THE ANCIENT
LIFE-HISTORY
OF THE EARTH
Chapter 7:
THE LAURENTIAN AND HURONIAN PERIODS.
The Laurentian Rocks constitute the base of the entire
stratified series, and are, therefore, the oldest sediments of
which we have as yet any knowledge. They are more largely and more
typically developed in North America, and especially in Canada,
than in any known part of the world, and they derive their title
from the range of hills which the old French geographers named
the "Laurentides." These hills are composed of Laurentian Rocks,
and form the watershed between the valley of the St Lawrence river
on the one hand, and the great plains which stretch northwards
to Hudson Bay on the other hand. The main area of these ancient
deposits forms a great belt of rugged and undulating country,
which extends from Labrador westwards to Lake Superior, and then
bends northwards towards the Arctic Sea. Throughout this extensive
area the Laurentian Rocks for the most part present themselves
in the form of low, rounded, ice-worn hills, which, if generally
wanting in actual sublimity, have a certain geological grandeur
from the fact that they "have endured the battles and the storms
of time longer than any other mountains" (Dawson). In some places,
however, the Laurentian Rocks produce scenery of the most magnificent
character, as in the great gorge cut through them by the river
Saguenay, where they rise at times into vertical precipices 1500
feet in height. In the famous group of the Adirondack mountains,
also, in the state of New York, they form elevations no less than
6000 feet above the level of the sea. As a general rule, the
character of the Laurentian region is that of a rugged, rocky,
rolling country, often
densely timbered, but rarely well fitted for agriculture, and
chiefly attractive to the hunter and the miner.
As regards its mineral characters, the Laurentian series is composed
throughout of metamorphic and highly crystalline rocks, which are
in a high degree crumpled, folded, and faulted. By the late Sir
William Logan the entire series was divided into two great groups,
the Lower Laurentian and the Upper Laurentian, of
which the latter rests unconformably upon the truncated edges
of the former, and is in turn unconformably overlaid by strata
of Huronian and Cambrian age (fig. 20).
The Lower Laurentian series attains the enormous thickness
Fig. 20.—Diagrammatic section of the Laurentian
Rocks in Lower Canada. a Lower Laurentian; b Upper
Laurentian, resting unconformably upon the lower series; c
Cambrian strata (Potsdam Sandstone), resting unconformably on
the Upper Laurentian.
of over 20,000 feet, and is composed mainly of great beds of gneiss,
altered sandstones (quartzites), mica-schist, hornblende-schist,
magnetic iron-ore, and hæmatite, together with masses of
limestone. The limestones are especially interesting, and have an
extraordinary development—three principal beds being known, of
which one is not less than 1500 feet thick; the collective
thickness of the whole being about 3500 feet.
The Upper Laurentian series, as before said, reposes
unconformably upon the Lower Laurentian, and attains a thickness of
at least 10,000 feet. Like the preceding, it is wholly metamorphic,
and is composed partly of masses of gneiss and quartzite; but it
is especially distinguished by the possession of great beds of
felspathic rock, consisting principally of "Labrador felspar."
Though typically developed in the great Canadian area already
spoken of, the Laurentian Rocks occur in other localities, both
in America and in the Old World. In Britain, the so-called
"fundamental gneiss" of the Hebrides and of Sutherlandshire is
probably of Lower Laurentian age, and the "hypersthene rocks"
of the Isle of Skye may, with great probability, be regarded
as referable to the Upper Laurentian. In other localities in
Great Britain (as in St David's, South Wales; the Malvern Hills;
and the North of Ireland) occur ancient metamorphic deposits
which also are probably referable to the Laurentian series. The
so-called "primitive gneiss" of Norway appears to belong to the
Laurentian, and the
ancient metamorphic rocks of Bohemia and Bavaria may be regarded
as being approximately of the same age.
