THE LAURENTIAN ROCKS.
As we descend in depth and time into the earth's crust, after passing
through nearly all the vast series of strata constituting the monuments
of geological history, we at length reach the Eozoic or Laurentian
rocks, deepest and oldest of all the formations known to the geologist,
and more thoroughly altered or metamorphosed by heat and heated
moisture than any others. These rocks, at one time known as Azoic,
being supposed destitute of all remains of living things, but now more
properly Eozoic, are those in which the first bright streaks of the
dawn of life make their appearance.[A]
[Footnote A: Dana has recently proposed the term "_Archæan_," on the
ground that some of these rocks are as yet unfossiliferous but as the
oldest known part of them contains fossils, there seems no need for
this new name.]
The name Laurentian, given originally to the Canadian development of
these rocks by Sir William Logan, but now applied to them throughout
the world, is derived from a range of hills lying north of the
St. Lawrence valley, which the old French geographers named the
Laurentides. In these hills the harder rocks of this old formation
rise to considerable heights, and form the highlands separating the
St. Lawrence valley from the great plain fronting on Hudson's Bay
and the Arctic Sea. At first sight it may seem strange that rocks so
ancient should anywhere appear at the surface, especially on the tops
of hills; but this is a necessary result of the mode of formation of
our continents. The most ancient sediments deposited in the sea were
those first elevated into land, and first altered and hardened by heat.
Upheaved in the folding of the earth's crust into high and rugged
ridges, they have either remained uncovered with newer sediments, or
have had such as were deposited on them washed away; and being of a
hard and resisting nature, they have remained comparatively unworn when
rocks much more modern have been swept off by denuding agencies.
But the exposure of the old Laurentian skeleton of mother earth is not
confined to the Laurentide Hills, though these have given the formation
its name. The same ancient rocks appear in the Adirondack mountains
of New York, and in the patches which at lower levels protrude from
beneath the newer formations along the American coast from Newfoundland
to Maryland. The older gneisses of Norway, Sweden, and the Hebrides,
of Bavaria and Bohemia, belong to the same age, and it is not unlikely
that similar rocks in many other parts of the old continent will be
found to be of as great antiquity. In no part of the world, however,
are the Laurentian rocks more extensively distributed or better
known than in North America; and to this as the grandest and most
instructive development of them, and that which first afforded organic
remains, we may more especially devote our attention. Their general
relations to the other formations of America may be learned from the
rough generalised section (fig. 1); in which the crumpled and contorted
Laurentian strata of Canada are seen to underlie unconformably the
comparatively flat Silurian beds, which are themselves among the oldest
monuments of the geological history of the earth.
[Illustration: Fig. 1. _General Section, showing the Relations of
the Laurentian and Palæozoic Rocks in Canada._ (L.) Laurentian. (1.)
Cambrian, or Primordial. (2.) Lower Silurian. (3.) Upper Silurian. (4.)
Devonian and Carboniferous.]
The Laurentian rocks, associated with another series only a little
younger, the Huronian, form a great belt of broken and hilly country,
extending from Labrador across the north of Canada to Lake Superior,
and thence bending northward to the Arctic Sea. Everywhere on the lower
St. Lawrence they appear as ranges of billowy rounded ridges on the
north side of the river; and as viewed from the water or the southern
shore, especially when sunset deepens their tints to blue and violet,
they present a grand and massive appearance, which, in the eye of
the geologist, who knows that they have endured the battles and the
storms of time longer than any other mountains, invests them with a
dignity which their mere elevation would fail to give. (Fig. 2.) In the
isolated mass of the Adirondacks, south of the Canadian frontier, they
rise to a still greater elevation, and form an imposing mountain group,
almost equal in height to their somewhat more modern rivals, the White
Mountains, which face them on the opposite side of Lake Champlain.
The grandeur of the old Laurentian ranges is, however, best displayed
where they have been cut across by the great transverse gorge of the
Saguenay, and where the magnificent precipices, known as Capes Trinity
and Eternity, look down from their elevation of 1500 feet on a fiord,
which at their base is more than 100 fathoms deep (see frontispiece).
The name Eternity applied to such a mass is geologically scarcely a
misnomer, for it dates back to the very dawn of geological time, and is
of hoar antiquity in comparison with such upstart ranges as the Andes
and the Alps.
[Illustration: Fig. 2. _Laurentian Hills opposite Kamouraska, Lower St.
Lawrence._
The islands in front are Primordial.]
