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.[19] 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 slicken-sided
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 Grenville series of Canada,
though they certainly underlie the Cambrian series of the St. John or
Acadian group, and are separated from it by beds having the character
of the Huronian, and thus come, approximately at least, into the same
geological position.
[Footnote 19: "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. The only apparent plant
of the Laurentian to which a name has been given, _Archæophyton_ of
Britton, from New Jersey, consists of ribbon-like strips, destitute
of apparent structure, and which, if they are of vegetable origin,
may have belonged to either of the leading divisions of the vegetable
kingdom. I have found similar flat frond-like objects in the limestone
of the Grenville series, at Lachute, in Canada.
"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."
[Illustration: Figs. 20 _and_ 21.--_Bent and dislocated Quartzite,
in contorted schists interstratified with Grenville Limestone, near
Montebello._
The Quartzites have been broken and displaced, while the schists have
been bent and twisted. In the immediate vicinity the same beds may be
seen slightly inclined and undisturbed.]
Let us take, in connection with all this, the fact that we are dealing
with the deposits of the earliest ocean known to us--an ocean warm and
abounding in the mineral matters suitable for the skeletons of humble
animals, and fitted to nourish aquatic plants. The conditions were
certainly favourable to an exuberant development of the lower forms
of marine life; and in later times, when such conditions prevail, we
generally find that life has been introduced to take advantage of them.
The prudent farmer does not usually allow his best pasture to remain
untenanted with flocks and herds, and the Great Husbandman of nature
has, so far as we know, been similarly careful.
I add two sections showing the local disturbances of beds of quartzite
and schist associated with the Grenville limestones (Figs. 20 and 21,
page 103).
_PROBABILITIES AS TO LAURENTIAN LIFE, AND
CONDITIONS OF ITS PRESERVATION_
V
_PROBABILITIES AS TO LAURENTIAN LIFE, AND
CONDITIONS OF ITS PRESERVATION_
We have seen that the mineral constitution of the Upper Laurentian
affords evidence that in this age there were already land and water,
and that the processes by which the land is being worn down, and its
materials deposited on the sea-bottom, were in full operation; while
the absence of any evidence of violent wave-action, and the presence of
thick deposits of limestone, coaly matter, iron ore, and fine-grained
beds of sediment, indicates a time of rest and quiescence. All these
conditions were favourable to the presence of life, and we should
expect to find in such a period some sign of its commencement.
But here we are met by a formidable difficulty. If the beds of the
Grenville series were originally deposits in a quiet sea, they are, as
now existing in the old Laurentian hills and valleys, very much changed
from their original condition. They have, in short, experienced the
changes known to geologists by the formidable word metamorphism,
whereby they have lost the more obvious characters of ordinary aqueous
deposits, and have assumed new and strange forms. Dr. Adams, of
Montreal, has taken the pains to collect a number of chemical analyses
of the gneisses and schists or crystalline slates of the Grenville
series, and finds that, however unlike to more modern shales and clays,
they have substantially the same chemical composition. Now if they were
originally such shales and clays, it has happened to them that the
ingredients of the clays have rearranged themselves in new forms and
become crystalline. We are familiar in a small way with such changes
when brick clay, over-heated in the kiln, becomes fused into slag
or vitrified; and if such slag were allowed to cool very slowly, it
would present different kinds of crystalline minerals. We actually see
changes of this kind in the substance of bricks which have been long
exposed to intense heat in the walls of furnaces. Now in the crust of
the earth, very old rocks, buried under newer deposits, and exposed
to the heat of the interior molten rocks, experience such changes on
a great scale; and there is one kind of influence present in the
bowels of the earth which we in our experiments cannot easily imitate
or understand, namely, the action of superheated water prevented by
pressure from escaping as steam, and permeating the whole substance of
deposits, which are thus baked at a high temperature in presence of
water, instead of being exposed to mere dry heat, as in our kilns and
furnaces. The study of the partial changes which have passed on later
sediments where in contact with volcanic masses once intensely heated,
enables us to understand the greater and more extensive metamorphism
of the oldest rocks. Thus a mere mud becomes glorified by metamorphic
crystallization into a micaceous schist. We have taken ordinary clay
as an example; but under the same processes sand has been converted
into a compact quartzite, ordinary limestone into crystalline marble,
clay-ironstone into magnetic iron ore, coal into graphite, and lavas or
volcanic ashes into hard crystalline granites, gneisses, or pyroxene
rocks or hornblendic schists, according to their original composition.
