THE PRESERVATION OF EOZOON.
Perhaps nothing excites more scepticism as to this ancient fossil
than the prejudice existing among geologists that no organism can be
preserved in rocks so highly metamorphic as those of the Laurentian
series. I call this a prejudice, because any one who makes the
microscopic structure of rocks and fossils a special study, soon learns
that fossils undergo the most remarkable and complete chemical changes
without losing their minute structure, and that calcareous rocks if
once fossiliferous are hardly ever so much altered as to lose all
trace of the organisms which they contained, while it is a most common
occurrence to find highly crystalline rocks of this kind abounding in
fossils preserved as to their minute structure.
Let us, however, look at the precise conditions under which this takes
place.
When calcareous fossils of irregular surface and porous or cellular
texture, such as Eozoon was or corals were and are, become imbedded
in clay, marl, or other soft sediment, they can be washed out and
recovered in a condition similar to that of recent specimens, except
that their pores or cells if open may be filled with the material of
the matrix, or if not so open that they can be thus filled, they may be
more or less incrusted with mineral deposits introduced by water, or
may even be completely filled up in this way. But if such fossils are
contained in hard rocks, they usually fail, when these are broken, to
show their external surfaces, and, breaking across with the containing
rock, they exhibit their internal structure merely,--and this more
or less distinctly, according to the manner in which their cells or
cavities have been filled. Here the microscope becomes of essential
service, especially when the structures are minute. A fragment of
fossil wood which to the naked eye is nothing but a dark stone, or a
coral which is merely a piece of gray or coloured marble, or a specimen
of common crystalline limestone made up originally of coral fragments,
presents, when sliced and magnified, the most perfect and beautiful
structure. In such cases it will be found that ordinarily the original
substance of the fossil remains, in a more or less altered state. Wood
may be represented by dark lines of coaly matter, or coral by its
white or transparent calcareous laminæ; while the material which has
been introduced and which fills the cavities may so differ in colour,
transparency, or crystalline structure, as to act differently on
light, and so reveal the structure. These fillings are very curious.
Sometimes they are mere earthy or muddy matter. Sometimes they are
pure and transparent and crystalline. Often they are stained with
oxide of iron or coaly matter. They may consist of carbonate of lime,
silica or silicates, sulphate of baryta, oxides of iron, carbonate of
iron, iron pyrite, or sulphides of copper or lead, all of which are
common materials. They are sometimes so complicated that I have seen
even the minute cells of woody structures, each with several bands of
differently coloured materials deposited in succession, like the coats
of an onyx agate.
A further stage of mineralization occurs when the substance of the
organism is altogether removed and replaced by foreign matter, either
little by little, or by being entirely dissolved or decomposed,
leaving a cavity to be filled by infiltration. In this state are some
silicified woods, and those corals which have been not filled with but
converted into silica, and can thus sometimes be obtained entire and
perfect by the solution in an acid of the containing limestone, or by
its removal in weathering. In this state are the beautiful silicified
corals obtained from the corniferous limestone of Lake Erie. It may be
well to present to the eye these different stages of fossilization. I
have attempted to do this in fig. 22, taking a tabulate coral of the
genus Favosites for an example, and supposing the materials employed to
be calcite and silica. Precisely the same illustration would apply to a
piece of wood, except that the cell-wall would be carbonaceous matter
instead of carbonate of lime. In this figure the dotted parts represent
carbonate of lime, the diagonally shaded parts silica or a silicate.
Thus we have, in the natural state, the walls of carbonate of lime
and the cavities empty. When fossilized the cavities may be merely
filled with carbonate of lime, or they may be filled with silica; or
the walls themselves may be replaced by silica and the cavities may
remain filled with carbonate of lime; or both the walls and cavities
may be represented by or filled with silica or silicates. The ordinary
specimens of Eozoon are in the third of these stages, though some exist
in the second, and I have reason to believe that some have reached to
the fifth. I have not met with any in the fourth stage, though this is
not uncommon in Silurian and Devonian fossils.
[Illustration: Fig. 22. _Diagram showing different States of
Fossilization of a Cell of a Tabulate Coral._
(_a._) Natural condition--walls calcite, cell empty. (_b._) Walls
calcite, cell filled with the same. (_c._) Walls calcite, cell filled
with silica or silicate. (_d._) Walls silicified, cell filled with
calcite. (_e._) Walls silicified, cell filled with silica or silicate.]
With regard to the calcareous organisms with which we have now to do,
when these are imbedded in pure limestone and filled with the same, so
that the whole rock, fossils and all, is identical in composition, and
when metamorphic action has caused the whole to become crystalline,
and perhaps removed the remains of carbonaceous matter, it may be very
difficult to detect any traces of fossils. But even in this case
careful management of light may reveal indications of structure, as in
some specimens of Eozoon described by the writer and Dr. Carpenter. In
many cases, however, even where the limestones have become perfectly
crystalline, and the cleavage planes cut freely across the fossils,
these exhibit their forms and minute structure in great perfection.
This is the case in many of the Lower Silurian limestones of Canada,
as I have elsewhere shown.[X] The gray crystalline Trenton limestone
of Montreal, used as a building stone, is an excellent illustration
of this. To the naked eye it is a gray marble composed of cleavable
crystals; but when examined in thin slices, it shows its organic
fragments in the greatest perfection, and all the minute structures
are perfectly marked out by delicate carbonaceous lines. The only
exception in this limestone is in the case of the Crinoids, in which
the cellular structure is filled with transparent calc-spar, perfectly
identical with the original solid matter, so that they appear solid
and homogeneous, and can be recognised only by their external forms.
The specimen represented in fig. 23, is a mass of Corals, Bryozoa, and
Crinoids, and shows these under a low power, as represented in the
figure; but to the naked eye it is merely a gray crystalline limestone.
