of rapid deposition, in which, in certain localities at least, great
thicknesses of rock with few organic remains were formed. We have
instances of this in the later Cambrian, in the Ordovician, and still
later in the Permian and Trias. Thus in the beds immediately underlying
the lowest Cambrian we may be passing through a tract of comparative
barrenness to find more fertile ground below.
It is also to be observed that there is evidence of disturbance
occurring in the interval between the lowest Cambrian and the highest
pre-Cambrian, which may involve the lapse of much time not recorded in
the localities hitherto explored, but of which monuments may be found
elsewhere.
We may now, taking some North American localities as our best available
guides, inquire as to the nature and contents of the beds next below
the Lower Cambrian.
[Illustration: Fig. 10.--_Section at Hanford Brook._ (After Matthew.)
Showing St. John group resting on Etcheminian, and this on Coldbrook
(Huronian).]
In Southern New Brunswick, Matthew indicated, several years ago, the
occurrence of certain conglomerates and sandy and slaty beds over the
rocks, mostly of igneous origin, constituting a great thickness of
beds under the Cambrian, and known locally as the "Coldbrook" series,
which is probably equivalent to the Huronian of Northern and Western
Canada, to be noticed later. These beds were at first regarded as an
upper member of the Huronian, but subsequently it was thought better
to unite them with the overlying Cambrian as basal Cambrian. The fact
that these problematical beds were ascertained to be unconformable to
the Cambrian, and the peculiarity of their fossils, led to their being
constituted a separate group under the name _Etcheminian_, which seems
to represent a time and conditions introductory to the Cambrian (Fig.
10). The fossils in these beds are few and hard to find. Matthew has
kindly furnished me with the following list.[7] The Trilobites are
conspicuous by their absence. Sea-worms have left burrows, trails, and
casts, which probably represent several species (Fig. 11). A single
little shell (Volborthella) is supposed to be a precursor of the
straight chambered shells allied to the modern nautilus, which become
so large and numerous in succeeding periods. There are a few univalve
shell-fishes allied to modern sea-snails, a brachiopod of the antique
genus Obolus, some fragments supposed to represent Cystideans, a
rudimentary type of the stalked sea-stars so abundant later, spicules
of sponges and minute Protozoa, with shells not unlike those of their
modern successors. This meagre list sums up the forms of life known
in the Etcheminian of this district, one in which the Cambrian beds
exhibit the rich and varied fauna of Trilobites and other animals
described and figured by Matthew in several successive volumes of the
"Transactions of the Royal Society of Canada" (Fig. 12).
[Footnote 7: "Transactions Royal Society of Canada," vol. vii.]
[Illustration: Fig. 11.--_Trails of Worms of two types (Psammchnites
and Planilites)._]
Beds in Newfoundland (the Signal Hill and Random Sound series),
underlying the Lower Cambrian, have afforded to Murray and Billings
some well-characterized worm-castings of spiral form, and a few
problematical forms known as Aspidella, which may be Crustaceans or
Mollusks allied to the limpets (Fig. 13).
[Illustration: Fig. 12.--_Group of pre-Cambrian (Etcheminian) Animals
from the Etcheminian._ (After Matthew.)]
The name "Etcheminian" is derived from that of an ancient Indian tribe
of New Brunswick.
(_a_) Volborthella, supposed to be a Cephalopod shell. (_b_)
Pelagiella. (_c_) Orthotheca, supposed to be Pteropods. (_d_) Primitia,
an Ostracod Crustacean, (_e_) Obolus, a Brachiopod shell. (_f_)
Platysolenites, probably fragment of a Cystidean. (_g_) Globigerinæ,
casts of Foraminiferal shells, Etcheminian, New Brunswick.
[Illustration: Fig. 13.--_Arenicolites (Spiroscolex) spirales_
(Billings) _and Aspidella tenanovica_ (Billings), _Signal Hill Series,
Newfoundland._]
[Illustration: Fig. 14.--_Fragment of Cryptozoon, Grand Cañon, Arizona._
Photograph from a specimen presented by Dr Walcott to the Peter Redpath
Museum.]
