Chapter V: 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.

(_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.]

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.

Upper Silurian Limestone, Pole Hill, New Brunswick. Magnified 25 diameters.]

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.

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.

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"

From a Photo. by Weston. Vincent Brooks, Day & Son Lith.

CANAL SYSTEM OF EOZOON.

SLICES OF THE FOSSIL (MAGNIFIED.)

_To face Chap. 6._]