THE EOZOIC AGES.
The dominion of heat has passed away; the excess of water has been
precipitated from the atmosphere, and now covers the earth as a
universal ocean. The crust has folded itself into long ridges, the bed
of the waters has subsided into its place, and the sea for the first
time begins to rave against the shores of the newly elevated land,
while the rain, washing the bare surfaces of rocky ridges, carries its
contribution of the slowly wasting rocks back into the waters whence
they were raised, forming, with the material worn from the crust by
the surf, the first oceanic sediments. Do we know any of these
earliest aqueous beds, or are they all hidden from view beneath newer
deposits, or have they been themselves worn away and destroyed by
denuding agencies? Whether we know the earliest formed sediments is,
and may always remain, uncertain; but we do know certain very ancient
rocks which may be at least their immediate successors.
[Illustration: Fig. 6.--The Laurentian nucleus of the American
continent.]
Deepest and oldest of all the rocks we are acquainted with in the
crust of the earth, are certain beds much altered and metamorphosed,
baked by the joint action of heat and heated moisture--rocks once
called Azoic, as containing no traces of life, but for which I have
elsewhere proposed the name "Eozoic," or those that afford the traces
of the earliest known living beings. These rocks are the Laurentian
Series of Sir William Logan, so named from the Laurentide hills, north
of the River St. Lawrence, which are composed of these ancient beds,
and where they are more largely exposed then in any other region. It
may seem at first sight strange that any of these ancient rocks should
be found at the surface of the earth; but this is a necessary result
of the mode of formation of the continents. The oldest rocks, thrown
up in places into high ridges, have either not been again brought
under the waters, or have lost by denudation the sediments once
resting on them; and being of a hard and resisting nature, still
remain; and often rise into hills of considerable elevation, showing
as it were portions of the skeleton of the earth protruding through
its superficial covering. Such rocks stretch along the north side of
the St. Lawrence river from Labrador to Lake Superior, and thence
northwardly to an unknown distance, constituting a wild and rugged
district often rising into hills 4000 feet high, and in the deep gorge
of the Saguenay forming cliffs 1,500 feet in sheer height from the
water's edge. South of this great ridge, the isolated mass of the
Adirondack Mountains rises to the height of 6,000 feet, rivalling the
newer, though still very ancient, chain of the White Mountains. Along
the eastern coast of North America, a lower ridge of Laurentian rock,
only appearing here and there from under the overlying sediments, is
seen in Newfoundland, in New Brunswick, possibly in Nova Scotia, and
perhaps farther south in Massachusetts, and as far as Maryland. In the
old world, rocks of this age do not, so far as known, appear so
extensively. They have been recognised in Norway and Sweden, in the
Hebrides, and in Bavaria, and may, no doubt, be yet discerned in other
localities. Still, the grandest and most instructive development of
these rocks is in North America; and it is there that we may best
investigate their nature, and endeavour to restore the conditions in
which they were deposited. It has been already stated that the oldest
wrinkles of the crust of the globe take the direction of great circles
of the earth tangent to the polar circle, forming north-east and
south-west, and north-west and south-east lines. To such lines are the
great exposures of Laurentian rock conformed, as may be well seen from
the map of North America (fig. 6), taken from Dana, with some
additions. The great angular Laurentian belt is evidently the nucleus
of the continent, and consists of two broad bands or ridges meeting in
the region of the great lakes. The remaining exposures are parallel to
these, and appear to indicate a subordinate coast-line of
comparatively little elevation. It is known that these Laurentian
exposures constitute the oldest part of the continent, a part which
was land before any of the rocks of the shaded portion of the map were
deposited in the bed of the ocean--all this shaded portion being
composed of rocks of various geological ages resting on the older
Laurentian. It is further to be observed that the beds occurring in
the Laurentian bands are crumpled and folded in a most remarkable
manner, and that these folds were impressed upon them before the
deposition of the rocks next in geological age.
