VII
THE DAWN OF LIFE
I
IN the Grenvillian system, as represented in the vicinity of the Ottawa River, perfect specimens of Eozoon are found in one only of the principal limestones there exposed, and in certain layers of that limestone, and they are associated with concretions and grains of the greenish mineral serpentine, which, as we shall see, has much to do with their preservation. As exposed on broken surfaces, the specimens consist of concentric layers of greenish serpentine and white calcite, not, however, even or uniform, as in ordinary concretions having concentric structure, but often approaching and uniting with each other, so as to constitute wide flat chambers, and forming patches from an inch to nearly a foot in diameter, while some of the larger patches seem to coalesce or to become confluent. On weathered surfaces the serpentine laminæ often become brown, owing to the rusting of the iron contained in them, and project above the general surface, in this case resembling very much the appearance of the layer-corals so plentiful in some limestones of later date.
The external forms of Eozoon are at first sight not very obvious, as they adhere very closely to the containing rock; but the smaller specimens, when entirely weathered out or disengaged by the solution of the limestone in an acid, usually present the form of a broad inverted cone, like some modern sponges or the broader turbinate fossil corals ([Fig. 32]). The limestone having, like the other beds of the formation, been much compressed and folded, the specimens of Eozoon are sometimes crumpled in these folds or broken across by small cracks or faults, which shift the laminæ slightly out of their places. The cracks thus formed are also sometimes filled with a fibrous variety of serpentine, known to mineralogists as chrysotile and popularly as "rock cotton" or "asbestus." It is finely fibrous, and of a silky lustre, and must have been deposited by water in the cracks and fissures formed by the fracturing of the rock and the contained fossils, by movements taking place after the whole was hardened. Accordingly these veins often cross not only the rock, but also the serpentine and calcite layers of the contained masses of Eozoon, without regard to the direction of their laminæ, though sometimes they run parallel to the structure, the rock having broken more easily in that direction.
Fig. 32.—Entire specimen of Eozoon, disengaged from the matrix and showing its turbinate form, enclosed in the outline of a larger specimen of similar form.
Both natural size, Côte St. Pierre. (Specimens in Peter Redpath Museum.)
Bearing in mind these general points of material form and appearance, we may now proceed to inquire as to the following points: (1) The structures visible in the specimens; (2) The manner in which they are represented by different mineral substances, and how these are to be accounted for; (3) The explanation of the whole on the supposition that we are dealing with an animal fossil.
(1) In regard to the first of these questions, I may quote here, with some slight alteration, from a recent memoir of my own[29]:—
[29] London Geological Magazine, 1895.
In recent years I have been disposed to attach more importance than formerly to the general form of Eozoon. The earlier examples studied were, for the most part, imbedded in the limestone in such a manner as to give little definite information as to external form; and at a later date, when Sir William Logan employed one of his assistants, Mr. Lowe, to quarry large specimens at Grenville and Côte St. Pierre, the attempt was made to secure the most massive blocks possible, in order to provide large slabs for showy museum specimens.
Fig. 33.—Weathered surface of Eozoon.
Showing sections of two funnels or tubes with limiting walls, Côte St. Pierre.
More recently, when collections have been made from the eroded and crumbling surfaces of the limestone in its wider exposures, it was found that specimens of moderate size had been weathered out, and could, either naturally or by treatment with acid, be entirely separated from the matrix. Such specimens sometimes showed, either on the surfaces or on the sides of "funnels" and tubes penetrating the mass (Figs. [33], [34]), a confluence of the laminæ, constituting a porous cortex or limiting structure. Specimens of this kind were figured in 1888, and I was enabled to add to the characters of the species that the original and proper form was "broadly turbinate with a depression or cavity above, and occasionally with oscula or pits penetrating the mass." The great flattened masses thus seemed to represent confluent or overgrown individuals, often contorted by the folding of the enclosing beds.
Fig. 34.—Section of the Base of a specimen of Eozoon.
This specimen shows an oscuilform, cylindrical funnel, cut in such a manner as to show its reticulated wall and the descent of the laminæ toward it. Two-thirds of natural size. From a photograph. Col. Carpenter, also in Redpath Museum.
[This illustration (from Prof. Prestwich's "Geology," vol. ii. p. 21) has been courteously lent by the Clarendon Press, Oxford.