By some geological writers the ancient and highly metamorphosed
sediments of the Laurentian and the succeeding Huronian series
have been spoken of as the "Azoic rocks" (Gr. a, without;
zoe, life); but even if we were wholly destitute of any
evidence of life during these periods, this name would be
objectionable upon theoretical grounds. If a general name be
needed, that of "Eozoic" (Gr. eos, dawn; zoe, life),
proposed by Principal Dawson, is the most appropriate. Owing
to their metamorphic condition, geologists long despaired of
ever detecting any traces of life in the vast pile of strata
which constitute the Laurentian System. Even before any direct
traces were discovered, it was, however, pointed out that there
were good reasons for believing that the Laurentian seas had been
tenanted by an abundance of living beings. These reasons are
briefly as follows:—(1) Firstly, the Laurentian series consists,
beyond question, of marine sediments which originally differed
in no essential respect from those which were subsequently laid
down in the Cambrian or Silurian periods. (2) In all formations
later than the Laurentian, any limestones which are present can
be shown, with few exceptions, to be organic rocks, and to
be more or less largely made up of the comminuted debris of marine
or fresh-water animals. The Laurentian limestones, in consequence
of the metamorphism to which they have been subjected, are so
highly crystalline (fig. 21) that the microscope fails to detect
Fig. 21.—Section of Lower Laurentian Limestone from Hull,
Ottawa; enlarged five diameters. The rock is very highly
crystalline, and contains mica and other minerals. The irregular
black masses in it are graphite. (Original.)
any organic structure in the rock, and no fossils beyond those
which will be spoken of immediately have as yet been discovered in
them. We know, however, of numerous cases in which limestones,
of later age, and undoubtedly organic to begin with, have been
rendered so intensely crystalline by metamorphic action that
all traces of organic structure have been obliterated. We have
therefore, by analogy, the strongest possible ground for believing
that the vast beds of Laurentian limestone have been originally
organic in their origin, and primitively composed, in the main,
of the calcareous skeletons
of marine
animals. It would, in fact, be a matter of great difficulty to
account for the formation of these great calcareous masses on any
other hypothesis. (3) The occurrence of phosphate of lime in the
Laurentian Rocks in great abundance, and sometimes in the form of
irregular beds, may very possibly be connected with the former
existence in the strata of the remains of marine animals of whose
skeleton this mineral is a constituent. (4) The Laurentian Rocks
contain a vast amount of carbon in the form of black-lead or
graphite. This mineral is especially abundant in the
limestones, occurring in regular beds, in veins or strings, or
disseminated through the body of the limestone in the shape of
crystals, scales, or irregular masses. The amount of graphite in
some parts of the Lower Laurentian is so great that it has been
calculated as equal to the quantity of carbon present in an equal
thickness of the Coal-measures. The general source of solid
carbon in the crust of the earth is, however, plant-life; and it
seems impossible to account for the Laurentian graphite, except
upon the supposition that it is metamorphosed vegetable matter.
(5) Lastly, the great beds of iron-ore (peroxide and magnetic
oxide) which occur in the Laurentian series interstratified with
the other rocks, point with great probability to the action of
vegetable life; since similar deposits in later formations can
commonly be shown to have been formed by the deoxidising power
of vegetable matter in a state of decay.
In the words of Principal Dawson, "anyone of these reasons might,
in itself, be held insufficient to prove so great and, at first
sight, unlikely a conclusion as that of the existence of abundant
animal and vegetable life in the Laurentian; but the concurrence
of the whole in a series of deposits unquestionably marine, forms
a chain of evidence so powerful that it might command belief
even if no fragment of any organic or living form or structure
had ever been recognised in these ancient rocks." Of late years,
however, there have been discovered in the Laurentian Rocks certain
bodies which are believed to be truly the remains of animals, and
of which by far the most important is the structure known under
the now celebrated name of Eozoön. If truly organic,
a very special and exceptional interest attaches itself to
Eozoön, as being the most ancient fossil animal of which
we have any knowledge; but there are some who regard it really
a peculiar form of mineral structure, and a severe, protracted,
and still unfinished controversy has been carried on as to its
nature. Into this controversy it is wholly unnecessary to enter
here; and it will be sufficient to briefly explain the structure
of Eozoön, as elucidated by the elaborate and masterly
investigations of Carpenter
and Dawson, from the standpoint that it is a genuine
organism—the balance of evidence up to this moment inclining
decisively to this view.
The structure known as Eozoön is found in various
localities in the Lower Laurentian limestones of Canada, in the form
of isolated masses or spreading layers, which are composed of thin
alternating laminæ, arranged more or less concentrically (fig.
22). The laminæ of these masses are usually of different colours
Fig. 22.—Fragment of Eozoön, of the natural size,
showing alternate laminæ of loganite and dolomite. (After
Dawson.)
and composition; one series being white, and composed of carbonate
of lime—whilst the laminæ of the second series alternate
with the preceding, are green in colour, and are found by chemical
analysis to consist of some silicate, generally serpentine or the
closely-related "loganite." In some instances, however, all the
laminæ are calcareous, the concentric arrangement still remaining
visible in consequence of the fact that the laminæ are composed
alternately of lighter and darker coloured limestone.