What then are these oldest rocks deposited by the sea--the first-born
of the reign of the waters? They are very different in their external
aspect from the silt and mud, the sand and gravel, and the shell and
coral rocks of the modern sea, or of the more recent geological
formations. Yet the difference is one in condition rather then
composition. The members of this ancient aristocracy of the rocks are
made of the same clay with their fellows, but have been subjected to a
refining and crystallizing process which has greatly changed their
condition. They have been, as geologists say, metamorphosed; and are
to ordinary rocks what a china vase is to the lump of clay from which
it has been made. Deeply buried in the earth under newer sediments,
they have been baked, until sandstones, gravels, and clays came out
bright and crystalline, as gneiss, mica-schist, hornblende-schist, and
quartzite--all hard crystalline rocks showing at first sight no
resemblance to their original material, except in the regularly
stratified or bedded arrangement which serves to distinguish them from
igneous or volcanic rocks. In like manner certain finer, calcareous
sediments have been changed into Labrador feldspar, sometimes gay with
a beautiful play of colour, and what were once common limestones
appear as crystalline marble. If the evidence of such metamorphoses is
asked for, this is twofold. In the first place, these rocks are
similar in structure to more modern beds which have been partially
metamorphosed, and in which the transition from the unaltered to the
altered state can be observed. Secondly, there are limited areas in
the Laurentian itself, in which the metamorphism has been so imperfect
as to permit traces of the original character of the rocks to remain.
It seems also quite certain, and this is a most important point for
our sketch, that the Laurentian ocean was not universal, but that
there were already elevated portions of the crust capable of yielding
sediment to the sea.
In North America these Laurentian rocks attain to an enormous
thickness. This has been estimated by Sir W. E. Logan at 30,000 feet,
so that the beds would, if piled on each other horizontally, be as
high as the highest mountains on earth. They appear to consist of two
great series, the Lower and Upper Laurentian. Even if we suppose that
in the earlier stages of the world's history erosion and deposition
were somewhat more rapid then at present, the formation of such
deposits, probably more widely spread then any that succeeded them,
must have required an enormous length of time.
Geologists long looked in vain for evidences of life in the Laurentian
period; but just as astronomers' have suspected the existence of
unknown planets from the perturbations due to their attraction,
geologists have guessed that there must have been some living things
on earth even at this early time. Dana and Sterry Hunt especially have
committed themselves to such speculations. The reasons for this belief
may be stated thus: (1.) In later formations limestone is usually an
organic rock, produced by the accumulation of shells, corals, and
similar calcareous organisms in the sea, and there are enormous
limestones in the Laurentian, constituting regular beds. (2.) In
later formations coaly matter is an organic substance, derived from
vegetables, and there are large quantities of Laurentian carbon in the
form of graphite. (3.) In later formations deposits of iron ores are
almost always connected with the deoxidising influence of organic
matters as an efficient cause of their accumulation, and the
Laurentian contains immense deposits of iron ore, occurring in layers
in the manner of later deposits of these minerals. (4.) The limestone,
carbon, and iron of the Laurentian exist in association with the other
beds in the same manner as in the later formations in which they are
known to be organic.
[Illustration: Fig. 7.--_Eozoon Canadense._ Dawson.
The oldest known animal. Portion of skeleton, two-thirds natural size,
(_a_) Tabulated cell-wall, magnified, (_b_) Portion of canal system,
magnified.]
In addition to this inferential evidence, however, one well-marked
animal fossil has at length been found in the Laurentian of Canada,
Eozoon Canadense, (fig. 7), a gigantic representative of one of the
lowest forms of animal life, which the writer had the honour of naming
and describing in 1865--its name of "Dawn-animal" having reference to
its great antiquity and possible connection with the dawn of life on
our planet. In the modern seas, among the multitude of low forms of
life with which they swarm, occur some in which the animal matter is a
mere jelly, almost without distinct parts or organs, yet
unquestionably endowed with life of an animal character. Some of these
creatures, the Foraminifera, have the power of secreting at the
surface of their bodies a calcareous shell, often divided into
numerous chambers, communicating with each other, and with the water
without, by pores or orifices through which, the animal can extend
soft and delicate prolongations of its gelatinous body, which, when
stretched out into the water, serve for arms and legs. In modern times
these creatures, though extremely abundant in the ocean, are usually
small, often microscopic; but in a fossil state there are others of
somewhat larger size, though few equaling the Eozoon, which seems to
been a sessile creature, resting on the bottom of the sea, and
covering its gelatinous body with a thin crust of carbonate of lime or
limestone, adding to this, as it grew in size, crust after crust,
attached to each other by numerous partitions, and perforated with
pores for the emission of gelatinous filaments. This continued growth
of gelatinous animal matter and carbonate of lime went on from age to
age, accumulating great beds of limestone, in some of which the entire
form and most minute structures of the creature are preserved, while
in other cases the organisms have been broken up, and the limestones
are a mere congeries of their fragments. It is a remarkable instance
of the permanence of fossils, that in these ancient organisms the
minutest pores through which the semi-fluid matter of these humble
animals passed, have been preserved in the most delicate perfection.
