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).
[Illustration: 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.]
The _Lower Laurentian_ series attains the enormous thickness 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.
[Illustration: 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.)]
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
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.
[Illustration: Fig. 22.--Fragment of _Eozoön_, of the natural
size, showing alternate laminæ of loganite and dolomite. (After
Dawson.)]
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
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:--
[Illustration: 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.)]
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 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, 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.
[Illustration: Fig. 24.--Portion of one of the calcareous layers
of _Eozoön_, magnified 100 diameters. a a, The proper wall
("Nummuline layer") of one of the chambers, showing the fine
vertical tubuli with which it is penetrated, and which are slightly
bent along the line a' a'. c c, The intermediate skeleton,
with numerous branched canals. The oblique lines are the cleavage
planes of the carbonate of lime, extending across both the
intermediate skeleton and the proper wall. (After Carpenter.)]
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. 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 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.
[Illustration: 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.]
[Illustration: 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 _Archoeosphoerinoe_ 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 _Globigerinoe_. 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.
[Footnote 10: In this and in all subsequently following
bibliographical lists, not only is the selection of works and
memoirs quoted necessarily extremely limited; but only such have,
as a general rule, been chosen for mention as are easily accessible
to students who are in the position of being able to refer to a good
library. Exceptions, however, are occasionally made to this rule,
in favour of memoirs or works of special historical interest. It
is also unnecessary to add that it has not been thought requisite
to insert in these lists the well-known handbooks of geological
and palæontological science; except in such instances as where
they contain special information on special points.]