When first discovered, the masses of Eozoön were supposed
to be of a mineral nature; but their striking general resemblance
to the undoubted fossils which will be subsequently spoken of under
the name of Stromatopora was recognised by Sir William
Logan, and specimens were submitted for minute examination, first
to Principal Dawson, and subsequently to Dr W. B. Carpenter.
After a careful microscopic examination, these two distinguished
observers came to the conclusion that Eozoön was truly
organic, and in this opinion they were afterwards corroborated
by other high authorities (Mr W. K. Parker, Professor Rupert
Jones, Mr H. B. Brady, Professor Gümbel, &c.) Stated briefly,
the structure of Eozoön, as exhibited by the microscope,
is as follows:—
The concentrically-laminated mass of Eozoön is composed
of numerous calcareous layers, representing the original skeleton
of the organism (fig. 23, b). These calcareous layers
Fig. 23.—Diagram of a portion of Eozoön
cut vertically. A, B, C, Three tiers of chambers communicating
with one another by slightly constricted apertures: a a,
The true shell-wall, perforated by numerous delicate tubes; b
b. The main calcareous skeleton ("intermediate skeleton");
c, Passage of communication ("stolon-passage") from one
tier of chambers to another; d, Ramifying tubes in the
calcareous skeleton. (After Carpenter.)
serve to separate and define a series of chambers arranged in
successive tiers, one above the other (fig. 23, A, B, C); and
they are perforated not only by passages (fig. 23, c),
which serve to place successive tiers of chambers in communication,
but also by a system of delicate branching canals (fig. 23,
d). Moreover, the central and principal portion of each
calcareous layer, with the ramified canal-system just spoken
of, is bounded both above and below by a thin lamina which has
a structure of its own, and which may be regarded as the proper
shell-wall (fig. 23, a a). This proper wall forms the
actual lining of the chambers, as well as the outer surface of
the whole mass; and it is perforated with numerous fine vertical
tubes (fig. 24, a a), opening into the chambers and on to
the surface by corresponding fine pores. From the resemblance
of this tubulated layer to similar structures in the shell of
the Nummulite, it is often spoken of as the "Nummuline layer."
The chambers are sometimes piled up one above the other in an
irregular manner; but they are more commonly arranged in regular
tiers, the separate chambers being marked off from one another
by projections of the wall in the form of partitions, which are
so far imperfect as to allow of a free communication between
contiguous chambers. In the original condition of the organism,
all these chambers, of course, must have been filled with
living-matter; but they are found in the present state of the
fossil to be generally filled with some silicate, such as serpentine,
which not only fills the actual chambers, but has also penetrated
the minute tubes of the proper wall and the branching canals of
the intermediate skeleton. In some cases
the chambers are simply filled with crystalline carbonate of lime.
When the originally porous fossil has been permeated by a silicate,
Fig. 24.—The animal of Nonionina, one of the
Foraminifera, after the shell has been removed by
a weak acid; b, Gromia, a single-chambered Foraminifer
(after Schultze), showing the shell surrounded by a network of
filaments derived from the body substance.
it is possible to dissolve away the whole of the calcareous skeleton
by means of acids, leaving an accurate and beautiful cast of the
chambers and the tubes connected with them in the insoluble silicate.
The above are the actual appearances presented by Eozoön
when examined microscopically, and it remains to see how far they
enable us to decide upon its true position in the animal kingdom.
Those who wish to study this interesting subject in detail must
consult the admirable memoirs by Dr W. B. Carpenter and Principal
Dawson: it will be enough here to indicate the results which
have been arrived at. The only animals at the present day which
possess a continuous calcareous skeleton, perforated by pores and
penetrated by canals, are certain organisms belonging to the
group of the Foraminifera. We have had occasion before
to speak of these animals, and as they are not conspicuous or
commonly-known forms of life, it may be well to say a few words as
to the structure of the living representatives of the group. The
Foraminifera are all inhabitants of the sea, and are mostly
of small or even microscopic dimensions. Their bodies are composed
of an apparently structureless animal
substance of an albuminous nature ("sarcode"), of a gelatinous
consistence, transparent, and exhibiting numerous minute granules
or rounded particles.
