appears to be, however, the best reason to recognise
a distinct Cambrian period, preceding the two great periods, those of
the second and third faunas of Barrande, to which the term Silurian is
usually applied. On the other hand, in so far as our knowledge extends
at present, a strongly marked line of separation exists between the
Laurentian and Primordial, the latter resting on the edges of the
former, which seems then to have been as much altered as now. Still a
break of this kind may be, perhaps must be, merely local; and may vary
in amount. Thus, in some places we find rocks of Silurian and later
ages resting directly on the Laurentian, without the intervention of
the Primordial. In any case, where a line of coast is steadily
sinking, each succeeding deposit will overlap that which went before;
and this seems to have been the case with the Laurentian shore when
the Primordial and Silurian were being deposited. Hence over large
spaces the Primordial is absent, being probably buried up, except
where exposed by denudation at the margin of the two formations.
This occurs in several parts of Canada, while the Laurentian rocks
have evidently been subjected to metamorphism and long-continued
weathering before the Lower Silurian were deposited; and in some
cases the latter rest on weather-worn and pitted surfaces, and are
filled with angular bits of the underlying rock, as well as with
drift-shells which have been cast on these old Laurentian shores;
while in other cases the Silurian rests on smooth water-worn
Laurentian rocks, and is filled at the junction with well-rounded
pebbles and grains of sand which have evidently been subjected to a
more thorough attrition then those of the present beach. With respect
to the line of division between the Primordial and the next succeeding
rocks, it will be seen that important movements of the continents
occurred at the close of the Cambrian, and in some places the Cambrian
rocks have been much disturbed before the deposition of the Lower
Silurian.
Seated on some ancient promontory of the Laurentian, and looking over
the plain which, in the Primordial and Lower Silurian periods was the
sea, I have often wished for some shred of vegetable matter to tell
what lived on that land when the Primordial surf beat upon its shore,
and washed up the Trilobites and Brachiopods of those old seas; but no
rock has yet taken up its parable to reveal the secret, and the
Primordial is vocal only with the old story: "And God said, Let the
waters swarm with swarming living things, and it was so." So our
picture of the period may represent a sea-bottom swarming with animals
of low grade, some sessile, some locomotive; and we may merely suppose
a distant shore with vegetation dimly seen, and active volcanoes; but
a shore on which no foot of naturalist has yet trod to scan its
productions. Very different estimates have been formed of the amount
of life in this period, according to the position given to its latest
limit. Taking some of the more modern views of this subject, we might
have included among the Primordial animals many additional creatures,
which we prefer noticing in the Silurian, since it may at least be
affirmed that their head-quarters were in that age, even if they had a
beginning in the Primordial. It may be interesting here, however, to
note the actual amount of life known to us in this period, taken in
its largest scope. In doing this, I shall take advantage of an
interesting table given by Dr. Bigsby,[G] and representing the state
of knowledge in 1868, and shall group the species in such a manner as
to indicate the relative abundance of distinct types of structure. We
find then--
Plants (all, or nearly all, supposed to be
sea-weeds, and some, probably, mere tracks
or trails of animals) 22 species.
Sponges, and similar creatures 27 "
Corals and their allies 6 "
Starfishes and their allies 4 "
Worms 29 "
Trilobites and other crustaceans 442 "
Lamp-shells and other molluscoids 193 "
Common bivalve mollusks 12 "
Common univalve mollusks and their allies 172 "
Higher mollusks, nautili, cuttle-fishes, etc. 65 "
---
In all 972 "
[G] "Thesaurus Siluricus."
Now in this enumeration we observe, in the first place, a
representation of all the lower or invertebrate groups of the waters.
We have next the remarkable fact that the Radiata of Cuvier, the
lowest and most plant-like of the marine animals, are comparatively
slenderly represented, yet that there are examples of their higher as
well as of their lower forms. We have the further fact that the
crustaceans, the highest marine animals of the annulose type, are
predominant in the waters; and that in the mollusks the highest and
lowest groups are most plentiful, the middle less so. The whole number
of species is small, and this may arise either from our having here
reached an early period in the history of life, or from our
information being defective. Both are probably true. Still, of the
animals known, we cannot say that the proportions of the different
kinds depend on defective knowledge. There is no reason, for example,
why corals should not have been preserved as well as Trilobites, or
why Brachiopods should have been preserved rather then ordinary
bivalves. The proportions, therefore, it may be more safe to reason
from then the aggregate. In looking at these proportions, and
comparing them with those of modern seas, we are struck with the great
number of species representing some types either now extinct or
comparatively rare: the Trilobites and Brachiopods more particularly.
We are astonished at the enormous preponderance of these two groups,
and especially of the Trilobites. Further, we observe that while some
forms, like Lingula and Nautilus, have persisted down to modern
times, others, like the Trilobites and Orthids, perished very early.
In all this we can dimly perceive a fitness of living things to
physical conditions, a tendency to utilise each type to the limit of
its capacities for modification, and then to abandon it for something
higher; a tendency of low types to appear first, but to appear in
their highest perfection and variety; a sudden apparition of totally
diverse plans of structure subserving similar ends simultaneously with
each other, as for instance those of the Mollusk and the Crustacean;
the appearance of optical and mechanical contrivances, as for example
the compound eyes of the Trilobite and the swimming float of the
Orthoceras, in all their perfection at first, just as they continue to
this day in creatures of similar grade. That these and other similar
things point to a uniform and far-reaching plan, no rational mind can
doubt; and if the world had stopped short in the Primordial period,
and attained to no further development, this would have been
abundantly apparent; though it shines forth more and more
conspicuously in each succeeding page of the stony record. How far
such unity and diversity can be explained by the modern philosophy of
a necessary and material evolution out of mere death and physical
forces, and how far it requires the intervention of a Creative mind,
are questions which we may well leave with the thoughtful reader, till
we have traced this history somewhat further.