ocean, and covering wide patches with more or less irregular masses,
it must have thrown up from its whole surface its pseudopods to seize
whatever floating particles of food the waters carried over it There
is also reason to believe, from the outline of certain specimens, that
it often grew upward in inverted, conical, or club-shaped forms, and
that only the broader patches were penetrated by the tubes or oscula
already mentioned, admitting the sea-water deeply into the substance of
the masses. In this way its growth might be rapid and continuous; but
it does not seem to have possessed the power of growing indefinitely
by new and living layers covering those that had died, in the manner
of some corals. Its life seems to have had a definite termination, and
when that was reached, an entirely new colony had to be commenced.
In this it had more affinity with the Foraminifera, as we now know
them, than with the corals, though practically it had the same power
with the coral polyps of accumulating limestone in the sea-bottom, a
power indeed still possessed by its foraminiferal successors. In the
case of coral limestones, we know that a large proportion of these
consist, not of continuous reefs, but of fragments of coral mixed with
other calcareous organisms, spread usually by waves and currents in
continuous beds over the sea-bottom. In like manner we find in the
limestones containing Eozoon, layers of fragmental matter which shows
in places the characteristic structures, and which evidently represents
the debris swept from the Eozoon masses and reefs by the action of
the waves. With this fragmental matter small rounded organisms to be
noticed in the sequel occur; and while they may be distinct animals
resembling the smaller modern species, they may also be the fry of
Eozoon, or small portions of its acervuline upper surface floated off
in a living state, and possibly capable of living independently and of
founding new colonies.
[Illustration: Fig. 47.--_Slice of Limestone (magnified),_
(_a_) Fragment of Eozoon with canals, (_b_) Fragments of granular
calcite, probably organic, (_c_) Structureless calcite with cleavage
lines (Côte St. Pierre).]
It is only by a somewhat wild poetical licence that Eozoon has been
represented as a "kind of enormous composite animal stretching from the
shores of Labrador to Lake Superior, and thence northward and southward
to an unknown distance, and forming masses 1,500 feet in depth." We may
discuss by-and-by the question of the composite nature of masses of
Eozoon, and we see in the corals evidence of the great size to which
composite animals of a higher grade can attain. In the case of Eozoon
we must imagine an ocean floor more uniform and level than that now
existing. On this the organism would establish itself in spots and
patches. These might finally become confluent over large areas, just
as massive corals do. As individual masses attained maturity and died,
their pores would be filled up with limestone or silicious deposits,
and thus could form a solid basis for new generations, and in this way
limestone to an indefinite extent might be produced. Further, wherever
such masses were high enough to be attacked by the breakers, or where
portions of the sea-bottom were elevated, the more fragile parts of the
surface would be broken up and scattered widely in beds of fragments
over the bottom of the sea, while here and there beds of mud or sand
or of volcanic debris would be deposited over the living or dead
organic mass, and would form the layers of gneiss and other schistose
rocks interstratified with the Laurentian limestone. In this way, in
short, Eozoon would perform a function combining that which corals and
Foraminifera perform in the modern seas; forming both reef limestones
and extensive chalky beds, and probably living both in the shallow and
the deeper parts of the ocean. If in connection with this we consider
the rapidity with which the soft, simple, and almost structureless
sarcode of these Protozoa can be built up, and the probability that
they were more abundantly supplied with food, both for nourishing their
soft parts and skeletons, than any similar creatures in later times, we
can readily understand the great volume and extent of the Laurentian
limestones which they aided in producing. I say aided in producing,
because I would not desire to commit myself to the doctrine that the
Laurentian limestones are wholly of this origin. There may have been
other animal limestone-builders than Eozoon, and there may have been
limestones formed by plants like the modern Nullipores or by merely
mineral deposition.
Its relations to modern animals of its type have been very clearly
defined by Dr. Carpenter. In the structure of its proper wall and its
fine parallel perforations, it resembles the _Nummulites_ and their
allies (Figs. 48, 49); and the organism may therefore be regarded as
an aberrant member of the Nummuline group, which affords some of the
largest and most widely distributed of the fossil Foraminifera. This
resemblance may be seen in Fig. 48.
