"Specimens from Long Lake, in the collection of the Geological Survey of Canada, exhibit white crystalline limestone with light green compact or septariiform[W]serpentine, and much resemble some of the serpentine limestones of Grenville. Under the microscope the calcareous matter presents a delicate areolated appearance, without lamination; but it is not an example of acervuline Eozoon, but rather of fragments of such a structure, confusedly aggregated together, and having the interstices and cell-cavities filled with serpentine. I have not found in any of these fragments a canal system similar to that of Eozoon Canadense, though there are casts of large stolons, and, under a high power, the calcareous matter shows in many places the peculiar granular or cellular appearance which is one of the characters of the supplemental skeleton of that species. In a few places a tubulated cell-wall is preserved, with structure similar to that of Eozoon Canadense.
"Specimens from Long Lake, in the collection of the Geological Survey of Canada, exhibit white crystalline limestone with light green compact or septariiform[W]serpentine, and much resemble some of the serpentine limestones of Grenville. Under the microscope the calcareous matter presents a delicate areolated appearance, without lamination; but it is not an example of acervuline Eozoon, but rather of fragments of such a structure, confusedly aggregated together, and having the interstices and cell-cavities filled with serpentine. I have not found in any of these fragments a canal system similar to that of Eozoon Canadense, though there are casts of large stolons, and, under a high power, the calcareous matter shows in many places the peculiar granular or cellular appearance which is one of the characters of the supplemental skeleton of that species. In a few places a tubulated cell-wall is preserved, with structure similar to that of Eozoon Canadense.
[W]I use the term “septariiform” to denote thecurdledappearance so often presented by the Laurentian serpentine.
[W]I use the term “septariiform” to denote thecurdledappearance so often presented by the Laurentian serpentine.
“Specimens of Laurentian limestone from Wentworth, in the collection of the Geological Survey, exhibit many rounded silicious bodies, some of which are apparently grains of sand, or small pebbles; but others, especially when freed from the calcareous matter by a dilute acid, appear as rounded bodies, with rough surfaces, either separate or aggregated in lines or groups, and having minute vermicular processes projecting from their surfaces. At first sight these suggest the idea of spicules; but I think it on the whole more likely that they are casts of cavities and tubes belonging to some calcareous Foraminiferal organism which has disappeared. Similar bodies, found in the limestone of Bavaria, have been described by Gümbel, who interprets them in the same way. They may also be compared with the silicious bodies mentioned in a former paper as occurring in the loganite filling the chambers of specimens ofEozoonfrom Burgess.”These specimens will be more fully referred to underChapter VI.
“Specimens of Laurentian limestone from Wentworth, in the collection of the Geological Survey, exhibit many rounded silicious bodies, some of which are apparently grains of sand, or small pebbles; but others, especially when freed from the calcareous matter by a dilute acid, appear as rounded bodies, with rough surfaces, either separate or aggregated in lines or groups, and having minute vermicular processes projecting from their surfaces. At first sight these suggest the idea of spicules; but I think it on the whole more likely that they are casts of cavities and tubes belonging to some calcareous Foraminiferal organism which has disappeared. Similar bodies, found in the limestone of Bavaria, have been described by Gümbel, who interprets them in the same way. They may also be compared with the silicious bodies mentioned in a former paper as occurring in the loganite filling the chambers of specimens ofEozoonfrom Burgess.”
These specimens will be more fully referred to underChapter VI.
(D.)Additional Structural Facts.
I may mention here a peculiar and interesting structure which has been detected in one of my specimens while these sheets were passing through the press. It is an abnormal thickening of the calcareous wall, extending across several layers, and perforated with large parallel cylindrical canals, filled with dolomite, and running in the direction of the laminæ; the intervening calcite being traversed by a very fine and delicate canal system. It makes a nearer approach to some of the Stromatoporæ mentioned inChapter VI.than any other Laurentian structure hitherto observed, and may be either an abnormal growth of Eozoon, consequent on some injury, or a parasitic mass of some Stromatoporoid organism overgrown by the laminæ of the fossil. The structure of the dolomite in this specimen indicates that it first lined the canals, and afterward filled them; an appearance which I have also observed recently in the larger canals filled with serpentine (Plate VIII., fig. 5). The cut below is an attempt, only partially successful, to show the Amœba-like appearance, when magnified, of the casts of the chambers of Eozoon, as seen on the decalcified surface of a specimen broken parallel to the laminæ.