Fig. 25.—The animal of Nonionina, one of the
Foraminifera, after the shell has been removed by
a weak acid; b, Gromia, a single-chambered Foraminifer
(after Schultze), showing the shell surrounded by a network of
filaments derived from the body substance.
The body-substance cannot be said in itself
to possess any definite form, except in so far as it may be bounded
by a shell; but it has the power, wherever it may be exposed, of
emitting long thread-like filaments ("pseudopodia"), which interlace
with one another to form a network (fig. 25, b). These
filaments can be thrown out at will, and to considerable distances,
and can be again retracted into the soft mass of the general
body-substance, and they are the agents by which the animal obtains
its food. The soft bodies of the Foraminifera are protected
by a shell, which is usually calcareous, but may be composed of
sand-grains cemented
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together; and it may consist
of a single chamber (fig. 26, a), or of many chambers arranged
in different ways (fig. 26, b-f). Sometimes the shell has but
one large opening into it—the mouth; and then it is from this
aperture that the animal protrudes the delicate net of filaments
with which it seeks its food. In other cases the entire shell
is perforated with minute pores (fig. 26, e), through
which the soft body-substance gains the exterior, covering the
whole shell with a gelatinous film of animal matter, from which
filaments can be emitted at any point. When the shell consists
of many chambers, all of these are placed in direct communication
with one another, and the actual substance of the shell is often
traversed by minute canals filled with living matter (e.g.,
in Calcarina and Nummulina). The shell, therefore,
may be regarded, in such cases, as a more or less completely
porous calcareous structure,
filled to its minutest internal recesses with the substance of the
living animal, and covered externally with a layer of the same
substance, giving off a network of interlacing filaments.
Fig. 26.—Shells of living Foraminifera.
a, Orbulina universa, in its perfect condition, showing
the tubular spines which radiate from the surface of the shell;
b, Globigerina bulloides, in its ordinary condition, the
thin hollow spines which are attached to the shell when perfect
having been broken off; c, Textularia variabilis; d, Peneroplis
planatus; e, Rotalia concamerata; f, Cristellaria subarcuatula.
[Fig. a is after Wyville Thomson; the others are after
Williamson. All the figures are greatly enlarged.
Such, in brief, is the structure of the living Foraminifera;
and it is believed that in Eozoön we have an extinct
example of the same group, not only of special interest from its
immemorial antiquity, but hardly less striking from its gigantic
dimensions. In its original condition, the entire chamber-system
of Eozoön is believed to have been filled with soft
structureless living matter, which passed from chamber to chamber
through the wide apertures connecting these cavities, and from
tier to tier by means of the tubuli in the shell-wall and the
branching canals in the intermediate skeleton. Through the
perforated shell-wall covering the outer surface the soft
body-substance flowed out, forming a gelatinous investment, from
every point of which radiated an interlacing net of delicate
filaments, providing nourishment for the entire colony. In its
present state, as before said, all the cavities originally
occupied by the body-substance have been filled with some mineral
substance, generally with one of the silicates of magnesia; and
it has been asserted that this fact militates strongly against
the organic nature of Eozoön, if not absolutely
disproving it. As a matter of fact, however—as previously
noticed—it is by no means very uncommon at the present day
to find the shells of living species of Foraminifera in
which all the cavities primitively occupied by the body-substance,
down to the minutest pores and canals, have been similarly
injected by some analogous silicate, such as glauconite.
Those, then, whose opinions on such a subject deservedly carry the
greatest weight, are decisively of opinion that we are presented
in the Eozoön of the Laurentian Rocks of Canada with an
ancient, colossal, and in some respects abnormal type of the
Foraminifera. In the words of Dr Carpenter, it is not
pretended that "the doctrine of the Foraminiferal nature of
Eozoön can be proved in the demonstrative sense;"
but it may be affirmed "that the convergence of a number of
separate and independent probabilities, all accordant with
that hypothesis, while a separate explanation must be invented
for each of them on any other hypothesis, gives it that high
probability on which we rest in the ordinary affairs of life,
in the verdicts of juries, and in the interpretation of geological
phenomena generally."