[Illustration: Fig. 48.--_Section of a Nummulite, from Eocene Limestone
of Syria._
Showing chambers, tubuli, and canals. Compare this and Fig. 49 with
Figs. 28 and 29.]
[Illustration: Fig. 49.--_Portion of Shell of Calcarina._
Magnified, after Carpenter, (_a_) Cells. (_b_) Original cell-wall with
tubuli. (_c_) Supplementary skeleton with canals.]
To the Nummulites it also conforms in its tendency to form a
supplemental or intermediate skeleton with canals, though the canals
themselves in their arrangement more nearly resemble Calcarina, which
is represented in Fig. 49. In its superposition of many layers, and in
its tendency to a heaped-up or acervuline irregular growth it resembles
_Carpenteria_, _Polytrema_ and _Tinoporus_, forms of a different group
in so far as shell-structure is concerned. The large and curious sandy
Foraminifer from the Pacific dredged by Alexander Agassiz, and named
by Goës, _Neusina Agassizi_, may also be mentioned as presenting some
points of resemblance.[38] It may thus be regarded as a composite
type, combining peculiarities now observed in two groups, or it may be
regarded as a representative in the Nummuline series of Polytrema and
Tinoporus in the Rotaline series. At the time when Dr. Carpenter stated
these affinities, it might be objected that Foraminifera of these
families are in the main found in the Modern and Tertiary periods. Dr.
Carpenter has since shown that the curious oval Foraminifer called
_Fusulina_, found in the coal formation, is in like manner allied to
both Nummulites and Rotalines; and still more recently Mr. Brady has
discovered a true Nummulite in the Lower Carboniferous of Belgium. This
group being now fairly brought down to the Palæozoic, we may hope
finally to trace it back to the Primordial, and thus to bring it still
nearer to Eozoon in time.
[Footnote 38: _Bulletin Mus. Comp. Zoology_, vol. xxiii., No. 5, Dec.,
1892.]
Though Eozoon was probably not the only animal of the Laurentian seas,
yet it was in all likelihood the most conspicuous and important as
a collector of calcareous matter, filling the same place afterwards
occupied by the reef-building corals. Though probably less efficient
than these as a constructor of solid limestones, from its less
permanent and continuous growth, it formed wide floors and patches
on the sea-bottom, and when these were broken up vast quantities of
limestone were formed from their debris. It must also be borne in
mind that Eozoon was not everywhere infiltrated with serpentine or
other silicious minerals; quantities of its substance were merely
filled with carbonate of lime, resembling the chamber-wall so closely
that it is nearly impossible to make out the difference, and thus is
likely to pass altogether unobserved by collectors, and to baffle
even the microscopist. Although therefore the layers which contain
well-characterized Eozoon are few and far between, there is reason to
believe that in the composition of the limestones of the Laurentian it
bore no small part; and as these limestones are some of them several
hundreds of feet in thickness, and extend over vast areas, Eozoon
may be supposed to have been as efficient a world-builder as the
Stromatoporæ of the Silurian and Devonian, the Globigerinæ and their
allies in the chalk, or the Nummulites and Miliolites in the Eocene. It
is a remarkable illustration of the constancy of natural causes and of
the persistence of animal types, that these humble Protozoans, which
began to secrete calcareous matter in the Laurentian period, have been
continuing their work in the ocean through all the geological ages,
and are still busy in accumulating those chalky muds with which recent
dredging operations in the deep sea have made us so familiar.
[Illustration: Fig. 50.--_Figures of Archæospherinæ._
(1) Specimen with tubulated wall. (2 to 5) Casts in serpentine, Côte
St. Pierre and Long Lake]
_CONTEMPORARIES OF EOZOON_
VIII
_CONTEMPORARIES OF EOZOON_
The name Eozoon, or Dawn-animal, raises the question whether we
shall ever know any earlier representative of animal life. Here I
think it necessary to explain that in suggesting the name Eozoon for
the earliest fossil, and Eozoic for the formation in which it is
contained, I had no intention to affirm that there may not have been
precursors of the Dawn-animal. By the similar term. Eocene, Lyell
did not mean to affirm that there may not have been modern types in
the preceding geological periods: and so the dawn of animal life may
have had its grey or rosy breaking at a time long anterior to that in
which Eozoon built its marble reefs. When the fossils of this early
auroral time shall be found, it will not be hard to invent appropriate
names for them. There are, however, two reasons that give propriety
to the name in the present state of our knowledge. One is, that the
Laurentian rocks are absolutely the oldest that have yet come under
the notice of geologists, and at the present moment it seems extremely
improbable that any older sediments exist, at least in a condition to
be recognised as such. The other is that Eozoon, as a member of the
group Protozoa, of gigantic size and comprehensive type, and oceanic in
its habitat, is as likely as any other creature that can be imagined
to have been the first representative of animal life on our planet.