In a thick series of pre-Cambrian beds in the Colorado Cañon in the
Western United States, Walcott has found a small roundish shell of
uncertain affinities,[8] a species of Hyolithes, probably a swimming
sea-snail or Pteropod, a small fragment which may possibly have
belonged to a Trilobite, and some laminated forms which, if organic,
are related to the Cryptozoon already mentioned (Fig. 14).
[Footnote 8: Discinoid or Patelloid.]
The Kewenian series of Lake Superior has yielded no fossils, but the
pipestone beds of Minnesota, supposed to be about the same age, have
afforded a small bivalve shell allied to Lingula;[9] and the black
shales of the head of Lake Superior contain some impressions supposed
to be trails of animals.[10]
[Footnote 9: Winchell.]
[Footnote 10: Selwyn and Matthew.]
It has been a question whether the beds above referred to should be
regarded as a downward continuation of the Cambrian, or as the upper
part of an older system. Matthew, whose opinion on such a subject is
of the highest authority, regards them as a distinct system, but as
belonging, with the Cambrian, to the great Palæozoic Period. Van
Hise, and some other United States authorities, would separate them
even from the Palæozoic, and unite them with the underlying Huronian,
as representing a "Proterozoic" or "Algonkian" Period. This is merely
a matter of classification, necessarily more or less arbitrary; but I
believe the facts to be stated subsequently show that it will be best
to unite the Etcheminian and its equivalents with the Palæozoic, and to
place the groups lower than this in one great division, equivalent to
Palæozoic, and for which many years ago I proposed the name "Eozoic,"
or that of the Dawn of Life.
Having thus hastily glanced at the slender fauna of the rocks
immediately below the Cambrian, we may now proceed to inquire a little
more in detail into its true value and import as leading toward the
beginning of life. I have already referred to the apparently sudden
drop in the number of groups and of species below the base of the
Cambrian, and have hinted that this may be an effect of temporary
local conditions of deposit or of defective information. Another fact
that strikes us is the diverse and miscellaneous character of the
fossils that remain to us; and this would suggest that we are either
dealing with a mere handful picked at random, as it were out of a
richer fauna, or that in the beginning of things the gaps and missing
links between different forms of life were even more pronounced than
at present. This, however, would be likely to occur if the plan of
creation was to represent at first different types, with few forms in
each; to produce, in short, a sort of type collection representing the
whole range of organization by a few characteristic things rather than
to give a complete series, with all the intermediate connections. Such
a mode of introduction of life is not _à priori_ improbable, however at
variance with some prevalent hypotheses.
Beginning with the higher Invertebrates, we must not conclude that
we have altogether lost the Trilobites. The fragments referred to
this group may represent at least a few species, and it would be very
interesting to know more of these as to their relations to their
successors, and whether they are tending to lower or more embryonic
forms. The bivalve Crustaceans (Ostracods) may be regarded as inferior
in rank to the Trilobites, but are still very complex, and specialized
animals and a specimen silicified in such a manner as to show the
interior organs testified that, as far back as the Carboniferous at
least, these creatures were as highly organized as at present,[11]
while their generally larger size in the earlier formations tends to
show that they have rather been degenerating in the lapse of geological
time.
[Footnote 11: _Palæocypris Edwardsi_, Brougniart, Coal Formation of St.
Etienne, France.]
In regard to the Sea-worms, the burrows, castings, and trails found
in the pre-Cambrian beds are scarcely, if at all, different from
those now seen on sandy and muddy shores, and would seem to indicate
that these highly organized and very sensitive and active creatures
swarmed in the muddy bottom of the pre-Cambrian Sea, and lived in the
same way as at present. It is impossible, however, to know anything
of the internal structures of these creatures, but the marks left by
their bristle-bearing feet seem to indicate that some of them at least
belong to the higher group of Sea-centipedes, creatures rivalling the
Crustaceans in complexity of organization, and near to them in plan
of structure, though at present usually widely separated from them in
current systems of classification. In the Ordovician system, next above
the Cambrian, Hinde has found many curiously formed jaws of animals of
this kind, which show at least that their alimentary arrangements were
similar to those now in force. If any of the problematical "Conodonts"
discovered by Pander in the Cambrian of Russia belonged to marine
worms, this inference would be extended back to the Lower Cambrian,
so that if the evidence of structure anywhere remains we may hope
to find that the pre-Cambrian worms were not inferior to their more
modern successors, perhaps even that in this early period, when they
probably played a more important part in nature, they were of higher
organization than in later times.