There may exist portions of these old rocks which have been exempt
from such alteration, but hitherto we have not been able to find them,
and they are probably under the ocean bed, or deeply burled beneath
later rocks, while the parts exposed are precisely those which have
by their crumpling and pressure, and the influence of internal heat,
become most hardened and altered, and have therefore best resisted
denudation. We need not therefore be astonished if any organic remains
originally present in such rocks should have perished, or should have
been subjected to such changes of composition and form as to have
altogether lost their original characters. The searcher for fossils
in such rocks has to expect that these can have been preserved only
under very rare and exceptional circumstances. We have now to consider
what these circumstances are, and for simplicity may suppose that we
are endeavouring to discover in a crystalline limestone the remains
of animals having a skeleton of limestone, as is the case with most
shell-fishes and corals, and with many Protozoa and marine worms. In
regard to these, we have to consider what may happen to them when they
are imbedded in calcareous marl or ooze, or the limestone which results
from the hardening of such materials; and we have to bear in mind that
such organisms usually consist of hard, stony walls or partitions,
enclosing cavities originally filled with the soft parts of the animal
which may be supposed to have disappeared by decay before or during the
mineralization of its skeleton.
So long as the imbedding mass continues soft and incoherent, shells,
corals, etc., can be recovered in a condition similar to that of recent
specimens, except that they may have become bleached in colour and
brittle in texture, owing to the removal of organic matter intimately
associated with the lime, and that their cavities may have been filled
with sand or silt washed into them, or with calcite or calcareous spar
introduced in solution in water. But if the containing mass has become
a hard stone, the material filling the interior of our shell or coral
has experienced a similar change; and when we break open the stone,
we may obtain the specimen, now hard, solid, and heavy, but still
showing more or less of its outer surface and markings, and possibly to
some extent also its internal structure when it is sliced and studied
under the microscope. But if the whole mass has been metamorphosed,
and has become crystalline, the contained fossil and its contents may
have experienced a similar change, and may have so coalesced with the
containing matrix that it is no longer separable from it. Even in this
case, however, if the whole is reduced to a thin transparent slice
and examined microscopically, some traces may be found of the external
and internal limiting lines of the fossil, and even of its minute
structures, which often cause it to present an appearance granular,
cellular, or otherwise different from that of the enclosing matrix. It
requires, however, both skill and care to detect organic remains in
such circumstances, and they may often escape observation, except when,
as in many old crystalline limestones, the fossils are darkened in
whole or in part with coaly matter derived from the decay of their own
organic substance. The crystalline Trenton limestone of Montreal, used
there as a building stone, is an excellent example (Fig. 22).
[Illustration: Fig. 22.--_Section of "Trenton Limestone" (magnified)._
Showing its composition of fragments of calcareous fossils.]
[Illustration: Fig. 23.--_Diagram of different States of Fossilization
of the Cell of a Tubulate Coral._
(_a_) Natural condition, (_b_) Cell filled with calcite. (_c_) Walls
calcite, filling silica. (_d_) Walls silica, filling calcite. (_e_)
Both walls and calcite silica. All these conditions are found in the
fossil corals of the corniferous Limestone of Canada--Middle Permian.]
It is otherwise, however, when the calcareous fossils have been filled
or injected with some mineral matter different from the matrix, as, for
example, silica or some silicate, oxide or sulphide of iron. In this
case the texture, colour, or hardness of the filling appear different
from those of the limestone, and may be seen in a fresh fracture or
polished slice; or when the rock is weathered, the hard mineralizing
substance may project from the surface of the specimens, or may be
disclosed by treating the surface with a weak acid. The figures here
given may suffice to show some of these conditions of mineralization
in ordinary limestones, and the effects which they produce (Fig. 23).