On a nearer acquaintance, the Laurentian country appears as a broken
and hilly upland and highland district, clad in its pristine state
with magnificent forests, but affording few attractions to the
agriculturist, except in the valleys, which follow the lines of its
softer beds, while it is a favourite region for the angler, the hunter,
and the lumberman. Many of the Laurentian townships of Canada are,
however, already extensively settled, and the traveller may pass
through a succession of more or less cultivated valleys, bounded by
rocks or wooded hills and crags, and diversified by running streams and
romantic lakes and ponds, constituting a country always picturesque
and often beautiful, and rearing a strong and hardy population. To
the geologist it presents in the main immensely thick beds of gneiss,
and similar metamorphic and crystalline rocks, contorted in the most
remarkable manner, so that if they could be flattened out they would
serve as a skin much too large for mother earth in her present state,
so much has she shrunk and wrinkled since those youthful days when the
Laurentian rocks were her outer covering. (Fig. 3.)
The elaborate sections of Sir William Logan show that these old rocks
are divisible into two series, the Lower and Upper Laurentian; the
latter being the newer of the two, and perhaps separated from the
former by a long interval of time; but this Upper Laurentian being
probably itself older than the Huronian series, and this again older
than all the other stratified rocks. The Lower Laurentian, which
attains to a thickness of more than 20,000 feet, consists of stratified
granitic rocks or gneisses, of indurated sandstone or quartzite, of
mica and hornblende schist, and of crystalline limestones or marbles,
and iron ores, the whole interstratified with each other. The Upper
Laurentian, which is 10,000 feet thick at least, consists in part of
similar rocks, but associated with great beds of triclinic feldspar,
especially of that peculiar variety known as labradorite, or Labrador
feldspar, and which sometimes by its wonderful iridescent play of
colours becomes a beautiful ornamental stone.
I cannot describe such rocks, but their names will tell something to
those who have any knowledge of the older crystalline materials of the
earth's crust. To those who have not, I would advise a visit to some
cliff on the lower St. Lawrence, or the Hebridean coasts, or the shore
of Norway, where the old hard crystalline and gnarled beds present
their sharp edges to the ever raging sea, and show their endless
alternations of various kinds and colours of strata often diversified
with veins and nests of crystalline minerals. He who has seen and
studied such a section of Laurentian rock cannot forget it.
[Illustration: Fig. 3. _Section from Petite Nation Seigniory to St.
Jerome_ (60 miles). _After Sir W. E. Logan._
(_a, b._) Upper Laurentian. (_c._) Fourth gneiss. (_d´._) Third
limestone. (_d._) Third gneiss. (_e´._) Second limestone. (_x._)
Porphyry. (_y._) Granite.]
All the constituents of the Laurentian series are in that state known
to geologists as metamorphic. They were once sandstones, clays, and
limestones, such as the sea now deposits, or such as form the common
plebeian rocks of everyday plains and hills and coast sections. Being
extremely old, however, they have been buried deep in the bowels of
the earth under the newer deposits, and hardened by the action of
pressure and of heat and heated water. Whether this heat was part
of that originally belonging to the earth when a molten mass, and
still existing in its interior after aqueous rocks had begun to form
on its surface, or whether it is a mere mechanical effect of the
intense compression which these rocks have suffered, may be a disputed
question; but the observations of Sorby and of Hunt (the former in
connection with the microscopic structure of rocks, and the latter
in connection with the chemical conditions of change) show that no
very excessive amount of heat would be required. These observations
and those of Daubrée indicate that crystallization like that of the
Laurentian rocks might take place at a temperature of not over 370° of
the centigrade thermometer.
The study of those partial alterations which take place in the
vicinity of volcanic and older aqueous masses of rock confirms these
conclusions, so that we may be said to know the precise conditions
under which sediments may be hardened into crystalline rocks, while
the bedded character and the alternations of different layers in the
Laurentian rocks, as well as the indications of contemporary marine
life which they contain, show that they actually are such altered
sediments. (See Note D.)
It is interesting to notice here that the Laurentian rocks thus
interpreted show that the oldest known portions of our continents were
formed in the waters. They are oceanic sediments deposited perhaps when
there was no dry land or very little, and that little unknown to us
except in so far as its debris may have entered into the composition
of the Laurentian rocks themselves. Thus the earliest condition of the
earth known to the geologist is one in which old ocean was already
dominant on its surface; and any previous condition when the surface
was heated, and the water constituted an abyss of vapours enveloping
its surface, or any still earlier condition in which the earth was
gaseous or vaporous, is a matter of mere inference, not of actual
observation. The formless and void chaos is a deduction of chemical and
physical principles, not a fact observed by the geologist. Still we
know, from the great dykes and masses of igneous or molten rock which
traverse the Laurentian beds, that even at that early period there were
deep-seated fires beneath the crust; and it is quite possible that
volcanic agencies then manifested themselves, not only with quite as
great intensity, but also in the same manner, as at subsequent times.
It is thus not unlikely that much of the land undergoing waste in the
earlier Laurentian time was of the same nature with recent volcanic
ejections, and that it formed groups of islands in an otherwise
boundless ocean.