The specimen represented in fig. 24 shows the Laurentian Eozoon in a
similar state of preservation. It is from a sketch by Dr. Carpenter,
and shows the delicate canals partly filled with calcite as clear and
colourless as that of the shell itself, and distinguishable only by
careful management of the light.
[Footnote X: _Canadian Naturalist_, 1859; Microscopic Structure of
Canadian Limestones.]
[Illustration: Fig. 23. _Slice of Crystalline Lower Silurian Limestone;
showing Crinoids, Bryozoa, and Corals in fragments._]
[Illustration: Fig. 24. _Wall of Eozoon penetrated with Canals. The
unshaded portions filled with Calcite._ (_After Carpenter._)]
In the case of recent and fossil Foraminifers, these--when not so
little mineralized that their chambers are empty, or only partially
filled, which is sometimes the case even with Eocene Nummulites
and Cretaceous forms of smaller size,--are very frequently filled
solid with calcareous matter, and as Dr. Carpenter well remarks,
even well preserved Tertiary Nummulites in this state often fail
greatly in showing their structures, though in the same condition
they occasionally show these in great perfection. Among the finest
I have seen are specimens from the Mount of Olives (fig. 19), and
Dr. Carpenter mentions as equally good those of the London clay of
Bracklesham. But in no condition do modern Foraminifera or those of
the Tertiary and Mesozoic rocks appear in greater perfection than when
filled with the hydrous silicate of iron and potash called glauconite,
and which gives by the abundance of its little bottle-green concretions
the name of "green-sand" to formations of this age both in Europe and
America. In some beds of green-sand every grain seems to have been
moulded into the interior of a microscopic shell, and has retained
its form after the frail envelope has been removed. In some cases the
glauconite has not only filled the chambers but has penetrated the
fine tubulation, and when the shell is removed, either naturally or
by the action of an acid, these project in minute needles or bundles
of threads from the surface of the cast. It is in the warmer seas,
and especially in the bed of the Ægean and of the Gulf Stream, that
such specimens are now most usually found. If we ask why this mineral
glauconite should be associated with Foraminiferal shells, the answer
is that they are both products of one kind of locality. The same sea
bottoms in which Foraminifera most abound are also those in which for
some unknown chemical reason glauconite is deposited. Hence no doubt
the association of this mineral with the great Foraminiferal formation
of the chalk. It is indeed by no means unlikely that the selection
by these creatures of the pure carbonate of lime from the sea-water
or its minute plants, may be the means of setting free the silica,
iron, and potash, in a state suitable for their combination. Similar
silicates are found associated with marine limestones, as far back as
the Silurian age; and Dr. Sterry Hunt, than whom no one can be a better
authority on chemical geology, has argued on chemical grounds that the
occurrence of serpentine with the remains of Eozoon is an association
of the same character.
However this may be, the infiltration of the pores of Eozoon with
serpentine and other silicates has evidently been one main means of
the preservation of its structure. When so infiltrated no metamorphism
short of the complete fusion of the containing rock could obliterate
the minutest points of structure; and that such fusion has not
occurred, the preservation in the Laurentian rocks of the most delicate
lamination of the beds shows conclusively; while, as already stated, it
can be shown that the alteration which has occurred might have taken
place at a temperature far short of that necessary to fuse limestone.
Thus has it happened that these most ancient fossils have been
handed down to our time in a state of preservation comparable, as Dr.
Carpenter states, to that of the best preserved fossil Foraminifera
from the more recent formations that have come under his observation in
the course of all his long experience.
Let us now look more minutely at the nature of the typical specimens
of Eozoon as originally observed and described, and then turn to those
preserved in other ways, or more or less destroyed and defaced. Taking
a polished specimen from Petite Nation, like that delineated in Plate
V., we find the shell represented by white limestone, and the chambers
by light green serpentine. By acting on the surface with a dilute
acid we etch out the calcareous part, leaving a cast in serpentine
of the cavities occupied by the soft parts; and when this is done in
polished slices these may be made to print their own characters on
paper, as has actually been done in the case of Plate V., which is an
electrotype taken from an actual specimen, and shows both the laminated
and acervuline parts of the fossil. If the process of decalcification
has been carefully executed, we find in the excavated spaces delicate
ramifying processes of opaque serpentine or transparent dolomite, which
were originally imbedded in the calcareous substance, and which are
often of extreme fineness and complexity. (Plate VI. and fig. 10.)
These are casts of the canals which traversed the shell when still
inhabited by the animal. In some well preserved specimens we find the
original cell-wall represented by a delicate white film, which under
the microscope shows minute needle-like parallel processes representing
its still finer tubuli. It is evident that to have filled these tubuli
the serpentine must have been introduced in a state of actual solution,
and must have carried with it no foreign impurities. Consequently we
find that in the chambers themselves the serpentine is pure; and if we
examine it under polarized light, we see that it presents a singularly
curdled or irregularly laminated appearance, which I have designated
under the name septariiform, as if it had an imperfectly crystalline
structure, and had been deposited in irregular laminæ, beginning at
the sides of the chambers, and filling them toward the middle, and
had afterward been cracked by shrinkage, and the cracks filled with a
second deposit of serpentine. Now, serpentine is a hydrous silicate of
magnesia, and all that we need to suppose is that in the deposits of
the Laurentian sea magnesia was present instead of iron and potash,
and we can understand that the Laurentian fossil has been petrified
by infiltration with serpentine, as more modern Foraminifera have
been with glauconite, which, though it usually has little magnesia,
often has a considerable percentage of alumina. Further, in specimens
of Eozoon from Burgess, the filling mineral is loganite, a compound
of silica, alumina, magnesia and iron, with water, and in certain
Silurian limestones from New Brunswick and Wales, in which the delicate
microscopic pores of the skeletons of stalked star-fishes or Crinoids
have been filled with mineral deposits, so that when decalcified
these are most beautifully represented by their casts, Dr. Hunt has
proved the filling mineral to be a silicate of alumina, iron, magnesia
and potash, intermediate between serpentine and glauconite. We have,
therefore, ample warrant for adhering to Dr. Hunt's conclusion that
the Laurentian serpentine was deposited under conditions similar to
those of the modern green-sand. Indeed, independently of Eozoon, it is
impossible that any geologist who has studied the manner in which this
mineral is associated with the Laurentian limestones could believe it
to have been formed in any other way. Nor need we be astonished at
the fineness of the infiltration by which these minute tubes, perhaps
1/10000 of an inch in diameter, are filled with mineral matter. The
micro-geologist well knows how, in more modern deposits, the finest
pores of fossils are filled, and that mineral matter in solution
can penetrate the smallest openings that the microscope can detect.