The mineral matters which thus aid in preserving fossils are of various
kinds, and the whole subject is a very curious one; but for the present
we may content ourselves with two kinds of mineralization--that by
silicates and that by magnesian limestone or dolomite.
From the bottom of modern seas the dredge often brings up multitudes
of minute shells, especially those of the simple gelatinous Protozoa,
known as Foraminifera, whose internal cavities and pores have been
filled with a greenish mineral composed of silica, iron and potash,
combined with water (or, chemically speaking, a hydrous silicate of
iron and potassium), which is named _glauconite_ from its bluish-green
colour--a name which we shall do well to remember. In such compounds,
bases of similar chemical properties often replace one another, so
that various glauconites differ somewhat in composition, the iron
being in part often replaced by alumina or magnesia, and the potash by
soda. The combined water also differs somewhat in its percentage. When
minute shells fossilized in this way are treated with an acid so as to
remove the calcareous shell itself, the enclosed silicate remains as a
beautiful cast or core, representing all the forms of the interior,
and any pores that may have penetrated the walls, and also perfectly
representing the soft gelatinous body of the animal which once tenanted
the shells (Fig. 24). (See also Fig. 25 at end of chapter.)
[Illustration: Fig. 24.--_Cast of Cavities of Polystomella in
Glauconite (magnified)._
After a photograph from Dr. Carpenter, and mounted specimens from his
collection.]
When we examine oceanic sediments of older date, we find similar
fillings in limestones, chalks, and sandstones of various ages, some
of the latter containing glauconite so abundantly as to bear the name
of green-sands, from their colour; and in these older examples we more
frequently find alumina and magnesia occupying a large place in the
mineralizing silicate. Fig. 24A gives two illustrations of this--one
a crinoidal stem from the Silurian of New Brunswick, injected with a
silicate of alumina, iron, magnesia and potash; the other a spiral
shell from more ancient perhaps Cambrian rocks in Wales, filled with a
silicate apparently more nearly related to serpentine. Further examples
will be referred to in an appended note.
[Illustration: Fig. 24A.--(_a_) _Joint of Crinoid injected with a
Hydrous Silicate, Silurian, Pole Hill, New Brunswick._ (× 25.)
(_b_) _Spiral Shell injected with a Hydrous Silicate allied to
Serpentine, near Llangwyllog, North Wales,_ (× 25.)]
We may now consider shortly the relation of dolomite, or the mixed
carbonates of lime and magnesia, to the preservation of fossils. The
presence of dolomite or magnesian limestone in these beds does not
affect the conclusion as to their probable organic origin. This form of
limestone occurs abundantly in later formations, and is even forming in
connection with coral deposits in the modern ocean.
Dana has shown this by his observations on the occurrence of dolomite
in the elevated coral island of Matea in Polynesia,[20] under
circumstances which show that it was formed in the lagoon of an ancient
coral atoll, or ring-shaped island, while he finds that coral and
coral sands of the same elevated reef contain very little magnesia.
He concludes that the introduction of magnesia into the consolidating
under-water coral sand or mud has apparently taken place--"(1) In
sea-water at the ordinary temperature; and (2) without the agency of
any other mineral water except that of the ocean"; but the sand and
mud were those of a lagoon in which the saline matter was in process
of concentration by evaporation under the solar heat. Klement has
more recently taken up this fact in the way of experiment, and finds
that, while in the case of ordinary calcite this action is slow
and imperfect, with the aragonite which constitutes the calcareous
framework of certain corals,[21] and at temperatures of 60° or over,
it is very rapid and complete, producing a mixture of calcium and
magnesium carbonates, from which a pure dolomite more or less mixed
with calcite may subsequently result.[22]
[Footnote 20: "Corals and Coral Islands," p. 356, etc.]