It only remains to be added, that whilst Eozoön is by far
the most important organic body hitherto found in the Laurentian,
and has been here treated at proportionate length, other
traces of life have been detected, which may subsequently prove of
great interest and importance. Thus, Principal Dawson has recently
described under the name of Archœosphœrinœ
certain singular rounded bodies which he has discovered in the
Laurentian limestones, and which he believes to be casts of the
shells of Foraminifera possibly somewhat allied to the
existing Globigerinœ. The same eminent
palæontologist has also described undoubted worm-burrows
from rocks probably of Laurentian age. Further and more extended
researches, we may reasonably hope, will probably bring to light
other actual remains of organisms in these ancient deposits.
THE HURONIAN PERIOD.
The so-called Huronian Rocks, like the Laurentian, have
their typical development in Canada, and derive their name from
the fact that they occupy an extensive area on the borders of
Lake Huron. They are wholly metamorphic, and consist principally
of altered sandstones or quartzites, siliceous, felspathic, or
talcose slates, conglomerates, and limestones. They are largely
developed on the north shore of Lake Superior, and give rise
to a broken and hilly country, very like that occupied by the
Laurentians, with an abundance of timber, but rarely with sufficient
soil of good quality for agricultural purposes. They are, however,
largely intersected by mineral veins, containing silver, gold,
and other metals, and they will ultimately doubtless yield a rich
harvest to the miner. The Huronian Rocks have been identified,
with greater or less certainty, in other parts of North America,
and also in the Old World.
The total thickness of the Huronian Rocks in Canada is estimated
as being not less than 18,000 feet, but there is considerable
doubt as to their precise geological position. In their typical
area they rest unconformably on the edges of strata of Lower
Laurentian age; but they have never been seen in direct contact
with the Upper Laurentian, and their exact relations to
this series are therefore doubtful. It is thus open to question
whether the Huronian Rocks constitute a distinct formation, to
be intercalated in point of time between the Laurentian and the
Cambrian groups; or whether, rather, they should not be considered
as the metamorphosed representatives of the Lower Cambrian Rocks
of other regions.
As regards the fossils of the Huronian Rocks, little can be said.
Some of the specimens of Eozoön Canadense which have
been discovered in Canada are thought to come
from rocks which are probably of Huronian age. In Bavaria,
Dr Gümbel has described a species of Eozoön under
the name of Eozoön Bavaricum, from certain metamorphic
limestones which he refers to the Huronian formation. Lastly, the
late Mr Billings described, from rocks in Newfoundland apparently
referable to the Huronian, certain problematical limpet-shaped
fossils, to which he gave the name of Aspidella.
LITERATURE.
Amongst the works and memoirs which the student may consult with
regard to the Laurentian and Huronian deposits may be mentioned
the following:[10]—
| (1) |
'Report of Progress of the Geological Survey of Canada
from its Commencement to 1863,' pp. 38-49, and pp. 50-66. |
| (2) |
'Manual of Geology.' Dana. 2d Ed. 1875. |
| (3) |
'The Dawn of Life.' J. W, Dawson. 1876. |
| (4) |
"On the Occurrence of Organic Remains in the Laurentian
Rocks of Canada." Sir W. E. Logan. 'Quart. Journ. Geol.'
Soc.,' xxi. 45-50.' |
| (5) |
"On the Structure of Certain Organic Remains in the
Laurentian Limestones of Canada." J. W. Dawson. 'Quart.
Journ. Geol. Soc.,' xxi. 51-59. |
| (6) |
"Additional Note on the Structure and Affinities of
Eozoön Canadense." W. B, Carpenter. 'Quart. Journ.
Geol. Soc.,' xxi. 59-66. |
| (7) |
"Supplemental Notes on the Structure and Affinities of
Eozoön' Canadense," W. B. Carpenter, 'Quart. Journ.
Geol. Soc.,' xxii. 219-228. |
| (8) |
"On the So-Called Eozoönal Rocks." King & Rowney.
'Quart. Journ. Geol. Soc.,' xxii. 185-218. |
| (9) |
'Chemical and Geological Essays.' Sterry Hunt. |
The above list only includes some of the more important memoirs
which may be consulted as to the geological and chemical features
of the Laurentian and Huronian Rocks, and as to the true nature
of Eozoön. Those who are desirous of studying the later
phases of the controversy with regard to Eozoön must consult
the papers of Carpenter, Carter, Dawson, King & Rowney, Hahn,
and others, in the 'Quart. Journ. of the Geological Society,'
the 'Proceedings of the Royal Irish Academy,' the 'Annals of
Natural History,' the 'Geological Magazine,' &c. Dr Carpenter's
'Introduction to the Study of the Foraminifera' should also be
consulted.
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