The evidence as to pre-Cambrian mollusks, so far as it goes, is
even more curious. The little shell called Volborthella, so far as
can be judged from its form and internal structure, is a miniature
representative of these straight Nautili, the Orthoceratites of
the Ordovician and later Palæozoic rocks; and no one doubts that
these latter belong to the highest class of the Mollusks, a class
approaching in the development of nerve system and sensory organs to
the Vertebrates themselves. This tiny member of the great class of
Cuttle-fishes may perhaps have been more nearly allied to the modern
Spirula than to the Nautilus. In any case, if, as seems altogether
probable it was, a mollusk, it must have been one of advanced type, and
with a highly complex structure, as well as the singular apparatus for
flotation implied in a chambered shell with a siphuncle.
Next to this among these primitive Mollusks are straight and spiral
shells representing those delicate and beautiful animals of the modern
seas, the Pteropods, or wing-footed Sea-snails, beautiful and graceful
creatures, the butterflies of the sea, and moving in the water with
the greatest ease and beauty by the aid of membranous fins, or wings,
sometimes brightly coloured. These creatures abound in all latitudes
in the modern ocean, and their delicate shells sometimes accumulate in
beds of "Pteropod sand." They very early entered on the arena of marine
life, and have continued to this day.
We miss here the two great Molluscan groups of the creeping Sea-snails
like the limpet and whelk, and of the ordinary bivalves like the
oyster and cockle. Both are present in the lowest Cambrian, though in
small numbers compared with their present abundance. Possibly they had
not yet appeared in the Etcheminian Sea, though the muddy and sandy
bottoms, evidenced by its slates and sandstones, would seem to have
afforded favourable habitats, and warrant the expectation that species
may yet be found.
The case was different with the little group of the Lamp-shells, or
Brachiopods. These creatures, somewhat resembling the ordinary bivalves
in their shelly coverings, were very dissimilar in their internal
structure, and once settled on the bottom they were attached for life,
not having even the limited means of locomotion possessed by the
Sea-snails and common bivalves. They collected their food wholly by
means of currents of water produced by cilia, or movable threads, on
arms or processes within their shells. In this they resembled the young
or embryo stages of some of the more ordinary Mollusks, though they are
so remote from these in their adult condition that they have usually
been placed in a distinct class, and some naturalists have thought
it best to separate them from the Mollusks altogether. Their history
is peculiar. Coming into existence at a very early date, they became
very abundant in early Palæozoic times, then gradually gave place to
the ordinary bivalves, and in the modern seas are represented by very
few species. Yet while in the middle period of their history they are
represented by very many peculiar specific and generic forms. Some of
the earliest types, like Obolus and Lingula, persist very long, and
the latter has continued without change from the Early Cambrian to the
Modern period.
The great group of the Sea-stars and Sea-urchins appears only in a
few of its lower forms, and seems to be the only class represented by
embryonic types. The coral animals are absent, so far as known. The
Jelly-fishes and their allies cannot be preserved as fossils, but some
peculiar markings, at one time regarded as plants, are now supposed to
be trails made by the tentacles of creatures of this kind moving over
muddy bottoms. A few spicules indicate Sponges, and the ubiquitous
groups of the marine Protozoa, the Foraminifera and the Radiolaunus,
are represented by shells scarcely distinguishable from those of modern
species. The great and peculiar forms represented at this early time by
Cryptozoon and its allies seem long ago to have perished, and we shall
have to return to them in a later stage of our inquiry.