Wherever the fluids of the living body can penetrate, there also
mineral substances can be carried, and this natural injection, effected
under great pressure and with the advantage of ample time, can surpass
any of the feats of the anatomical manipulator. Fig. 25 represents
a microscopic joint of a Crinoid from the Upper Silurian of New
Brunswick, injected with the hydrous silicate already referred to, and
fig. 26 shows a microscopic chambered or spiral shell, from a Welsh
Silurian limestone, with its cavities filled with a similar substance.
[Illustration: Fig. 25. _Joint of a Crinoid, having its pores injected
with a Hydrous Silicate._
Upper Silurian Limestone, Pole Hill, New Brunswick. Magnified 25
diameters.]
[Illustration: Fig. 26. _Shell from a Silurian Limestone, Wales; its
cavity filled with a Hydrous Silicate._
Magnified 25 diameters.]
It is only necessary to refer to the attempts which have been made to
explain by merely mineral deposits the occurrence of the serpentine
in the canals and chambers of Eozoon, and its presenting the form it
does, to see that this is the case. Prof. Rowney, for example, to avoid
the force of the argument from the canal system, is constrained to
imagine that the whole mass has at one time been serpentine, and that
this has been partially washed away, and replaced by calcite. If so,
whence the deposition of the supposed mass of serpentine, which has to
be accounted for in this way as well as in the other? How did it happen
to be eroded into so regular chambers, leaving intermediate floors and
partitions. And, more wonderful still, how did the regular dendritic
bundles, so delicate that they are removed by a breath, remain perfect,
and endure until they were imbedded in calcareous spar? Further, how
does it happen that in some specimens serpentine and pyroxene seem to
have encroached upon the structure, as if they and not calcite were the
eroding minerals? How any one who has looked at the structures can for
a moment imagine such a possibility, it is difficult to understand. If
we could suppose the serpentine to have been originally deposited as
a cellular or laminated mass, and its cavities filled with calcite in
a gelatinous or semi-fluid state, we might suppose the fine processes
of serpentine to have grown outward into these cavities in the mass,
as fibres of oxide of iron or manganese have grown in the silica of
moss-agate; but this theory would be encompassed with nearly as great
mechanical and chemical difficulties. The only rational view that any
one can take of the process is, that the calcareous matter was the
original substance, and that it had delicate tubes traversing it which
became injected with serpentine. The same explanation, and no other,
will suffice for those delicate cell-walls, penetrated by innumerable
threads of serpentine, which must have been injected into pores. It is
true that there are in some of the specimens cracks filled with fibrous
serpentine or chrysotile, but these traverse the mass in irregular
directions, and they consist of closely packed angular prisms,
instead of a matrix of limestone penetrated by cylindrical threads of
serpentine. (Fig. 27.) Here I must once for all protest against the
tendency of some opponents of Eozoon to confound these structures and
the canal system of Eozoon with the acicular crystals, and dendritic
or coralloidal forms, observed in some minerals. It is easy to make
such comparisons appear plausible to the uninitiated, but practised
observers cannot be so deceived, the differences are too marked and
essential. In illustration of this, I may refer to the highly magnified
canals in figs. 28 and 29. Further, it is evident from the examination
of the specimens, that the chrysotile veins, penetrating as they often
do diagonally or transversely across both chambers and walls, must have
originated subsequently to the origin and hardening of the rock and its
fossils, and result from aqueous deposition of fibrous serpentine in
cracks which traverse alike the fossils and their matrix. In specimens
now before me, nothing can be more plain than this entire independence
of the shining silky veins of fibrous serpentine, and the fact of their
having been formed subsequently to the fossilization of the Eozoon;
since they can be seen to run across the lamination, and to branch off
irregularly in lines altogether distinct from the structure. This,
while it shows that these veins have no connection with the fossil,
shows also that the latter was an original ingredient of the beds when
deposited, and not a product of subsequent concretionary action.
[Illustration: Fig. 27. _Diagram showing the different appearances
of the cell-wall of Eozoon and of a vein of Chrysotile, when highly
magnified._]
[Illustration: Fig. 28. _Casts of Canals of Eozoon in Serpentine,
decalcified and highly magnified._]
[Illustration: Fig. 29. _Canals of Eozoon._
Highly magnified.]
Taking the specimens preserved by serpentine as typical, we now turn
to certain other and, in some respects, less characteristic specimens,
which are nevertheless very instructive. At the Calumet some of
the masses are partly filled with serpentine and partly with white
pyroxene, an anhydrous silicate of lime and magnesia. The two minerals
can readily be distinguished when viewed with polarized light; and in
some slices I have seen part of a chamber or group of canals filled
with serpentine and part with pyroxene. In this case the pyroxene
or the materials which now compose it, must have been introduced by
infiltration, as well as the serpentine. This is the more remarkable as
pyroxene is most usually found as an ingredient of igneous rocks; but
Dr. Hunt has shown that in the Laurentian limestones and also in veins
traversing them, it occurs under conditions which imply its deposition
from water, either cold or warm. Gümbel remarks on this:--"Hunt, in
a very ingenious manner, compares this formation and deposition of
serpentine, pyroxene, and loganite, with that of glauconite, whose
formation has gone on uninterruptedly from the Silurian to the Tertiary
period, and is even now taking place in the depths of the sea; it being
well known that Ehrenberg and others have already shown that many of
the grains of glauconite are casts of the interior of foraminiferal
shells. In the light of this comparison, the notion that the serpentine
and such like minerals of the primitive limestones have been formed,
in a similar manner, in the chambers of Eozoic Foraminifera, loses any
traces of improbability which it might at first seem to possess."