There are also in well-preserved specimens certain constant properties of the calcite and serpentine layers. The former are continuous, and connected at intervals, so that if the silicious filling of the chambers could be removed, the calcareous portion would form a continuous skeleton, while the serpentine filling the chambers, when the calcareous plates are dissolved out by an acid, forms a continuous cast of the animal matter filling the chambers ([Fig. 36]). This cast of the sarcodous material, when thus separated, is very uniformly and beautifully mammillated on the surfaces of the laminæ, and this tuberculation gradually passes upward into smaller chambers having amœboid outlines, and finally into rounded chamberlets. It is also a very constant point of structure that the lower laminæ of calcite are thicker than those above, and have the canal-systems larger and coarser. There is thus in the more perfect specimens a definite plan of macroscopical structure ([Fig. 35]).
Fig. 35.—Structure of small specimen of Eozoon, calcareous matter removed.
1. Natural size. 2. Acervuline cells of upper part. 3. Group of the same coalescing into a lamina with tuberculated surface. 4. Laminæ with tuberculated surfaces in section. (See also [Fig. 36].)
Fig. 36.—Decalcified Eozoon, in section, slightly enlarged. Showing the character of the sarcodous laminæ now replaced by Serpentine.
The normal mode of mineralization at Côte St. Pierre and Grenville is that the laminæ of the test remain as calcite, while the chambers and larger canals are filled with serpentine of a light green or olive colour, and the finer tubuli are injected with dolomite. It may also be observed that the serpentine in the larger cavities often shows a banded structure, as if it had been deposited in successive coats, and the canals are sometimes lined with a tubular film of serpentine, with a core or axis of dolomite, which also extends into the finer tubuli of the surfaces of the laminæ. This, on the theory of animal origin, is the most perfect state of preservation, and it equals anything I have seen in calcareous organisms of later periods. This state of perfection is, however, naturally of infrequent occurrence.
Fig. 37.—Finest Tubuli filled with Dolomite (magnified).
The finer tubuli are rarely perfect or fully infiltrated. Even the coarser canals are not infrequently imperfect, while the laminæ themselves are sometimes crumpled, crushed, faulted, or penetrated with veins of chrysotile or of calcite. In some instances the calcareous laminæ are replaced by dolomite, in which case the canal-systems are always imperfect or obsolete. The laminæ of the test itself are also in some cases replaced by serpentine in a flocculent form. At the opposite extreme are specimens, or portions of specimens, in which the chambers are obliterated by pressure, or occupied only with calcite. In such cases the general structure is entirely lost to view, and scarcely appears in weathering. It can be detected only by microscopic examination of slices, in parts where the granular structure or the tubulation of the calcite layers has been preserved. All palæontologists who have studied silicified fossils in the older rocks are familiar with such appearances.
Fig. 38.—Plan of arrangement of Canals in Lamina of Eozoon.
It has been alleged by Möbius and others that the canal-systems and tubes present no organic regularity. This difficulty, however, arises solely from imperfect specimens or inattention to the necessary results of slicing any system of ramifying canals. In Eozoon the canals form ramifying groups in the middle planes of the laminæ, and proceed at first almost horizontally, dividing into smaller branches, which ultimately give off brushes of minute tubuli running nearly at right angles to the surfaces of the lamina, and forming the extremely fine tubulation which Dr. Carpenter regarded as the proper wall (Figs. [38], [39]).
Fig. 39.—Cross section of minute Tubuli, about 5 microms in diameter (magnified).
In my earlier description I did not distinguish this from the canal-system, with which its tubuli are inwardly continuous. Dr. Carpenter, however, understood this arrangement, and has represented it in his figures[30] (see also [Fig. 28]). It is evident that in a structure like this a transverse or oblique section will show truncated portions of the larger tubes apparently intermixed with others much finer and not continuous with them, except very rarely. Good specimens and many slices and decalcified portions are necessary to understand the arrangement This consideration alone, I think, entirely invalidates the criticisms of Möbius, and renders his large and costly figures of little value, though his memoir is, as I have elsewhere shown, liable to other and fatal objections.[31]
[30] "Ann. and Mag. Nat. Hist.," ser. 4, xiii., p. 456, figs. 3, 4.
[31] "Museum Memoir," pp. 50 et seq.
It has been pretended that the veins of chrysotile, when parallel to the laminæ, cannot be distinguished from the minute tubuli terminating on the surfaces of the laminæ. I feel confident, however, that no microscopist who has seen both, under proper conditions of preservation and study, could confound them. The fibres of chrysotile are closely appressed parallel prisms, with the optical properties of serpentine. The best preserved specimens of the "proper wall" contain no serpentine, but are composed of calcite with extremely minute parallel cylinders of dolomite about five to ten microms. in diameter, and separated by spaces greater than their own diameter (Figs. [40], [41]). In the rare cases where the cylinders are filled with serpentine, they are, of course, still more distinct and beautiful. At the same time, I do not doubt that observers who have not seen the true tubulation may have been misled by chrysotile veins when these fringe the laminæ. Möbius, for instance, figures the true and false structure as if they were the same.