The existence of such creatures supposes that of other organisms,
probably microscopic plants, on which they could feed. No traces of
these have been observed, though the great quantity of carbon in the
beds probably implies the existence of larger sea-weeds. No other form
of animal has yet been distinctly recognized in the Laurentian
limestones, but there are fragments of calcareous matter which may
have belonged to organisms distinct from Eozoon. Of life on the
Laurentian land we know nothing, unless the great beds of iron ore
already referred to may be taken as a proof of land vegetation.[C]
[C] It is proper to state here that some geologists and naturalists
still doubt the organic nature of Eozoon. Their objections however, so
far as stated publicly, have been shown to depend on misapprehension
as to the structures observed and their state of preservation; and
specimens recently found in comparatively unaltered rocks have
indicated the true character of those more altered by metamorphism.
To an observer in the Laurentian period, the earth would have
presented an almost boundless ocean, its waters, perhaps, still warmed
with the internal heat, and sending up copious exhalations to be
condensed in thick clouds and precipitated in rain. Here and there
might be seen chains of rocky islands, many of them volcanic, or
ranges of bleak hills, perhaps clothed with vegetation the forms of
which are unknown to us. In the bottom of the sea, while sand and mud
and gravel were being deposited in successive layers in some portions
of the ocean floor, in others great reefs of Eozoon were growing up in
the manner of reefs of coral. If we can imagine the modern Pacific,
with its volcanic islands and reefs of coral, to be deprived of all
other forms of life, 'we should have a somewhat accurate picture of
the Eozoic time as it appears to us now. I say as it appears to us
now; for we do not know what new discoveries remain to be made. More
especially the immense deposits of carbon and iron in the Laurentian
would seem to bespeak a profusion of plant life in the sea or on the
land, or both, second to that of no other period that succeeded,
except that of the great coal formation. Perhaps no remnant of this
primitive vegetation exists retaining its form or structure; but we
may hope for better things, and cherish the expectation that some
fortunate discovery may still reveal to us the forms of the vegetation
of the Laurentian time.
It is remarkable that the humbly organized living things which built
up the Laurentian limestones have continued to exist unchanged, save
in dimensions, up to modern times; and here and there throughout the
geological series we find beds of Foraminiferous limestone, similar,
except in the species of Foraminifera composing them, to that of the
Laurentian. It is true that other kinds of creatures, the coral
animals more particularly, have been introduced, and have proved
equally efficient builders of limestones; but in the deeper parts of
the sea the Foraminifera continue to assert their pre-eminence in this
respect, and the dredge reveals in the depths of our modern oceans
beds of calcareous matter which may be regarded as identical in origin
with the limestones formed in the period which is to us the dawn of
organic life.
Many inquiries suggest themselves to the zoologist in connection with
the life of the Laurentian period. Was Eozoon the first creature in
which the wondrous forces of animal life were manifested, when, in
obedience to the Divine fiat, the waters first "swarmed with
swarmers," as the terse and expressive language of the Mosaic record
phrases it? If so, in contemplating this organism we are in the
presence of one of the greatest of natural wonders--brought nearer
then in any other case to the actual workshop of the Almighty Maker.