Vegetable life may have preceded it, nay probably did so by at least
one great creative æon, and may have accumulated previous stores of
organic matter; but if any older forms of animal life existed, it is
certain at least that they cannot have belonged to much simpler or more
comprehensive types. It is also to be observed that such forms of life,
if they did exist, may have been naked protozoa, which may have left no
sign of their existence except a minute trace of carbonaceous matter,
and perhaps not even this.
But if we do not know, and perhaps are not likely to know, any
animals older than Eozoon, may we not find traces of some of its
contemporaries, either in the Eozoon limestones themselves, or other
rocks associated with them? Here we must admit that a deep-sea
Foraminiferal limestone may give a very imperfect indication of the
fauna of its time. A dredger who should have no other information as
to the existing population of the world, except what he could gather
from the deposits formed under several hundred fathoms of water,
would necessarily have very inadequate conceptions of the matter. In
like manner a geologist who should have no other information as to
the animal life of the Mesozoic ages than that furnished by some of
the thick beds of white chalk, might imagine that he had reached a
period when the simplest kinds of protozoa predominated over all other
forms of life; but this impression would at once be corrected by the
examination of other deposits of the same age: so our inferences as to
the life of the Laurentian from the contents of its oceanic limestones
may be very imperfect, and it may yet yield other and various fossils.
Its possibilities are, however, limited by the fact that before we
reach this great depth in the earth's crust, we have already left
behind in much newer formations all traces of animal life except a
few of the lower forms of aquatic invertebrates; so that we are not
surprised to find only a limited number of living things, and those of
very low type. Do we then know in the Laurentian even a few distinct
species, or is our view limited altogether to Eozoon Canadense? In
answering this question, we must bear in mind that the Laurentian
itself was of vast duration, and that important changes of life may
have taken place even between the deposition of the Eozoon limestones
and that of those rocks in which we find the comparatively rich fauna
of the Primordial age. This subject was discussed by the writer as
early as 1865, and I may repeat here what could be said in relation to
it at that time:--
"In connection with these remarkable remains, it appeared desirable to
ascertain, if possible, what share these or other organic structures
may have had in the accumulation of the limestones of the Laurentian
series. Specimens were therefore selected by Sir W. E. Logan, and
slices were prepared under his direction. On microscopic examination,
a number of these were found to exhibit merely a granular aggregation
of crystals, occasionally with particles of graphite and other foreign
minerals, or a laminated mixture of calcareous and other matters, in
the manner of some more modern sedimentary limestones. Others, however,
were evidently made up almost entirely of fragments of Eozoon, or of
mixtures of these with other calcareous and carbonaceous fragments
which afford more or less evidence of organic origin. The contents of
these organic limestones may be considered under the following heads:--
1. Remains of Eozoon.
2. Other calcareous bodies, probably organic.
3. Objects imbedded in the serpentine.
4. Carbonaceous matters.
"(1) The more perfect individuals of Eozoon do not constitute the mass
of any of the larger specimens in our collections; but considerable
portions of some of them are made up of material of similar minute
structure, destitute of lamination, and irregularly arranged. Some of
this material gives the impression that there may have been organisms
similar to Eozoon, but growing in an irregular or acervuline manner
without lamination. Of this, however, I cannot be certain; and, on the
other hand, there is distinct evidence of the aggregation of fragments
of Eozoon in some of these specimens. In some they constitute the
greater part of the mass. In others they are imbedded in calcareous
matter of a different character, or in serpentine or granular pyroxene.