To sum up the little that we know of this earliest Palæozoic life:--It
was perfect of its kind, equally pregnant with evidences of design, and
of the nicest and most delicate contrivance as the animal life of any
later time, and it presupposed vegetable life and multitudes of minute
organic beings altogether unknown to us to nourish the creatures we do
know. As an example of this, a little Brachiopod or sponge nourished
by the currents produced by its cilia, or a Jelly-fish gathering
food by its thread-like tentacles, or a Globigerina selecting its
nourishment by its delicate gelatinous pseudopods, required an ocean
swarming with minute forms of life, which probably can never be known
to us, but every one of which must have been an inscrutable miracle of
organization and vital function.
Lastly, with reference to our present subject, the Etcheminian fossils
carry life backward one whole great period earlier than the Lower
Cambrian, and appear to indicate that we are approaching a beginning
of living things in the Palæozoic world. Much no doubt remains to be
discovered, but it would seem that any future discoveries must fail to
negative this conclusion.
The Huronian.
In whatever way the rocks immediately below the Cambrian may be
classified, it is certain that the next system in descending order
is that to which Logan long ago gave the name Huronian, from its
development on Lake Huron[12]--a name to which it is still entitled,
though there may, perhaps, be some grounds for dividing it into an
upper and lower member.[13] To this sub-division, however, we need
not for the present give any special attention. In the typical area
of Lake Huron the Huronian consists of quartzites, which are merely
hardened sandstones, of slates which are muddy or volcanic-ash beds,
of conglomerates or pebble-rocks, and of coarse earthy limestone.
With these rocks are deposits of igneous material which represent
contemporary volcanic eruptions. In other districts, as in New
Brunswick, Newfoundland, etc., the beds have been considerably altered,
and are locally more mixed with igneous products. The physical picture
presented to us by the Huronian is that of a shore deposit, formed
under circumstances in which beds of pebbles and sand were intermixed
with the products of neighbouring volcanoes.
[Footnote 12: Dr. G. M. Dawson, F.R.S., the present Director of the
Geological Survey of Canada, whose judgment in this matter should be of
the highest value, holds that the original simple arrangement of Logan
still holds, notwithstanding the multitude of new names proposed by the
Western Geologists of the United States.]
[Footnote 13: Van Hise, "Pre-Cambrian Rocks of North America." _Comptes
Rendus_, 5th Session International Geol. Congress 1891, p. 134. Also
"Report U.S. Geol. Survey, 1895."]
[Illustration: Fig. 15.--_Annelid Burrows, Hastings Series, Madoc._
1. _Transverse section of Worm-burrow_--magnified, as a transparent
object. (_a_) Calcareo-silicious rock. (_b_) Space filled with
calcareous spar, (_c_) Sand agglutinated and stained black. (_d_) Sand
less agglutinated and uncoloured. 2. Transverse section of Worm-burrow
on weathered surface, natural size. 3. The same, magnified.]
Such a formation is not likely to afford fossils in any considerable
number and variety, even if deposited at a time of abundant marine
life. It is therefore not wonderful that we find little evidence of
living beings in the Huronian. In Canada I can point to nothing of this
kind, except a few cylindrical burrows, probably of worms (Fig. 15),
and spicules possibly of silicious sponges, which occur in nodules of
chert in the limestones, traces of laminated forms like Cryptozoon
or Eozoon (Fig. 17), and minute carbonaceous fragments which may be
debris of sea-weeds or Zoophytes. In rocks of similar age in the United
States, Gresley has recently discovered worm-burrows, and in Brittany
there are quartzite beds in which Barrois and Cayeux believe that they
have found tests of Radiolarians, Foraminifera and spicules of sponges,
but their organic nature has been denied by Rauff, of Bonn. The casts
of Foraminifera, however, at least appear to be organic (Fig. 16), and
it is quite likely that Cayeux may be able to verify his Radiolarians
and sponges as well. Matthew's observations in New Brunswick in any
case establish their probability. Gümbel also recognises a species of
Eozoon in the equivalent rocks of Bavaria (see p. 213).