However this may be, the distribution and extent of these
pre-Laurentian lands is, and probably ever must be, unknown to us; for
it was only after the Laurentian rocks had been deposited, and after
the shrinkage of the earth's crust in subsequent times had bent and
contorted them, that the foundations of the continents were laid. The
rude sketch map of America given in fig. 4 will show this, and will
also show that the old Laurentian mountains mark out the future form of
the American continent.
[Illustration: Fig. 4. _The Laurentian Nucleus of the American
Continent._]
Rocks so highly altered as the Laurentian beds can scarcely be expected
to hold well characterized fossil remains, and those geologists who
entertained any hope that such remains might have been preserved, long
looked in vain for their actual discovery. Still, as astronomers have
suspected the existence of unknown planets from observing perturbations
not accounted for, and as voyagers have suspected the approach to
unknown regions by the appearance of floating wood or stray land birds,
anticipations of such discoveries have been entertained and expressed
from time to time. Lyell, Dana, and Sterry Hunt more especially, have
committed themselves to such speculations. The reasons assigned may be
stated thus:--
Assuming the Laurentian rocks to be altered sediments, they must, from
their great extent, have been deposited in the ocean; and if there had
been no living creatures in the waters, we have no reason to believe
that they would have consisted of anything more than such sandy and
muddy debris as may be washed away from wasting rocks originally of
igneous origin. But the Laurentian beds contain other materials than
these. No formations of any geological age include thicker or more
extensive limestones. One of the beds measured by the officers of the
Geological Survey, is stated to be 1500 feet in thickness, another is
1250 feet thick, and a third 750 feet; making an aggregate of 3500
feet.[B] These beds may be traced, with more or less interruption,
for hundreds of miles. Whatever the origin of such limestones, it
is plain that they indicate causes equal in extent, and comparable
in power and duration, with those which have produced the greatest
limestones of the later geological periods. Now, in later formations,
limestone is usually an organic rock, accumulated by the slow gathering
from the sea-water, or its plants, of calcareous matter, by corals,
foraminifera, or shell-fish, and the deposition of their skeletons,
either entire or in fragments, in the sea-bottom. The most friable
chalk and the most crystalline limestones have alike been formed in
this way. We know of no reason why it should be different in the
Laurentian period. When, therefore, we find great and conformable
beds of limestone, such as those described by Sir William Logan in
the Laurentian of Canada, we naturally imagine a quiet sea-bottom, in
which multitudes of animals of humble organization were accumulating
limestone in their hard parts, and depositing this in gradually
increasing thickness from age to age. Any attempts to account otherwise
for these thick and greatly extended beds, regularly interstratified
with other deposits, have so far been failures, and have arisen either
from a want of comprehension of the nature and magnitude of the
appearances to be explained, or from the error of mistaking the true
bedded limestones for veins of calcareous spar.
[Footnote B: Logan: _Geology of Canada_, p. 45.]
The Laurentian rocks contain great quantities of carbon, in the form of
graphite or plumbago. This does not occur wholly, or even principally,
in veins or fissures, but in the substance of the limestone and gneiss,
and in regular layers. So abundant is it, that I have estimated the
amount of carbon in one division of the Lower Laurentian of the Ottawa
district at an aggregate thickness of not less than twenty to thirty
feet, an amount comparable with that in the true coal formation itself.
Now we know of no agency existing in present or in past geological
time capable of deoxidizing carbonic acid, and fixing its carbon as
an ingredient in permanent rocks, except vegetable life. Unless,
therefore, we suppose that there existed in the Laurentian age a vast
abundance of vegetation, either in the sea or on the land, we have no
means of explaining the Laurentian graphite.
The Laurentian formation contains great beds of oxide of iron,
sometimes seventy feet in thickness. Here again we have an evidence of
organic action; for it is the deoxidizing power of vegetable matter
which has in all the later formations been the efficient cause in
producing bedded deposits of iron. This is the case in modern bog and
lake ores, in the clay iron-stones of the coal measures, and apparently
also in the great ore beds of the Silurian rocks. May not similar
causes have been at work in the Laurentian period?
Any one 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 and living form or structure
had ever been recognised in these ancient rocks.
Such was the condition of the matter until the existence of supposed
organic remains was announced by Sir W. Logan, at the American
Association for the Advancement of Science, in Springfield, in 1859;
and we may now proceed to narrate the manner of this discovery, and how
it has been followed up.