Still we cannot affirm that other creatures even more humble may not
have preceded Eozoon, since such humble organisms are known in the
present world. Attempts have often been made, and very recently have
been renewed with much affirmation of success, to prove that such low
forms of life may originate spontaneously from their materials in the
waters; but so far these attempts merely prove that the invisible
germs of the lower animals and plants exist everywhere, and that they
have marvellous powers of resisting extreme heat and other injurious
influences. We need not, therefore, be surprised if even lower forms
then Eozoon may have preceded that creature, or if some of these may
be found, like the organisms said to live in modern boiling springs,
to have had the power of existing even at a time when the ocean may
have been almost in a state of ebullition. Another problem is that of
means of subsistence for the Eozoic Foraminifera. A similar problem
exists in the case of the modern ocean, in whose depths live
multitudes of creatures, where, so far as we know, vegetable matter,
ordinarily the basis of life, cannot exist in a living condition. It
is probable, however, from the researches of Dr. Wyville Thompson,
that this is to be accounted for by the abundance of life at the
surface and in the shallower parts of the sea, and by the consequent
diffusion through the water of organic matter in an extremely tenuous
state, but yet sufficient to nourish these creatures. The same may
have been the case in the Eozoic sea, where, judging from the vast
amount of residual carbon, there must have been abundance of organic
matter, either growing at the bottom, or falling upon it from the
surface; and as the Eozoon limestones are usually free from such
material, we may assume that the animal life in them was sufficient to
consume the vegetable pabulum. On the other hand, as detached
specimens of Eozoon occur in graphitic limestones, we suppose that in
some cases the vegetable matter was in excess of the animal, and this
may have been either because of its too great exuberance, or because
the water was locally too shallow to permit Eozoon and similar
creatures to nourish. These details we must for the present fill up
conjecturally; bu the progress of discovery may give us further light
as to the precise conditions of the beginning of life in the "great
and wide sea wherein are moving things innumerable" and which is as
much a wonder now as in the days of the author of the "Hymn of
Creation"[D] in regard to the life that swarms in all its breadth and
depth, the vast variety of that life, and its low and simple types, of
which we can affirm little else then that they move.
[D] Psalm civ.
The enormous accumulations of sediment on the still thin crust of the
earth in the Laurentian period--accumulations probably arranged in
lines parallel to the directions of disturbance already
indicated--weighed down the surface, and caused great masses of the
sediment to come within the influence of the heated interior nucleus.
Thus, extensive metamorphism took place, and at length the tension
becoming too great to be any longer maintained, a second great
collapse occurred, crumpling and disturbing the crust, and throwing up
vast masses of the Laurentian itself, probably into lofty
mountains--many of which still remain of considerable height, though
they have been subjected to erosion throughout all the extent of
subsequent geological time.
The Eozoic age, whose history we have thus shortly sketched, is
fertile in material of thought for the geologist and the naturalist.
Until the labours of Murchison, Sedgwick, Hall, and Barrande had
developed the vast thickness and organic richness of the Silurian and
Cambrian rocks, no geologist had any idea of the extent to which life
had reached backward in time. But when this new and primitive world of
Siluria was unveiled, men felt assured that they had now at last
reached to the beginnings of life. The argument on this side of the
Question was thus put by one of the most thoughtful of English
geologists, Professor Phillips: "It is ascertained that in passing
downwards through the lower Palæozoic strata, the forms of life grow
fewer and fewer, until in the lowest Cambrian rocks they vanish
entirely. In the thick series of these strata in the Longmynd, hardly
any traces of life occur, yet these strata are of such a kind as might
be expected to yield them.... The materials are fine-grained or
arenaceous, with or without mica, in laminae or beds quite distinct,
and of various thicknesses, by no means unlikely to retain
impressions of a delicate nature, such as those left by graptolites,
or mollusks, or annulose crawlers. Indeed, one or two such traces are
supposed to have been recognised, so that the almost total absence of
the traces of life in this enormous series is best understood by the
supposition that in these parts of the sea little or no life existed.