In most of the specimens the cells of the fossils are more or less
filled with these minerals; and in some instances it would appear that
the calcareous matter of fragments of Eozoon has been in part replaced
by serpentine."
[I may add here that in the limestone at Côte St. Pierre there are in
some of the beds successive laminæ with grains of serpentine and others
with crystals of dolomite, and that both contain fragments of Eozoon.
It thus seems as if the magnesia associated with the limestone, at some
stages of deposition took the form of silicate, and in others that of
carbonate. I may also observe here that I have detected fragments of
Eozoon in Laurentian limestone from New Brunswick, from Chelmsford in
Massachusetts, from Warren County, New York, from Brazil, and from the
Alps.]
"(2) Intermixed with the fragments of Eozoon above referred to are
other calcareous matters apparently fragmentary. They are of various
angular and rounded forms, and present several kinds of structure. The
most frequent of these is a strong lamination varying in direction
according to the position of the fragments, but corresponding, as
far as can be ascertained, with the diagonal of the rhombohedral
cleavage. This structure, though crystalline, is highly characteristic
of crinoidal remains when preserved in altered limestones. The more
dense parts of Eozoon, destitute of tubuli, also sometimes show
this structure, though less distinctly. Other fragments are compact
and structureless, or show only a fine granular appearance; and
these sometimes include grains, patches, or fibres of graphite. In
Cambro-Silurian limestones, fragments of corals and shells which have
been partially infiltrated with bituminous matter, show a structure
like this. On comparison with altered organic limestones of the
Cambro-Silurian system, these appearances would indicate that, in
addition to the debris of Eozoon, other calcareous structures, more
like those of crinoids, corals, and shells, have contributed to the
formation of the Laurentian limestones.
"(3) In the hydrous silicate (Loganite) filling the chambers of a
large specimen of Eozoon from Burgess, there are numerous small pieces
of foreign matter; and the silicate itself is laminated, indicating
its sedimentary nature. Some of the included fragments appear to be
carbonaceous, others calcareous; but no distinct organic structure
can be detected in them. There are, however, in the Loganite, many
minute silicious grains of a bright green colour, resembling green-sand
concretions; and the manner In which these are occasionally arranged
in lines and groups suggests the supposition that they may possibly
be casts of the interior of minute Foraminiferal shells. They may,
however, be concretionary in their origin (Fig. 51).
[Illustration: Fig. 51.--Archæospherinæ from Burgess Eozoon. Grains
included in Loganite.
(Magnified.)]
"(4) In some of the Laurentian limestones submitted to me by Sir W. E.
Logan, and in others from Arnprior on the Ottawa, there are fibres and
granules of carbonaceous matter which do not conform to the crystalline
structure, and present appearances quite similar to those which in
more modern limestones result from the decomposition of the algæ, etc.
Though retaining mere traces of organic structure, little doubt would
be entertained as to their vegetable origin if they were found in
fossiliferous limestones. In limestones of Upper Laurentian age, near
St. John, New Brunswick, more distinct fibres occur, and associated
with these beds Matthew has found what seem to be spicules of sponges,
some simple and others hexactinelled like those of Protospongia of the
Cambrian.
Though the abundance and wide distribution of Eozoon, and the important
part it seems to have acted in the accumulation of limestone, indicate
that it was one of the most prevalent forms of animal existence
in the seas of the Laurentian period, the non-existence of other
organic beings is not implied. On the contrary, independently of the
indications afforded by the limestones themselves, it is evident that
in order to the existence and growth of these large Rhizopods, the
waters must have swarmed with more minute animal or vegetable organisms
on which they could subsist. On the other hand, though this is a less
certain inference, the dense calcareous skeleton of Eozoon may indicate
that it also was liable to the attacks of animal enemies. It is also
possible that the growth of Eozoon or the deposition of the serpentine
and pyroxene in which its remains have been preserved, or both, may
have been connected with certain oceanic depths and conditions, and
that we have as yet revealed to us the life of only certain stations in
the Laurentian seas. Whatever conjectures we may form on these more
problematic points, the observations above detailed appear to establish
the following conclusions:--
First, that in the Laurentian period, as in subsequent geological
epochs, the Rhizopods were important agents in the accumulation of
beds of limestone; and secondly, that in this early period these low
forms of animal life attained to a development, in point of magnitude
and complexity, unexampled, in so far as yet known, in the succeeding
ages of the earth's history. This early culmination of the Rhizopods is
in accordance with one of the great laws of the succession of living
beings, ascertained from the study of the introduction and progress of
other groups; and, should it prove that these great Protozoans were
really the dominant type of animals in the Laurentian period, this fact
might be regarded as an indication that in these ancient rocks we may
actually have the records of the first appearance of animal life on our
planet.