Before doing so, however, let us visit Eozoon in one of its haunts
among the Laurentian Hills. One of the most noted repositories of its
remains is the great Grenville band of limestone (see section, fig. 3,
and map), the outcrop of which may be seen in our map of the country
near the Ottawa, twisting itself like a great serpent in the midst of
the gneissose rocks; and one of the most fruitful localities is at a
place called Côte St. Pierre on this band. Landing, as I did, with
Mr. Weston, of the Geological Survey, last autumn, at Papineauville,
we find ourselves on the Laurentian rocks, and pass over one of the
great bands of gneiss for about twelve miles, to the village of St.
André Avelin. On the road we see on either hand abrupt rocky ridges,
partially clad with forest, and sometimes showing on their flanks the
stratification of the gneiss in very distinct parallel bands, often
contorted, as if the rocks, when soft, had been wrung as a washer-woman
wrings clothes. Between the hills are little irregular valleys, from
which the wheat and oats have just been reaped, and the tall Indian
corn and yellow pumpkins are still standing in the fields. Where not
cultivated, the land is covered with a rich second growth of young
maples, birches, and oaks, among which still stand the stumps and tall
scathed trunks of enormous pines, which constituted the original
forest. Half way we cross the Nation River, a stream nearly as large as
the Tweed, flowing placidly between wooded banks, which are mirrored in
its surface; but in the distance we can hear the roar of its rapids,
dreaded by lumberers in their spring drivings of logs, and which we
were told swallowed up five poor fellows only a few months ago. Arrived
at St. André, we find a wider valley, the indication of the change to
the limestone band, and along this, with the gneiss hills still in view
on either hand, and often encroaching on the road, we drive for five
miles more to Côte St. Pierre. At this place the lowest depression of
the valley is occupied by a little pond, and, hard by, the limestone,
protected by a ridge of gneiss, rises in an abrupt wooded bank by
the roadside, and a little further forms a bare white promontory,
projecting into the fields. Here was Mr. Love's original excavation,
whence some of the greater blocks containing Eozoon were taken, and a
larger opening made by an enterprising American on a vein of fibrous
serpentine, yielding "rock cotton," for packing steam pistons and
similar purposes. (Figs. 5 and 6.)
[Illustration: Fig. 5. _Attitude of Limestone at St. Pierre._
(_a._) Gneiss band in the Limestone. (_b._) Limestone with Eozoon.
(_c._) Diorite and Gneiss.]
[Illustration: Fig. 6. _Gneiss and Limestone at St. Pierre._
(_a._) Limestone. (_b._) Gneiss and Diorite.]
The limestone is here highly inclined and much contorted, and in all
the excavations a thickness of about 100 feet of it may be exposed.
It is white and crystalline, varying much however in coarseness in
different bands. It is in some layers pure and white, in others it
is traversed by many gray layers of gneissose and other matter, or
by irregular bands and nodules of pyroxene and serpentine, and it
contains subordinate beds of dolomite. In one layer only, and this
but a few feet thick, does the Eozoon occur in any abundance in a
perfect state, though fragments and imperfectly preserved specimens
abound in other parts of the bed. It is a great mistake to suppose
that it constitutes whole beds of rock in an uninterrupted mass.
Its true mode of occurrence is best seen on the weathered surfaces
of the rock, where the serpentinous specimens project in irregular
patches of various sizes, sometimes twisted by the contortion of the
beds, but often too small to suffer in this way. On such surfaces
the projecting patches of the fossil exhibit laminæ of serpentine so
precisely like the _Stromatoporæ_ of the Silurian rocks, that any
collector would pounce upon them at once as fossils. In some places
these small weathered specimens can be easily chipped off from the
crumbling surface of the limestone; and it is perhaps to be regretted
that they have not been more extensively shown to palæontologists, with
the cut slices which to many of them are so problematical. One of the
original specimens, brought from the Calumet, and now in the Museum
of the Geological Survey of Canada, was of this kind, and much finer
specimens from Côte St. Pierre are now in that collection and in my
own. A very fine example is represented, on a reduced scale, in Plate
III., which is taken from an original photograph.[C] In some of the
layers are found other and more minute fossils than Eozoon, and these,
together with its fragmental remains, as ingredients in the limestone,
will be discussed in the sequel. We may merely notice here that the
most abundant layer of Eozoon at this place, occurs near the base of
the great limestone band, and that the upper layers in so far as seen
are less rich in it. Further, there is no necessary connection between
Eozoon and the occurrence of serpentine, for there are many layers
full of bands and lenticular masses of that mineral without any Eozoon
except occasional fragments, while the fossil is sometimes partially
mineralized with pyroxene, dolomite, or common limestone. The section
in fig. 5 will serve to show the attitude of the limestone at this
place, while the more general section, fig. 3, taken from Sir William
Logan, shows its relation to the other Laurentian rocks, and the sketch
in fig. 6 shows its appearance as a feature on the surface of the
country.
[Footnote C: By Mr. Weston, of the Geological Survey of Canada.]