In many parts of the skeleton of Eozoon, and even in the best
infiltrated serpentine specimens, there are portions of the cell-wall
and canal system which have been filled with calcareous spar or with
dolomite, so similar to the skeleton that it can be detected only under
the most favourable lights and with great care. (Fig. 24, _supra_.)
The same phenomena may be observed in joints of Crinoids from the
Palæozoic rocks, and they constitute proofs of organic origin even
more irrefragable than the filling with serpentine. Dr. Carpenter has
recently, in replying to the objections of Mr. Carter, made excellent
use of this feature of the preservation of Eozoon. It is further to
be remarked that in all the specimens of true Eozoon, as well as
in many other calcareous fossils preserved in ancient rocks, the
calcareous matter, even when its minute structures are not preserved
or are obscured, presents a minutely granular or curdled appearance,
arising no doubt from the original presence of organic matter, and not
recognised in purely inorganic calcite.
Another style of these remarkable fossils is that of the Burgess
specimens. In these the walls have been changed into dolomite
or magnesian limestone, and the canals seem to have been wholly
obliterated, so that only the laminated structure remains. The material
filling the chambers is also an aluminous silicate named loganite; and
this seems to have been introduced, not so much in solution, as in
the state of muddy slime, since it contains foreign bodies, as grains
of sand and little groups of silicious concretions, some of which are
not unlikely casts of the interior of minute foraminiferal shells
contemporary with Eozoon, and will be noticed in the sequel.
[Illustration: Fig. 30. _Eozoon from Tudor._
Two-thirds natural size. (_a._) Tubuli. (_b._) Canals. Magnified. _a_
and _b_ from another specimen.]
Still another mode of occurrence is presented by a remarkable specimen
from Tudor in Ontario, and from beds probably on the horizon of the
Upper Laurentian or Huronian.[Y] It occurs in a rock scarcely at all
metamorphic, and the fossil is represented by white carbonate of lime,
while the containing matrix is a dark-coloured coarse limestone. In
this specimen the material filling the chambers has not penetrated
the canals except in a few places, where they appear filled with dark
carbonaceous matter. In mode of preservation these Tudor specimens
much resemble the ordinary fossils of the Silurian rocks. One of
the specimens in the collection of the Geological Survey (fig. 30)
presents a clavate form, as if it had been a detached individual
supported on one end at the bottom of the sea. It shows, as does
also the original Calumet specimen, the septa approaching each other
and coalescing at the margin of the form, where there were probably
orifices communicating with the exterior. Other specimens of fragmental
Eozoon from the Petite Nation localities have their canals filled with
dolomite, which probably penetrated them after they were broken up
and imbedded in the rock. I have ascertained with respect to these
fragments of Eozoon, that they occur abundantly in certain layers of
the Laurentian limestone, beds of some thickness being in great part
made up of them, and coarse and fine fragments occur in alternate
layers, like the broken corals in some Silurian limestones.
[Footnote Y: See Note B, Chap. III.]
Finally, on this part of the subject, careful observation of many
specimens of Laurentian limestone which present no trace of Eozoon
when viewed by the naked eye, and no evidence of structure when acted
on with acids, are nevertheless organic, and consist of fragments
of Eozoon, and possibly of other organisms, not infiltrated with
silicates, but only with carbonate of lime, and consequently revealing
only obscure indications of their minute structure. I have satisfied
myself of this by long and patient investigations, which scarcely admit
of any adequate representation, either by words or figures.
Every worker in those applications of the microscope to geological
specimens which have been termed micro-geology, is familiar with the
fact that crystalline forces and mechanical movements of material
often play the most fantastic tricks with fossilized organic matter.
In fossil woods, for example, we often have the tissues disorganized,
with radiating crystallizations of calcite and little spherical
concretions of quartz, or disseminated cubes and grains of pyrite,
or little veins filled with sulphate of barium or other minerals. We
need not, therefore, be surprised to find that in the venerable rocks
containing Eozoon, such things occur in the more highly crystalline
parts of the limestones, and even in some still showing traces of
the fossil. We find many disseminated crystals of magnetite, pyrite,
spinel, mica, and other minerals, curiously curved prisms of vermicular
mica, bundles of aciculi of tremolite and similar substances, veins of
calcite and crysolite or fibrous serpentine, which often traverse the
best specimens. Where these occur abundantly we usually find no organic
structures remaining, or if they exist they are in a very defective
state of preservation. Even in specimens presenting the lamination of
Eozoon to the naked eye, these crystalline actions have often destroyed
the minute structure; and I fear that some microscopists have been
victimised by having under their consideration only specimens in which
the actual characters had been too much defaced to be discernible. I
must here state that I have found some of the specimens sold under
the name of Eozoon Canadense by dealers in microscopical objects to
be almost or quite worthless, being destitute of any good structure,
and often merely pieces of Laurentian limestone with serpentine
grains only. I fear that the circulation of such specimens has done
much to cause scepticism as to the Foraminiferal nature of Eozoon. No
mistake can be greater than to suppose that any and every specimen
of Laurentian limestone must contain Eozoon. More especially have
I hitherto failed to detect traces of it in those carbonaceous or
graphitic limestones which are so very abundant in the Laurentian
country. Perhaps where vegetable matter was very abundant Eozoon
did not thrive, or on the other hand the growth of Eozoon may have
diminished the quantity of vegetable matter. It is also to be observed
that much compression and distortion have occurred in the beds of
Laurentian limestone and their contained fossils, and also that the
specimens are often broken by faults, some of which are so small as to
appear only on microscopic examination, and to shift the plates of the
fossil just as if they were beds of rock. This, though it sometimes
produces puzzling appearances, is an evidence that the fossils were
hard and brittle when this faulting took place, and is consequently
an additional proof of their extraneous origin. In some specimens it
would seem that the lower and older part of the fossil had been wholly
converted into serpentine or pyroxene, or had so nearly experienced
this change that only small parts of the calcareous wall can be
recognised. These portions correspond with fossil woods altogether
silicified, not only by the filling of the cells, but also by the
conversion of the walls into silica. I have specimens which manifestly
show the transition from the ordinary condition of filling with
serpentine to one in which the cell-walls are represented obscurely by
one shade of this mineral and the cavities by another.