I may mention here a peculiar and interesting structure which has been detected in one of my specimens while these sheets were passing through the press. It is an abnormal thickening of the calcareous wall, extending across several layers, and perforated with large parallel cylindrical canals, filled with dolomite, and running in the direction of the laminæ; the intervening calcite being traversed by a very fine and delicate canal system. It makes a nearer approach to some of the Stromatoporæ mentioned inChapter VI.than any other Laurentian structure hitherto observed, and may be either an abnormal growth of Eozoon, consequent on some injury, or a parasitic mass of some Stromatoporoid organism overgrown by the laminæ of the fossil. The structure of the dolomite in this specimen indicates that it first lined the canals, and afterward filled them; an appearance which I have also observed recently in the larger canals filled with serpentine (Plate VIII., fig. 5). The cut below is an attempt, only partially successful, to show the Amœba-like appearance, when magnified, of the casts of the chambers of Eozoon, as seen on the decalcified surface of a specimen broken parallel to the laminæ.
Fig. 21a.
Fig. 21a.
Plate V.Nature-print of Eozoon, showing laminated, acervuline, and fragmental portions.This is printed from an electrotype taken from an etched slab of Eozoon, and not touched with a graver except to remedy some accidental flaws in the plate. The diagonal white line marks the course of a calcite vein.
Plate V.
Nature-print of Eozoon, showing laminated, acervuline, and fragmental portions.This is printed from an electrotype taken from an etched slab of Eozoon, and not touched with a graver except to remedy some accidental flaws in the plate. The diagonal white line marks the course of a calcite vein.
Nature-print of Eozoon, showing laminated, acervuline, and fragmental portions.
This is printed from an electrotype taken from an etched slab of Eozoon, and not touched with a graver except to remedy some accidental flaws in the plate. The diagonal white line marks the course of a calcite vein.
CHAPTER V.THE PRESERVATION OF EOZOON.
Perhapsnothing 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 recentspecimens, 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 infig. 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.
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.
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 todetect 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 infig. 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 infig. 24shows 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.
[X]Canadian Naturalist, 1859; Microscopic Structure of Canadian Limestones.
[X]Canadian Naturalist, 1859; Microscopic Structure of Canadian Limestones.
Fig. 23.Slice of Crystalline Lower Silurian Limestone; showing Crinoids, Bryozoa, and Corals in fragments.
Fig. 23.Slice of Crystalline Lower Silurian Limestone; showing Crinoids, Bryozoa, and Corals in fragments.
Fig. 24.Wall of Eozoon penetrated with Canals. The unshaded portions filled with Calcite.(After Carpenter.)
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 chambersare 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 Foraminiferalshells, 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 havebeen 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 inPlate. 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 ofPlate. 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.andfig. 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 underthe 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, sothat 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. Norneed we be astonished at the fineness of the infiltration by which these minute tubes, perhaps110000of 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. 25represents a microscopic joint of a Crinoid from the Upper Silurian of New Brunswick, injected with the hydrous silicate already referred to, andfig. 26shows a microscopicchambered or spiral shell, from a Welsh Silurian limestone, with its cavities filled with a similar substance.
Fig. 25.Joint of a Crinoid, having its pores injected with a Hydrous Silicate.Upper Silurian Limestone, Pole Hill, New Brunswick. Magnified 25 diameters.
Fig. 25.Joint of a Crinoid, having its pores injected with a Hydrous Silicate.
Upper Silurian Limestone, Pole Hill, New Brunswick. Magnified 25 diameters.
Fig. 26.Shell from a Silurian Limestone, Wales; its cavity filled with a Hydrous Silicate.Magnified 25 diameters.
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 inthe 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 markedand 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. Inspecimens 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.
Fig. 27.Diagram showing the different appearances of the cell-wall of Eozoon and of a vein of Chrysotile, when highly magnified.
Fig. 27.Diagram showing the different appearances of the cell-wall of Eozoon and of a vein of Chrysotile, when highly magnified.
Fig. 28.Casts of Canals of Eozoon in Serpentine, decalcified and highly magnified.
Fig. 28.Casts of Canals of Eozoon in Serpentine, decalcified and highly magnified.
Fig. 29.Canals of Eozoon.Highly magnified.
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 ingeniousmanner, 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, presentsa 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.
Fig. 30.Eozoon from Tudor.Two-thirds natural size. (a.) Tubuli. (b.) Canals. Magnified.aandbfrom another specimen.
Fig. 30.Eozoon from Tudor.
Two-thirds natural size. (a.) Tubuli. (b.) Canals. Magnified.aandbfrom 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 werebroken 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.
[Y]SeeNote B, Chap. III.
[Y]SeeNote 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 venerablerocks 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 inthe 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 atpresent, 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 theJournal 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 softparts 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 contactof 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 pureserpentine; 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, inLower 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 ofForaminifera, but of spines ofEchinus, 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 ofForaminifera, 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 ofMillepores, and in the canals in the shells ofBalanus. According to him, the glauconite grains formed inForaminiferalose 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]
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 theJournal 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 softparts 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 contactof 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 pureserpentine; 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, inLower 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 ofForaminifera, but of spines ofEchinus, 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 ofForaminifera, 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 ofMillepores, and in the canals in the shells ofBalanus. According to him, the glauconite grains formed inForaminiferalose 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]