NOTES TO CHAPTER II.
(A.) Sir William E. Logan on the Laurentian System.
[_Journal of Geological Society of London_, February, 1865.]
After stating the division of the Laurentian series into the two
great groups of the Upper and Lower Laurentian, Sir William goes on
to say:--
"The united thickness of these two groups in Canada cannot be less
than 30,000 feet, and probably much exceeds it. The Laurentian of
the west of Scotland, according to Sir Roderick Murchison, also
attains a great thickness. In that region the Upper Laurentian or
Labrador series, has not yet been separately recognised; but from
Mr. McCulloch's description, as well as from the specimens collected
by him, and now in the Museum of the Geological Society of London,
it can scarcely be doubted that the Labrador series occurs in Skye.
The labradorite and hypersthene rocks from that island are identical
with those of the Labrador series in Canada and New York, and unlike
those of any formation at any other known horizon. This resemblance
did not escape the notice of Emmons, who, in his description of the
Adirondack Mountains, referred these rocks to the hypersthene rock
of McCulloch, although these observers, on the opposite sides of
the Atlantic, looked upon them as unstratified. In the _Canadian
Naturalist_ for 1862, Mr. Thomas Macfarlane, for some time resident
in Norway, and now in Canada, drew attention to the striking
resemblance between the Norwegian primitive gneiss formation, as
described by Naumann and Keilhau, and observed by himself, and the
Laurentian, including the Labrador group; and the equally remarkable
similarity of the lower part of the primitive slate formation
to the Huronian series, which is a third Canadian group. These
primitive series attain a great thickness in the north of Europe, and
constitute the main features of Scandinavian geology.
"In Bavaria and Bohemia there is an ancient gneissic series. After
the labours in Scotland, by which he was the first to establish a
Laurentian equivalent in the British Isles, Sir Roderick Murchison,
turning his attention to this central European mass, placed it on the
same horizon. These rocks, underlying Barrande's Primordial zone,
with a great development of intervening clay-slate, extend southward
in breadth to the banks of the Danube, with a prevailing dip towards
the Silurian strata. They had previously been studied by Gümbel and
Crejci, who divided them into an older reddish gneiss and a newer
grey gneiss. But, on the Danube, the mass which is furthest removed
from the Silurian rocks being a grey gneiss, Gümbel and Crejci
account for its presence by an inverted fold in the strata; while
Sir Roderick places this at the base, and regards the whole as a
single series, in the normal fundamental position of the Laurentian
of Scotland and of Canada. Considering the colossal thickness given
to the series (90,000 feet), it remains to be seen whether it may
not include both the Lower and Upper Laurentian, and possibly, in
addition, the Huronian.
"This third Canadian group (the Huronian) has been shown by my
colleague, Mr. Murray, to be about 18,000 feet thick, and to consist
chiefly of quartzites, slate-conglomerates, diorites, and limestones.
The horizontal strata which form the base of the Lower Silurian in
western Canada, rest upon the upturned edges of the Huronian series;
which, in its turn, unconformably overlies the Lower Laurentian. The
Huronian is believed to be more recent than the Upper Laurentian
series, although the two formations have never yet been seen in
contact.
"The united thickness of these three great series may possibly
far surpass that of all the succeeding rocks from the base of the
Palæozoic series to the present time. We are thus carried back to a
period so far remote, that the appearance of the so-called Primordial
fauna may by some be considered a comparatively modern event. We,
however, find that, even during the Laurentian period, the same
chemical and mechanical processes which have ever since been at
work disintegrating and reconstructing the earth's crust were in
operation as now. In the conglomerates of the Huronian series there
are enclosed boulders derived from the Laurentian, which seem to show
that the parent rock was altered to its present crystalline condition
before the deposit of the newer formation; while interstratified with
the Laurentian limestones there are beds of conglomerate, the pebbles
of which are themselves rolled fragments of a still older laminated
sand-rock, and the formation of these beds leads us still further
into the past.
"In both the Upper and Lower Laurentian series there are several
zones of limestone, each of sufficient volume to constitute an
independent formation. Of these calcareous masses it has been
ascertained that three, at least, belong to the Lower Laurentian. But
as we do not as yet know with certainty either the base or the summit
of this series, these three may be conformably followed by many more.
Although the Lower and Upper Laurentian rocks spread over more than
200,000 square miles in Canada, only about 1500 square miles have yet
been fully and connectedly examined in any one district, and it is
still impossible to say whether the numerous exposures of Laurentian
limestone met with in other parts of the province are equivalent to
any of the three zones, or whether they overlie or underlie them all."
(B.) Dr. Sterry Hunt on the Probable Existence of Life in the
Laurentian Period.