[Footnote 21: Aragonite, like ordinary limestone, is calcium carbonate,
but its atoms seem to be differently arranged, so as to make it a
less stable compound, and it has a different crystalline form. Some
calcareous organisms are composed of aragonite, others of ordinary
calcite.]
[Footnote 22:"Bulletin Geol. Soc. Belgium," vol. ix. (1895, p. 3). Also
notice in _Geol. Mag._, July, 1895, p. 329.]
I regard these observations as of the utmost importance in reference to
the relations of dolomite with fossiliferous limestones, and especially
with those of the Grenville series. The waters of the Laurentian ocean
must have been much richer in salts of magnesium than those of the
present seas, and the temperature was probably higher, so that chemical
changes now proceeding in limited lagoons might have occurred over
much larger areas. If at that time there were, as in later periods,
calcareous organisms composed of aragonite, these may have been
destroyed by conversion into dolomite, while others more resisting were
preserved, just as a modern _Polytrema_ or _Balanus_ might remain, when
a coral to which it might be attached would be dolomitized, or might
even be removed altogether by sea-water containing carbonic acid. There
is reason to believe that this last change sometimes takes place in
the deeper parts of the ocean at present. This would account for the
persistence of Eozoon and its fragments, when other organisms may have
perished, and also for the frequent filling of the canals and tubuli
with the magnesian carbonate.
The main point here, however, for our present purpose is that, when a
calcareous shell or skeleton has been thus infiltrated with a silicate,
it becomes imperishable, so that any amount of alteration of the
containing limestone short of its absolute fusion would not suffice
to destroy an organism once injected with silicious matter. Thus the
occasional persistence of silicified fossils in highly metamorphosed
limestones is in no respect contradictory to the general fact, that
when not preserved by silicious infiltration, they have perished, and
this more especially in the case of those whose skeletons are composed
of aragonite.
Carrying these facts with us, the next question is, What manner of
fossil remains should we expect to find in the Upper Laurentian rocks,
supposing that any such are therein preserved? The answer to this
question follows at once from the facts as to the succession of life
noticed above. Only the marine invertebrates have been traced as far
back as the oldest Cambrian, and only Worms, Sponges, and Protozoa
into the Huronian. We should therefore have no expectation of finding
remains of any vertebrate animals or of any of the land invertebrates;
and even allowing for the more favourable conditions, as compared
with the Huronian, evidenced by the great limestones and the abundant
carbon, we could scarcely expect anything higher than some of the
lower types of invertebrate life, such as Worms, Hydroids, Corals and
Protozoa. We have next to inquire what forms, possibly organic, have
actually been found, and what information we can derive from them as to
the beginnings of life. Since, however, such discoveries as have been
made have been the result of much labour and scientific skill brought
to bear on these old rocks, and are connected with the reputations of
several eminent men, now deceased, we may first refer shortly to the
history of the discovery of supposed fossils in the Laurentian rocks of
Canada.
[Illustration: Fig. 25.--_Nature-print of an etched Specimen of Eozoon._
Showing the laminæ, a part of the natural margin, near which passes a
diagonal calcite vein, and at the upper right-hand corner, fragmental
material with casts of Archæospherinæ. The dark lines represent the
chambers filled with serpentine, the white the calcite wall.]
_THE HISTORY OF A DISCOVERY_
VI
_THE HISTORY OF A DISCOVERY_
When Mr. Logan, afterwards Sir William Logan, entered on the Geological
Survey of Canada, in 1840, he found that vast and little-explored
regions in the northern part of that country were occupied with
gneissic rocks, similar to the oldest gneisses of Scotland and
Scandinavia, and to which the name Azoic had been given by Murchison,
as rocks destitute of fossils, while they had been the "fundamental
granite" or ur-gneiss of most European geologists. They were
unquestionably below and more ancient than the oldest fossiliferous
Cambrian rocks both in Europe and North America, and geologists had for
the most part contented themselves with regarding them as primitive
rocks, destitute of any geological interest, much as some United States
geologists of the present day call them the "Archæan complex," a name
which the late Prof Dana has well characterized as a "term of despair."