The above considerations as to mode of preservation of Eozoon concur
with those in previous chapters in showing its oceanic character;
but the ocean of the Eozoic period may not have been so deep as at
present, and its waters were probably warm and well stocked with
mineral matters derived from the newly formed land, or from hot springs
in its own bottom. On this point the interesting investigations of
Dr. Hunt with reference to the chemical conditions of the Silurian
seas, allow us to suppose that the Laurentian ocean may have been much
more richly stored, more especially with salts of lime and magnesia,
than that of subsequent times. Hence the conditions of warmth, light,
and nutriment, required by such gigantic Protozoans would all be
present, and hence, also no doubt, some of the peculiarities of its
mineralization.
NOTES TO CHAPTER V.
(A.) Dr. Sterry Hunt on the Mineralogy of Eozoon and the containing
Rocks.
It was fortunate for the recognition of Eozoon that Dr. Hunt had,
before its discovery, made so thorough researches into the chemistry
of the Laurentian series, and was prepared to show the chemical
possibilities of the preservation of fossils in these ancient
deposits. The following able summary of his views was appended to the
original description of the fossil in the _Journal of the Geological
Society_.
"The details of structure have been preserved by the introduction
of certain mineral silicates, which have not only filled up the
chambers, cells, and canals left vacant by the disappearance of the
animal matter, but have in very many cases been injected into the
tubuli, filling even their smallest ramifications. These silicates
have thus taken the place of the original sarcode, while the
calcareous septa remain. It will then be understood that when the
replacement of the Eozoon by silicates is spoken of, this is to be
understood of the soft parts only; since the calcareous skeleton is
preserved, in most cases, without any alteration. The vacant spaces
left by the decay of the sarcode may be supposed to have been filled
by a process of infiltration, in which the silicates were deposited
from solution in water, like the silica which fills up the pores of
wood in the process of silicification. The replacing silicates, so
far as yet observed, are a white pyroxene, a pale green serpentine,
and a dark green alumino-magnesian mineral, which is allied in
composition to chlorite and to pyrosclerite, and which I have
referred to loganite. The calcareous septa in the last case are found
to be dolomitic, but in the other instances are nearly pure carbonate
of lime. The relations of the carbonate and the silicates are well
seen in thin sections under the microscope, especially by polarized
light. The calcite, dolomite, and pyroxene exhibit their crystalline
structure to the unaided eye; and the serpentine and loganite are
also seen to be crystalline when examined with the microscope. When
portions of the fossil are submitted to the action of an acid, the
carbonate of lime is dissolved, and a coherent mass of serpentine is
obtained, which is a perfect cast of the soft parts of the Eozoon.
The form of the sarcode which filled the chambers and cells is
beautifully shown, as well as the connecting canals and the groups
of tubuli; these latter are seen in great perfection upon surfaces
from which the carbonate of lime has been partially dissolved. Their
preservation is generally most complete when the replacing mineral is
serpentine, although very perfect specimens are sometimes found in
pyroxene. The crystallization of the latter mineral appears, however,
in most cases to have disturbed the calcareous septa.
"Serpentine and pyroxene are generally associated in these specimens,
as if their disposition had marked different stages of a continuous
process. At the Calumet, one specimen of the fossil exhibits the
whole of the sarcode replaced by serpentine; while, in another one
from the same locality, a layer of pale green translucent serpentine
occurs in immediate contact with the white pyroxene. The calcareous
septa in this specimen are very thin, and are transverse to the plane
of contact of the two minerals; yet they are seen to traverse both
the pyroxene and the serpentine without any interruption or change.
Some sections exhibit these two minerals filling adjacent cells,
or even portions of the same cell, a clear line of division being
visible between them. In the specimens from Grenville on the other
hand, it would seem as if the development of the Eozoon (considerable
masses of which were replaced by pyroxene) had been interrupted, and
that a second growth of the animal, which was replaced by serpentine,
had taken place upon the older masses, filling up their interstices."
[Details of chemical composition are then given.]
"When examined under the microscope, the loganite which replaces the
Eozoon of Burgess shows traces of cleavage-lines, which indicate a
crystalline structure. The grains of insoluble matter found in the
analysis, chiefly of quartz-sand, are distinctly seen as foreign
bodies imbedded in the mass, which is moreover marked by lines
apparently due to cracks formed by a shrinking of the silicate, and
subsequently filled by a further infiltration of the same material.
This arrangement resembles on a minute scale that of septaria.
Similar appearances are also observed in the serpentine which
replaces the Eozoon of Grenville, and also in a massive serpentine
from Burgess, resembling this, and enclosing fragments of the fossil.
In both of these specimens also grains of mechanical impurities are
detected by the microscope; they are however, rarer than in the
loganite of Burgess.