Dr. Hunt's views on this subject were expressed in the _American
Journal of Science_, [2], vol. xxxi., p. 395. From this article,
written in 1861, after the announcement of the existence of laminated
forms supposed to be organic in the Laurentian, by Sir W. E. Logan,
but before their structure and affinities had been ascertained, I
quote the following sentences:--
"We see in the Laurentian series beds and veins of metallic
sulphurets, precisely as in more recent formations; and the extensive
beds of iron ore, hundreds of feet thick, which abound in that
ancient system, correspond not only to great volumes of strata
deprived of that metal, but, as we may suppose, to organic matters
which, but for the then great diffusion of iron-oxyd in conditions
favourable for their oxidation, might have formed deposits of mineral
carbon far more extensive than those beds of plumbago which we
actually meet in the Laurentian strata. All these conditions lead us
then to conclude the existence of an abundant vegetation during the
Laurentian period."
(C.) The Graphite of the Laurentian.
The following is from a paper by the author, in the _Journal of the
Geological Society_, for February, 1870:--
"The graphite of the Laurentian of Canada occurs both in beds and in
veins, and in such a manner as to show that its origin and deposition
are contemporaneous with those of the containing rock. Sir William
Logan states[D] that 'the deposits of plumbago generally occur in the
limestones or in their immediate vicinity, and granular varieties
of the rock often contain large crystalline plates of plumbago. At
other times this mineral is so finely disseminated as to give a
bluish-gray colour to the limestone, and the distribution of bands
thus coloured, seems to mark the stratification of the rock.' He
further states:--'The plumbago is not confined to the limestones;
large crystalline scales of it are occasionally disseminated in
pyroxene rock or pyrallolite, and sometimes in quartzite and in
feldspathic rocks, or even in magnetic oxide of iron.' In addition
to these bedded forms, there are also true veins in which graphite
occurs associated with calcite, quartz, orthoclase, or pyroxene,
and either in disseminated scales, in detached masses, or in
bands or layers 'separated from each other and from the wall rock
by feldspar, pyroxene, and quartz.' Dr. Hunt also mentions the
occurrence of finely granular varieties, and of that peculiarly
waved and corrugated variety simulating fossil wood, though really a
mere form of laminated structure, which also occurs at Warrensburgh,
New York, and at the Marinski mine in Siberia. Many of the veins
are not true fissures, but rather constitute a network of shrinkage
cracks or segregation veins traversing in countless numbers the
containing rock, and most irregular in their dimensions, so that
they often resemble strings of nodular masses. It has been supposed
that the graphite of the veins was originally introduced as a liquid
hydrocarbon. Dr. Hunt, however, regards it as possible that it
may have been in a state of aqueous solution;[E] but in whatever
way introduced, the character of the veins indicates that in the
case of the greater number of them the carbonaceous material must
have been derived from the bedded rocks traversed by these veins,
while there can be no doubt that the graphite found in the beds has
been deposited along with the calcareous matter or muddy and sandy
sediment of which these beds were originally composed.
[Footnote D: _Geology of Canada_, 1863.]
[Footnote E: _Report of the Geological Survey of Canada_, 1866.]
"The quantity of graphite in the Lower Laurentian series is enormous.
In a recent visit to the township of Buckingham, on the Ottawa
River, I examined a band of limestone believed to be a continuation
of that described by Sir W. E. Logan as the Green Lake Limestone.
It was estimated to amount, with some thin interstratified bands
of gneiss, to a thickness of 600 feet or more, and was found to
be filled with disseminated crystals of graphite and veins of the
mineral to such an extent as to constitute in some places one-fourth
of the whole; and making every allowance for the poorer portions,
this band cannot contain in all a less vertical thickness of pure
graphite than from twenty to thirty feet. In the adjoining township
of Lochaber Sir W. E. Logan notices a band from twenty-five to thirty
feet thick, reticulated with graphite veins to such an extent as
to be mined with profit for the mineral. At another place in the
same district a bed of graphite from ten to twelve feet thick, and
yielding twenty per cent. of the pure material, is worked. When it
is considered that graphite occurs in similar abundance at several
other horizons, in beds of limestone which have been ascertained by
Sir W. E. Logan to have an aggregate thickness of 3500 feet, it is
scarcely an exaggeration to maintain that the quantity of carbon in
the Laurentian is equal to that in similar areas of the Carboniferous
system. It is also to be observed that an immense area in Canada
appears to be occupied by these graphitic and Eozoon limestones, and
that rich graphitic deposits exist in the continuation of this system
in the State of New York, while in rocks believed to be of this age
near St. John, New Brunswick, there is a very thick bed of graphitic
limestone, and associated with it three regular beds of graphite,
having an aggregate thickness of about five feet.[F]
[Footnote F: Matthew, in _Quart. Journ. Geol. Soc._, vol. xxi., p. 423.