[Illustration: Fig. 16.--_Casts of Foraminifera, from the Huronian of
Brittany._ (After Cayeux.)
Compare with Globigerinæ on Fig. 12 and Archæospherinæ, Figs. 50-54.]
[Illustration: Fig. 17.--_Cryptozoon or Eozoon from the Hastings
Series, Tudor, Ontario_ (natural size).
From a specimen collected by the late Mr. Vennor, and now in the
collection of the Geological Survey, Ottawa. (See also Frontispiece and
figure of _Eozoon Bavaricum_, p. 213.)]
It is evident that here we have approached the limit of the higher
forms of marine invertebrate life, having as yet nothing to show except
worms and Protozoa. It is to be observed, however, that there may be
somewhere Huronian deposits formed in deep and quiet waters, which may
give better results, and that the unconformity between the Huronian and
overlying Kewenian may indicate a lapse of time, of which monuments may
yet be found.
The Laurentian.
Last of all we have the widely distributed Laurentian system of
Logan, the oldest known to geologists, and which with the Huronian
constitutes the great Archæan group of formations of Dana and others.
In its lowest part this consists entirely of the stratified granitic
rock known as gneiss, inter-bedded in some places with dark-coloured
crystalline rocks or schists. This may be a part of the first-formed
crust of our globe, produced under conditions different from those
of any later rocks, and incompatible with the existence of life. The
upper part of the Laurentian system, however, known in Canada as the
"Grenville Series," shows evidence of ordinary marine deposition in
quiet waters, which may have been not unfavourable to the lower forms
of marine life; and though its beds have been greatly changed by heat
and pressure, we can still to some extent realize the conditions of a
time of comparative quiescence intervening between the underlying Lower
Laurentian and the succeeding Huronian. This part of the system still
contains gneisses, bedded diorites, and other rocks which may have been
volcanic; but it has also quartzites and quartzose gneisses which must
have been sandstones or shales, thick limestones, beds of carbon now
in the state of graphite or plumbago, and large beds of iron ore. Such
rocks were in all succeeding formations produced under water and by
accumulations of the remains of plants and the hard parts of animals,
in strictly sedimentary beds, usually formed slowly and without
mechanical disturbance. Hence we may infer that aquatic life at least
existed in this early period, and as there must have been land and
water, shallows and deep seas, there may have been scope for various
kinds of living beings. The Grenville period is, however, separated
from the succeeding Huronian by a great interval, occupied mainly by
volcanic ejections and earth-movements; so that our Grenville series,
if it contains organic remains, may be supposed to afford species
differing from those of the Huronian, and to form a sort of oasis in
the desert of the early pre-Cambrian world. We find that the limestones
of this age actually contain remains supposed to be of animal origin.
They were first found in Canada, which contains the largest and best
exposed area of these rocks in the world, and were brought under the
notice of geologists by the late Sir William E. Logan, the first
director of the Geological Survey of that country.
In anticipation of details to be given later, the story of this
discovery and its announcement may here be given in brief
As early as 1858, Sir William Logan had begun to suspect that certain
laminated bodies found in the Laurentian limestones of the Grenville
series might be of organic origin. The points which struck him were
these: They differed from any known laminated concretions; they
resembled the "Stromatoporæ" or layer-corals of the lower Palæozoic
rocks next in succession to the Laurentian and Huronian; the forms were
similar in all the specimens, while the mineralizing substances were
different; they were found only in the limestone, and specially in one
of the three great beds known in the formation, the upper limestone
of the Grenville system. He exhibited specimens, and mentioned these
probabilities at the meeting of the American Association in 1859. In
1862 it was suggested to Logan that the microscopic structure of some
of the best preserved examples should be studied, and slices were
accordingly prepared and submitted to the writer for examination.
They revealed in the calcareous laminæ of the specimens complicated
systems of canals or tubes filled with mineral matter, which appeared
to be similar to those that Carpenter had recognised in the thickened
parts of the shells of modern Foraminifera. This clew being followed,
large numbers of slices of the supposed fossils and of the containing
limestone and of similar limestones from other parts of the world were
examined.