"From the above facts it may be concluded that the various silicates
which now constitute pyroxene, serpentine, and loganite were directly
deposited in waters in the midst of which the Eozoon was still
growing, or had only recently perished; and that these silicates
penetrated, enclosed, and preserved the calcareous structure
precisely as carbonate of lime might have done. The association
of the silicates with the Eozoon is only accidental; and large
quantities of them, deposited at the same time, include no organic
remains. Thus, for example, there are found associated with the
Eozoon limestones of Grenville, massive layers and concretions of
pure serpentine; and a serpentine from Burgess has already been
mentioned as containing only small broken fragments of the fossil.
In like manner large masses of white pyroxene, often surrounded
by serpentine, both of which are destitute of traces of organic
structure, are found in the limestone at the Calumet. In some cases,
however, the crystallization of the pyroxene has given rise to
considerable cleavage-planes, and has thus obliterated the organic
structures from masses which, judging from portions visible here and
there, appear to have been at one time penetrated by the calcareous
plates of Eozoon. Small irregular veins of crystalline calcite, and
of serpentine, are found to traverse such pyroxene masses in the
Eozoon limestone of Grenville.
"It appears that great beds of the Laurentian limestones are
composed of the ruins of the Eozoon. These rocks, which are white,
crystalline, and mingled with pale green serpentine, are similar in
aspect to many of the so-called primary limestones of other regions.
In most cases the limestones are non-magnesian, but one of them
from Grenville was found to be dolomitic. The accompanying strata
often present finely crystallized pyroxene, hornblende, phlogopite,
apatite, and other minerals. These observations bring the formation
of silicious minerals face to face with life, and show that their
generation was not incompatible with the contemporaneous existence
and the preservation of organic forms. They confirm, moreover, the
view which I some years since put forward, that these silicated
minerals have been formed, not by subsequent metamorphism in
deeply buried sediments, but by reactions going on at the earth's
surface.[Z] In support of this view, I have elsewhere referred to
the deposition of silicates of lime, magnesia, and iron from natural
waters, to the great beds of sepiolite in the unaltered Tertiary
strata of Europe; to the contemporaneous formation of neolite (an
aluimino-magnesian silicate related to loganite and chlorite in
composition); and to glauconite, which occurs not only in Secondary,
Tertiary, and Recent deposits, but also, as I have shown, in Lower
Silurian strata.[AA] This hydrous silicate of protoxide of iron
and potash, which sometimes includes a considerable proportion of
alumina in its composition, has been observed by Ehrenberg, Mantell,
and Bailey, associated with organic forms in a manner which seems
identical with that in which pyroxene, serpentine, and loganite
occur with the Eozoon in the Laurentian limestones. According to the
first of these observers, the grains of green-sand, or glauconite,
from the Tertiary limestone of Alabama, are casts of the interior
of Polythalamia, the glauconite having filled them by 'a species of
natural injection, which is often so perfect that not only the large
and coarse cells, but also the very finest canals of the cell-walls
and all their connecting tubes, are thus petrified and separately
exhibited.' Bailey confirmed these observations, and extended them.
He found in various Cretaceous and Tertiary limestones of the United
States, casts in glauconite, not only of _Foraminifera_, but of
spines of _Echinus_, and of the cavities of corals. Besides, there
were numerous red, green, and white casts of minute anastomosing
tubuli, which, according to Bailey, resemble the casts of the holes
made by burrowing sponges (_Cliona_) and worms. These forms are seen
after the dissolving of the carbonate of lime by a dilute acid.
He found, moreover, similar casts of _Foraminifera_, of minute
mollusks, and of branching tubuli, in mud obtained from soundings in
the Gulf Stream, and concluded that the deposition of glauconite is
still going on in the depths of the sea.[AB] Pourtales has followed
up these investigations on the recent formation of glauconite in
the Gulf Stream waters. He has observed its deposition also in
the cavities of _Millepores_, and in the canals in the shells
of _Balanus_. According to him, the glauconite grains formed in
_Foraminifera_ lose after a time their calcareous envelopes, and
finally become 'conglomerated into small black pebbles,' sections
of which still show under a microscope the characteristic spiral
arrangement of the cells.[AC]
[Footnote Z: _Silliman's Journal_ [2], xxix., p. 284; xxxii., p. 286.
_Geology of Canada_, p. 577.]
[Footnote AA: _Silliman's Journal_ [2], xxxiii., p. 277. _Geology of
Canada_, p. 487.]
[Footnote AB: _Silliman's Journal_ [2], xxii., p. 280.]
[Footnote AC: _Report of United States Coast-Survey_, 1858, p. 248.]
"It appears probable from these observations that glauconite is
formed by chemical reactions in the ooze at the bottom of the sea,
where dissolved silica comes in contact with iron oxide rendered
soluble by organic matter; the resulting silicate deposits itself in
the cavities of shells and other vacant spaces. A process analogous
to this in its results, has filled the chambers and canals of the
Laurentian _Foraminifera_ with other silicates; from the comparative
rarity of mechanical impurities in these silicates, however, it would
appear that they were deposited in clear water. Alumina and oxide of
iron enter into the composition of loganite as well as of glauconite;
but in the other replacing minerals, pyroxene and serpentine, we
have only silicates of lime and magnesia, which were probably formed
by the direct action of alkaline silicates, either dissolved in
surface-waters, or in those of submarine springs, upon the calcareous
and magnesian salts of the sea-water."
[As stated in the text, the canals of Eozoon are sometimes filled
with dolomite, or in part with serpentine and in part with dolomite.]
(B.) Silurian Limestones holding Fossils infiltrated with Hydrous
Silicate.
Since my attention has been directed to this subject, many
illustrations have come under my notice of Silurian limestones in
which the pores of fossils are infiltrated with hydrous silicates
akin to glauconite and serpentine. A limestone of this kind,
collected by Mr. Robb, at Pole Hill, in New Brunswick, afforded not
only beautiful specimens of portions of Crinoids preserved in this
way, but a sufficient quantity of the material was collected for an
exact analysis, a note on which was published in the Proceedings of
the Royal Irish Academy, 1871.