_Acadian Geology_, p. 662.]
"It may fairly be assumed that in the present world and in those
geological periods with whose organic remains we are more familiar
than with those of the Laurentian, there is no other source of
unoxidized carbon in rocks than that furnished by organic matter,
and that this has obtained its carbon in all cases, in the first
instance, from the deoxidation of carbonic acid by living plants. No
other source of carbon can, I believe, be imagined in the Laurentian
period. We may, however, suppose either that the graphitic matter
of the Laurentian has been accumulated in beds like those of coal,
or that it has consisted of diffused bituminous matter similar to
that in more modern bituminous shales and bituminous and oil-bearing
limestones. The beds of graphite near St. John, some of those in the
gneiss at Ticonderoga in New York, and at Lochaber and Buckingham
and elsewhere in Canada, are so pure and regular that one might
fairly compare them with the graphitic coal of Rhode Island. These
instances, however, are exceptional, and the greater part of the
disseminated and vein graphite might rather be compared in its mode
of occurrence to the bituminous matter in bituminous shales and
limestones.
"We may compare the disseminated graphite to that which we find in
those districts of Canada in which Silurian and Devonian bituminous
shales and limestones have been metamorphosed and converted into
graphitic rocks not dissimilar to those in the less altered portions
of the Laurentian.[G] In like manner it seems probable that the
numerous reticulating veins of graphite may have been formed by
the segregation of bituminous matter into fissures and planes of
least resistance, in the manner in which such veins occur in modern
bituminous limestones and shales. Such bituminous veins occur in
the Lower Carboniferous limestone and shale of Dorchester and
Hillsborough, New Brunswick, with an arrangement very similar to that
of the veins of graphite; and in the Quebec rocks of Point Levi,
veins attaining to a thickness of more than a foot, are filled with a
coaly matter having a transverse columnar structure, and regarded by
Logan and Hunt as an altered bitumen. These palæozoic analogies would
lead us to infer that the larger part of the Laurentian graphite
falls under the second class of deposits above mentioned, and that,
if of vegetable origin, the organic matter must have been thoroughly
disintegrated and bituminized before it was changed into graphite.
This would also give a probability that the vegetation implied was
aquatic, or at least that it was accumulated under water.
[Footnote G: Granby, Melbourne, Owl's Head, etc., _Geology of Canada_,
1863, p. 599.]
"Dr. Hunt has, however, observed an indication of terrestrial
vegetation, or at least of subaërial decay, in the great beds of
Laurentian iron ore. These, if formed in the same manner as more
modern deposits of this kind, would imply the reducing and solvent
action of substances produced in the decay of plants. In this case
such great ore beds as that of Hull, on the Ottawa, seventy feet
thick, or that near Newborough, 200 feet thick,[H] must represent
a corresponding quantity of vegetable matter which has totally
disappeared. It may be added that similar demands on vegetable matter
as a deoxidizing agent are made by the beds and veins of metallic
sulphides of the Laurentian, though some of the latter are no doubt
of later date than the Laurentian rocks themselves.
[Footnote H: _Geology of Canada_, 1863.]
"It would be very desirable to confirm such conclusions as those
above deduced by the evidence of actual microscopic structure. It is
to be observed, however, that when, in more modern sediments, algæ
have been converted into bituminous matter, we cannot ordinarily
obtain any structural evidence of the origin of such bitumen, and in
the graphitic slates and limestones derived from the metamorphosis of
such rocks no organic structure remains. It is true that, in certain
bituminous shales and limestones of the Silurian system, shreds of
organic tissue can sometimes be detected, and in some cases, as in
the Lower Silurian limestone of the La Cloche mountains in Canada,
the pores of brachiopodous shells and the cells of corals have been
penetrated by black bituminous matter, forming what may be regarded
as natural injections, sometimes of much beauty. In correspondence
with this, while in some Laurentian graphitic rocks, as, for
instance, in the compact graphite of Clarendon, the carbon presents
a curdled appearance due to segregation, and precisely similar to
that of the bitumen in more modern bituminous rocks, I can detect in
the graphitic limestones occasional fibrous structures which may be
remains of plants, and in some specimens vermicular lines, which I
believe to be tubes of Eozoon penetrated by matter once bituminous,
but now in the state of graphite.