But the same remark of the excessive rarity of life in the lower
deposits is made in North America, in Norway, and in Bohemia,
countries well searched for this very purpose, so that all our
observations lead to the conviction that the lowest of all the strata
are quite deficient of organic remains. The absence is general--it
appears due to a general cause. Is it not probable that during these
very early periods the ocean and its sediments were nearly devoid of
plants and animals, and in the earliest time of all, which is
represented by sediments, quite deprived of such?" These words were
written ten years ago, and about the same time were published in
America those anticipations of the probability of life in the
Laurentian already referred to, and Lyell was protesting against the
name Primordial, on the ground that it implied that we had reached the
beginning of life, when this was not proved. Yet there were elements
of truth in both views. It is true now, as then, that the Primordial
seems to be a morning hour of life, having, as we shall see in our
next paper, unmistakable signs about it of that approach to the
beginning to which Phillips refers. It is also true that it is not so
early a morning hour as one who has not risen with the dawn might
suppose, since with its apparently small beginnings of life it is
almost as far removed from the Eozoon reefs of the early Laurentian on
the one hand, as it is from the modern period on the other. The dawn
of life seems to have been a very slow and protracted process, and it
may have required as long a time between the first appearance of
Eozoon and the first of those primordial Trilobites which the next
period will introduce to our notice, as between these and the advent
of Adam. Perhaps no lesson is more instructive then this as to the
length of the working days of the Almighty.
Another lesson lies ready for us in these same facts. Theoretically,
plants should have preceded animals; and this also is the assertion of
the first chapter of Genesis; but the oldest fossil certainly known to
us is an animal. What if there were still earlier plants, whose
remains are still to be discovered? For my own part, I can see no
reason to despair of the discovery of an _Eophytic_ period preceding
the Eozoic; perhaps preceding it through ages of duration to us almost
immeasurable, though still within the possible time of the existence
of the crust of the earth. It is even possible that in a warm and
humid condition of the atmosphere, before it had been caused "to rain
upon the earth" and when dense "mists ascended from the earth and
watered the whole surface of the ground,"[E] vegetation may have
attained to a profusion and grandeur unequalled in the periods whose
flora is known to us.
[E] Genesis ii. 5. For a description of this Eophytic period of
Genesis, see the Author's "Archaia," pp. 160 _et seq._
But while Eozoon thus preaches of progress and of development, it has
a tale to tell of unity and sameness Just as Eozoon lived in the
Laurentian sea, and was preserved for us by the infiltration of its
canals with siliceous mineral matters, so its successors and
representatives have gone on through all the ages accumulating
limestone in the sea bottom. To-day they are as active as they were
then, and are being fossilised in the same way. The English chalk and
the chalky modern mud of the Atlantic sea-bed, are precisely similar
in origin to the Eozoic limestones. There is also a strange
parallelism in the fact that in the modern seas Foraminifera can live
under conditions of deprivation of light and vital air, and of
enormous pressure, under which few organisms of greater complexity
could exist, and that in like manner Eozoon could live in seas which
were perhaps as yet unfit for most other forms of life.
It has been attempted to press the Eozoic Foraminifers into the
service of those theories of evolution which would deduce the animals
of one geological period by descent with modification from those of
another; but it must be confessed that Eozoon proves somewhat
intractable in this connection. In the first place, the creature is
the grandest of his class, both in form and structure; and if, on the
hypothesis of derivation, it has required the whole lapse of
geological time to disintegrate Eozoon into Orbulina, Globigerina,
and other comparatively simple Foraminifers of the modern seas, it may
have taken as long, probably much longer, to develop Eozoon from such
simple forms in antecedent periods. Time fails for such a process.
Again, the deep sea has been the abode of Foraminifers from the first.
In this deep sea they have continued to live without improvement, and
with little material change. How little likely is it that in less
congenial abodes they could have improved into higher grades of being;
especially since we know that the result in actual fact of any such
struggle for existence is merely the production of depauperated
Foraminifers? Further, there is no link of connection known to us
between Eozoon and any of the animals of the succeeding Primordial,
which are nearly all essentially new types, vastly more different from
Eozoon then it is from many modern creatures. Any such connection is
altogether imaginary and unsupported by proof. The laws of creation
actually illustrated by this primeval animal are only these: First,
that there has been a progress in creation from few, low, and
generalised types of life to more numerous, higher, and more
specialised types; and secondly, that every type, low or high, was
introduced at first in its best and highest form, and was, as a type,
subject to degeneracy, and to partial or total replacement by higher
types subsequently introduced. I do not mean that we could learn all
this from Eozoon alone; but that, rightly considered, it illustrates
these laws, which we gather from the subsequent progress of the
creative work. As to the mystery of the origin of living beings from
dead matter, or any changes which they may have undergone after their
creation, it is absolutely silent.