The limestone of Pole Hill is composed almost wholly of organic
fragments, cemented by crystalline carbonate of lime, and traversed
by slender veins of the same mineral. Among the fragments may be
recognised under the microscope portions of Trilobites, and of
brachiopod and gastropod shells, and numerous joints and plates
of Crinoids. The latter are remarkable for the manner in which
their reticulated structure, which is similar to that of modern
Crinoids, has been injected with a silicious substance, which is
seen distinctly in slices, and still more plainly in decalcified
specimens. This filling is precisely similar in appearance to the
serpentine filling the canals of Eozoon, the only apparent difference
being in the forms of the cells and tubes of the Crinoids, as
compared with those of the Laurentian fossil; the same silicious
substance also occupies the cavities of some of the small shells,
and occurs in mere amorphous pieces, apparently filling interstices.
From its mode of occurrence, I have not the slightest doubt that
it occupied the cavities of the crinoidal fragments while still
recent, and before they had been cemented together by the calcareous
paste. This silicious filling is therefore similar on the one hand
to that effected by the ancient serpentine of the Laurentian, and
on the other to that which results from the depositions of modern
glauconite. The analysis of Dr. Hunt, which I give below, fully
confirms these analogies.
I may add that I have examined under the microscope portions of the
substance prepared by Dr. Hunt for analysis, and find it to retain
its form, showing that it is the actual filling of the cavities. I
have also examined the small amount of insoluble silica remaining
after his treatment with acid and alkaline solvents, and find it to
consist of angular and rounded grains of quartzose sand.
The following are Dr. Hunt's notes:--
"The fossiliferous limestone from Pole Hill, New Brunswick, probably
of Upper Silurian age, is light gray and coarsely granular. When
treated with dilute hydrochloric acid, it leaves a residue of 5·9 per
cent., and the solution gives 1·8 per cent. of alumina and oxide of
iron, and magnesia equal to 1·35 of carbonate--the remainder being
carbonate of lime. The insoluble matter separated by dilute acid,
after washing by decantation from a small amount of fine flocculent
matter, consists, apart from an admixture of quartz grains, entirely
of casts and moulded forms of a peculiar silicate, which Dr. Dawson
has observed in decalcified specimens filling the pores of crinoidal
stems; and which when separated by an acid, resembles closely under
the microscope the coralloidal forms of arragonite known as _flos
ferri_, the surfaces being somewhat rugose and glistening with
crystalline faces. This silicate is sub-translucent, and of a pale
green colour, but immediately becomes of a light reddish brown when
heated to redness in the air, and gives off water when heated in a
tube, without however, changing its form. It is partially decomposed
by strong hydrochloric acid, yielding a considerable amount of
protosalt of iron. Strong hot sulphuric acid readily and completely
decomposes it, showing it to be a silicate of alumina and ferrous
oxide, with some magnesia and alkalies, but with no trace of lime.
The separated silica, which remains after the action of the acid,
is readily dissolved by a dilute solution of soda, leaving behind
nothing but angular and partially rounded grains of sand, chiefly
of colourless vitreous quartz. An analysis effected in the way just
described on 1·187 grammes gave the following results, which give, by
calculation, the centesimal composition of the mineral:--
Silica ·3290 38·93 = 20·77 oxygen·
Alumina ·2440 28·88 = 13·46 "
Protoxyd of iron ·1593 18·86}
Magnesia ·0360 4·25} = 6·29 "
Potash ·0140 1·69}
Soda ·0042 ·48}
Water ·0584 6·91 = 6·14 "
Insoluble, quartz ·3420
------ ------
1·1869 100·00
"A previous analysis of a portion of the mixture by fusion with
carbonate of soda gave, by calculation, 18·80 p. c. of protoxide of
iron, and amounts of alumina and combined silica closely agreeing
with those just given.
"The oxygen ratios, as above calculated, are nearly as 3 : 2 : 1 : 1.
This mineral approaches in composition to the jollyte of Von Kobell,
from which it differs in containing a portion of alkalies, and only
one half as much water. In these respects it agrees nearly with the
silicate found by Robert Hoffman, at Raspenau, in Bohemia, where it
occurs in thin layers alternating with picrosmine, and surrounding
masses of Eozoon in the Laurentian limestones of that region;[AD]
the Eozoon itself being there injected with a hydrous silicate which
may be described as intermediate between glauconite and chlorite in
composition. The mineral first mentioned is compared by Hoffman to
fahlunite, to which jollyte is also related in physical characters as
well as in composition. Under the names of fahlunite, gigantolite,
pinite, etc., are included a great class of hydrous silicates, which
from their imperfectly crystalline condition, have generally been
regarded, like serpentine, as results of the alteration of other
silicates. It is, however, difficult to admit that the silicate
found in the condition described by Hoffman, and still more the
present mineral, which injects the pores of palæozoic Crinoids, can
be any other than an original deposition, allied in the mode of its
formation, to the serpentine, pyroxene, and other minerals which have
injected the Laurentian Eozoon, and the serpentine and glauconite,
which in a similar manner fill Tertiary and recent shells."
[Footnote AD: _Journ. für Prakt. Chemie_, Bd. 106 (Erster Jahrgang,
1869), p. 356.]
(C.) Various Minerals filling Cavities of Fossils in the Laurentian.