With reference to the first of the above heads, I have now to state
that it seems quite certain that the upper and younger portions of
the masses of Eozoon often passed into the acervuline form, and the
period in which this change took place seems to have depended on
circumstances. In some specimens there are only a few regular layers,
and then a heap of irregular cells. In other cases a hundred or more
regular layers were formed; but even in this case little groups of
irregular cells occurred at certain points near the surface. I have
also found some masses clearly not fragmental which consist altogether
of acervuline cells. A specimen of this kind is represented in Fig. 52.
It is oval in outline, enclosed in a nodule of serpentine, about three
inches in length, wholly made up of rounded or cylindrical cells, the
walls of which have a beautiful tubular structure, but there is little
or no supplemental skeleton. Whether this is a portion accidentally
broken off from the top of a mass of Eozoon, or a peculiar varietal
form, or a distinct species, it would be difficult to determine. In
the meantime I have described it as a variety, "_acervulina_" of the
species Eozoon Canadense. It admits of comparison with a fragment
figured by Dr. Carpenter, which he compares with the chamberlets and
tubes of _Nummulites lævigata_ of the Eocene.[39] Another variety
also, from Petite Nation, shows extremely thin laminæ, closely placed
together and very massive, and with little supplemental skeleton. This
may be allied to the last, and may be named variety "_minor_."[40]
[Footnote 39: _Proceedings of Geological Society_, 1875.]
[Footnote 40: _Annals and Magazine of Natural History_, Sen 4, vol.
xiii. p. 457.]
All this, however, has nothing to do with the layers of fragments of
Eozoon which are scattered through the Laurentian limestones. In these
the fossil is sometimes preserved in the ordinary manner, with its
cavities filled with serpentine, and the thicker parts of the skeleton
having their canals filled with this substance. In this case the
chambers may have been occupied with serpentine before it was broken
up. At St. Pierre there are distinct layers of this kind, from half an
inch to several inches in thickness, regularly interstratified with
the ordinary limestone. In other layers no serpentine occurs, but the
interstices of the fragments are filled with crystalline dolomite or
magnesian limestone, which has also penetrated the canals; and there
are indications, though less manifest, that some at least of the layers
of pure limestone are composed of fragmental Eozoon.
[Illustration: Fig. 52.--_Acervuline Variety of Eozoon, Côte St.
Pierre._
(_a_) General form, half natural size. (_b_) Portion of cellular
interior, magnified, showing the course of the tubuli.]
[Illustration: Fig. 53.--_Archæospherinæ from Côte St. Pierre._
(_a_) Specimens dissolved out by acid, the lower one showing interior
septa. (_b_) Specimens seen in section.]
In the Laurentian limestone of Wentworth, belonging apparently to the
same band with that of St. Pierre, there are many small rounded pieces
of limestone, evidently the debris of some older rock, broken up and
rounded by attrition. In some of these fragments the structure of
Eozoon may be plainly perceived. This shows that still older limestones
composed of Eozoon were at that time undergoing waste, and carries our
view of the existence of this fossil back to the very beginning of the
Grenville series of the Laurentian.
With respect to organic fragments not showing the structure of Eozoon,
I have not as yet been able to refer these to any definite origin. Some
of them may be simply thick portions of the shell of Eozoon with their
pores filled with calcite, so as to present a homogeneous appearance.
Others have much the appearance of fragments of such Primordial forms
as _Archæocyathus_, now usually regarded as corals or sponges; but
after much careful search, I have thus far been unable to say more than
I could say in 1865.