"When palæozoic land-plants have been converted into graphite,
they sometimes perfectly retain their structure. Mineral charcoal,
with structure, exists in the graphitic coal of Rhode Island. The
fronds of ferns, with their minutest veins perfect, are preserved
in the Devonian shales of St. John, in the state of graphite; and
in the same formation there are trunks of Conifers (_Dadoxylon
ouangondianum_) in which the material of the cell-walls has been
converted into graphite, while their cavities have been filled with
calcareous spar and quartz, the finest structures being preserved
quite as well as in comparatively unaltered specimens from the
coal-formation.[I] No structures so perfect have as yet been detected
in the Laurentian, though in the largest of the three graphitic beds
at St. John there appear to be fibrous structures which I believe may
indicate the existence of land-plants. This graphite is composed of
contorted and slickensided laminæ, much like those of some bituminous
shales and coarse coals; and in these there are occasional small
pyritous masses which show hollow carbonaceous fibres, in some cases
presenting obscure indications of lateral pores. I regard these
indications, however, as uncertain; and it is not as yet fully
ascertained that these beds at St. John are on the same geological
horizon with the Lower Laurentian of Canada, though they certainly
underlie the Primordial series of the Acadian group, and are
separated from it by beds having the character of the Huronian.
[Footnote I: _Acadian Geology_, p. 535. In calcified specimens the
structures remain in the graphite after decalcification by an acid.]
"There is thus no absolute impossibility that distinct organic
tissues may be found in the Laurentian graphite, if formed from
land-plants, more especially if any plants existed at that time
having true woody or vascular tissues; but it cannot with certainty
be affirmed that such tissues have been found. It is possible,
however, that in the Laurentian period the vegetation of the land may
have consisted wholly of cellular plants, as, for example, mosses and
lichens; and if so, there would be comparatively little hope of the
distinct preservation of their forms or tissues, or of our being able
to distinguish the remains of land-plants from those of Algæ.
"We may sum up these facts and considerations in the following
statements:--First, that somewhat obscure traces of organic structure
can be detected in the Laurentian graphite; secondly, that the
general arrangement and microscopic structure of the substance
corresponds with that of the carbonaceous and bituminous matters
in marine formations of more modern date; thirdly, that if the
Laurentian graphite has been derived from vegetable matter, it has
only undergone a metamorphosis similar in kind to that which organic
matter in metamorphosed sediment of later age has experienced;
fourthly, that the association of the graphitic matter with organic
limestone, beds of iron ore, and metallic sulphides, greatly
strengthens the probability of its vegetable origin; fifthly, that
when we consider the immense thickness and extent of the Eozoonal
and graphitic limestones and iron ore deposits of the Laurentian, if
we admit the organic origin of the limestone and graphite, we must
be prepared to believe that the life of that early period, though it
may have existed under low forms, was most copiously developed, and
that it equalled, perhaps surpassed, in its results, in the way of
geological accumulation, that of any subsequent period."
(D.) Western and other Laurentian Rocks, etc.
In the map of the Laurentian nucleus of America (fig. 4,) I have
not inserted the Laurentian rocks believed to exist in the Rocky
Mountains and other western ranges. Their distribution is at present
uncertain, as well as the date of their elevation. They may indicate
an old line of Laurentian fracture or wrinkling, parallel to the west
coast, and defining its direction. In the map there should be a patch
of Laurentian in the north of Newfoundland, and it should be wider at
the west end of lake Superior.
Full details as to the Laurentian rocks of Canada and sectional
lists of their beds will be found in the _Reports of the Geological
Survey_, and Dr. Hunt has discussed very fully their chemical
characters and metamorphism in his _Chemical and Geological Essays_.
The recent reports of Hitchcock on New Hampshire, and Hayden on
the Western Territories, contain some new facts of interest.
The former recognises in the White Mountain region a series of
gneisses and other altered rocks of Lower Laurentian age, and,
resting unconformably on these, others corresponding to the Upper
Laurentian; while above the latter are other pre-silurian formations
corresponding to the Huronian and probably to the Montalban series of
Hunt. These facts confirm Logan's results in Canada; and Hitchcock
finds many reasons to believe in the existence of life at the time of
the deposition of these old rocks. Hayden's report describes granitic
and gneissose rocks, probably of Laurentian age, as appearing over
great areas in Colorado, Arizona, Utah, and Nevada--showing the
existence of this old metamorphic floor over vast regions of Western
America.
The metamorphism of these rocks does not imply any change of
their constituent elements, or interference with their bedded
arrangement. It consists in the alteration of the sediments by merely
molecular changes re-arranging their particles so as to render them
crystalline, or by chemical reactions producing new combinations of
their elements. Experiment shows that the action of heat, pressure,
and waters containing alkaline carbonates and silicates, would
produce such changes. The amount and character of change would depend
on the composition of the sediment, the heat applied, the substances
in solution in the water, and the lapse of time. (See _Hunt's
Essays_, p. 24.)
[Illustration:
Plate III.
From a Photo by Weston. Vincent Brooks, Day & Son, Lith.
WEATHERED SPECIMEN OF EOZOON CANADENSE.
(ONE-HALF NATURAL SIZE.)
_To face Chap. 3_]