The following on this subject is from a memoir by Dr. Hunt in the
_Twenty-first Report of the Regents of the University of New York_,
1874:--
"Recent investigations have shown that in some cases the
dissemination of certain of these minerals through the crystalline
limestones is connected with organic forms. The observations
of Dr. Dawson and myself on the Eozoon Canadense showed that
certain silicates, namely serpentine, pyroxene, and loganite,
had been deposited in the cells and chambers left vacant by the
disappearance of the animal matter from the calcareous skeleton of
the foraminiferous organism; so that when this calcareous portion is
removed by an acid there remains a coherent mass, which is a cast of
the soft parts of the animal, in which, not only the chambers and
connecting canals, but the minute tubuli and pores are represented
by solid mineral silicates. It was shown that this process must have
taken place immediately after the death of the animal, and must have
depended on the deposition of these silicates from the waters of the
ocean.
"The train of investigation thus opened up, has been pursued by
Dr. Gümbel, Director of the Geological Survey of Bavaria, who, in
a recent remarkable memoir presented to the Royal Society of that
country, has detailed his results.
"Having first detected a fossil identical with the Canadian Eozoon
(together with several other curious microscopic organic forms not
yet observed in Canada), replaced by serpentine in a crystalline
limestone from the primitive group of Bavaria, which he identified
with the Laurentian system of this country, he next discovered a
related organism, to which he has given the name of Eozoon Bavaricum.
This occurs in a crystalline limestone belonging to a series of rocks
more recent than the Laurentian, but older than the Primordial zone
of the Lower Silurian, and designated by him the Hercynian clay slate
series, which he conceives may represent the Cambrian system of Great
Britain, and perhaps correspond to the Huronian series of Canada and
the United States. The cast of the soft parts of this new fossil is,
according to Gümbel, in part of serpentine, and in part of hornblende.
"His attention was next directed to the green hornblende (pargasite)
which occurs in the crystalline limestone of Pargas in Finland, and
remains when the carbonate of lime is dissolved as a coherent mass
closely resembling that left by the irregular and acervuline forms
of Eozoon. The calcite walls also sometimes show casts of tubuli....
A white mineral, probably scapolite was found to constitute some
tubercles associated with the pargasite, and the two mineral species
were in some cases united in the same rounded grain.
"Similar observations were made by him upon specimens of coccolite
or green pyroxene, occurring in rounded and wrinkled grains in a
Laurentian limestone from New York. These, according to Gümbel,
present the same connecting cylinders and branching stems as the
pargasite, and are by him supposed to have been moulded in the
same manner.... Very beautiful evidences of the same organic
structure consisting of the casts of tubuli and their ramifications,
were also observed by Gümbel in a purely crystalline limestone,
enclosing granules of chondrodite, hornblende, and garnet, from
Boden in Saxony. Other specimens of limestone, both with and without
serpentine and chondrodite, were examined without exhibiting any
traces of these peculiar forms; and these negative results are
justly deemed by Gümbel as going to prove that the structure of
the others is really, like that of Eozoon, the result of the
intervention of organic forms. Besides the minerals observed in the
replacing substance of Eozoon in Canada, viz., serpentine, pyroxene,
and loganite, Gümbel adds chondrodite, hornblende, scapolite, and
probably also pyrallolite, quartz, iolite, and dichroite."
(D.) Glauconites.
The following is from a paper by Dr. Hunt in the _Report of the
Survey of Canada_ for 1866:--
"In connection with the Eozoon it is interesting to examine more
carefully into the nature of the matters which have been called
glauconite or green-sand. These names have been given to substances
of unlike composition, which, however, occur under similar
conditions, and appear to be chemical deposits from water, filling
cavities in minute fossils, or forming grains in sedimentary rocks
of various ages. Although greenish in colour, and soft and earthy
in texture, it will be seen that the various glauconites differ
widely in composition. The variety best known, and commonly regarded
as the type of the glauconites, is that found in the green-sand of
Cretaceous age in New Jersey, and in the Tertiary of Alabama; the
glauconite from the Lower Silurian rocks of the Upper Mississippi is
identical with it in composition. Analysis shows these glauconites to
be essentially hydrous silicates of protoxyd of iron, with more or
less alumina, and small but variable quantities of magnesia, besides
a notable amount of potash. This alkali is, however, sometimes
wanting, as appears from the analysis of a green-sand from Kent in
England, by that careful chemist, the late Dr. Edward Turner, and
in another examined by Berthier, from the _calcaire grossier_, near
Paris, which is essentially a serpentine in composition, being a
hydrous silicate of magnesia and protoxyd of iron. A comparison of
these last two will show that the loganite, which fills the ancient
Foraminifer of Burgess, is a silicate nearly related in composition.
I. Green-sand from the _calcaire grossier_, near Paris. Berthier
(cited by Beudant, _Mineralogie_, ii., 178).
II. Green-sand from Kent, England. Dr. Edward Turner (cited by
Rogers, Final Report, Geol. N. Jersey, page 206).
III. Loganite from the Eozoon of Burgess.
IV. Green-sand, Lower Silurian; Red Bird, Minnesota.
V. Green-sand, Cretaceous, New Jersey.
VI. Green-sand, Lower Silurian, Orleans Island.
The last four analyses are by myself.
I. II. III. IV. V. VI.
Silica 40·0 48·5 35·14 46·58 50·70 50·7
Protoxyd of iron 24·7 22·0 8·60 20·61 22·50 8·6
Magnesia 16·6 3·8 31·47 1·27 2·16 3·7
Lime 3·3 .... .... 2·49 1·11 ....
Alumina 1·7 17·0 10·15 11·45 8·03 19·8
Potash .... traces. .... 6·96 5·80 8·2
Soda .... .... .... ·98 ·75 ·5
Water 12·6 7·0 14·64 9·66 8·95 8·5
---- ---- ------ ------ ------ -----
98·9 98·3 100·00 100·00 100·00 100·0"
[Illustration:
Plate VI.
From a Photo. by Weston. Vincent Brooks, Day & Son Lith.
CANAL SYSTEM OF EOZOON.
SLICES OF THE FOSSIL (MAGNIFIED.)
_To face Chap. 6._]