Fig. 40.—Cross section of similar Tubuli to those in [Fig. 39], more highly magnified, and showing granular character of the test.
(From camera tracings.)
Fig. 41.—Comparison of Tubulate Wall and Prisms of Chrysotile in perspective.
|
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| Canals of Eozoon. (After Möbius.) | Finer Canals of Eozoon. (After Möbius.) |
|
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| Canals of modern Calcarina. (After Carpenter.) | Canals and Tubule of Tertiary Nummulina. (After Möbius.) |
| Fig. 42. Figures selected from Möbius, to show the resemblance of structures of Eozoon to those of modern Foraminifera. | |
Protest should here be made against that mode of treating ancient fossils which regards the most obscure or defaced specimens as typical, and those better preserved as mere accidents, of mineral structure. In Tertiary Nummulites injected with glauconite it is rare to find the tubuli perfectly filled, except in tufts here and there; yet no one doubts that these patches represent a continuous structure.
I have remarked on previous occasions that the calcite constituting the laminæ of Eozoon often has a minutely granular appearance, different from that of the surrounding limestone. Under a high power it resolves itself into extremely minute dots or flocculi, somewhat uniformly diffused. Whether these dots are particles of carbon, iron, apatite, or silicious matter, or the remains of a porous structure, I do not know; but similar appearances occur in the calcareous fossils contained in altered limestones of later date. Wherever they occur in crystalline limestones, supposed to be organic, the microscopist should examine them with care. I have sometimes by this appearance detected fragments of Eozoon which afterward revealed their canals.
(2) The second question requires us to consider the nature and origin of the substances constituting the specimens. Reference has already been made to these in our fifth chapter, but they may be more particularly noticed here in connection with the forms as above described.
The calcareous laminæ are usually composed of clear translucent calcite or calcium carbonate, though, as in the case of many later fossils, sometimes replaced by dolomite. It often has the fine granular appearance above referred to, but is nearly always crystalline, and traversed by cleavage planes visible under the microscope.[32] This crystalline structure, as every student of fossils knows, is very common in calcareous fossils of all geological ages. In the thicker laminæ the canals traversing them and branching out in their substance are usually visible under a low power, except when they are filled with calcite similar to that of the laminæ themselves. In this case they can be seen only by very careful management of an oblique and subdued light. When occupied with serpentine, this presents, in a thin slice under transmitted light, a yellowish or brownish colour, and in a specimen decalcified with an acid an opaque white appearance. In some of the larger threads of serpentine, as already stated, this mineral forms a thin outer cylinder with a core of calcite or dolomite within; but this appearance is not common. Here and there, especially in the lower layers, a portion of a tube is filled with the harder mineral pyroxene, which is in some respects similar to serpentine, except that it contains lime as well as magnesia, and is destitute of water as an ingredient The finer tubuli into which the canals ramify are most usually filled with dolomite or magnesian limestone, which has a glossy appearance and higher lustre than the surrounding calcite, and so may be distinguished even in a transparent slice; but these fine dolomite threads are best seen when the surface of a slice is treated with a dilute acid in the cold, in which circumstances the calcite is dissolved, while the dolomite remains as tufts of delicate cylindrical hairs, presenting often a very beautiful appearance under the microscope. Thus, as in many other fossils, what are supposed to have been tubes and tubuli are found not empty, but filled with matter even harder and more resisting than the shell itself.
[32] Especially when the specimen has been heated or jarred in the process of grinding or polishing.
Serpentine is a mineral which has been produced in different ways. Some igneous or volcanic rocks consist largely of compounds of silica and magnesia (olivine, etc.). When these rocks have become cold and are exposed to the action of water, they sometimes absorb this and become hydrated, thus passing into a kind of serpentine. When such rocks are pulverized and dispersed as volcanic ash, this falling into the sea may be there hydrated, and may form serpentinous layers, or in a fine paste or in solution may pass into the pores and cavities of shells and other organic things, acting, as we have seen, in the same manner with ordinary glauconite. In like manner serpentine of this origin may form nodules or grains in limestones, in consequence of its particles being aggregated together by concretionary attraction. We have already seen that some comparatively modern so-called glauconites are essentially of the nature of serpentine, and we know that in the old Laurentian sea, salts of magnesia and magnesian minerals were abundant, so that serpentinous minerals might play a greater part than they do in the modern seas. Loganite, the mineralizing substance of the Burgess Eozoon, is different from serpentine, yet closely allied to the glauconites. The presence of pyroxene may be explained in a similar way. It is a frequent constituent of bedded volcanic rocks and of volcanic ashes, and beds of it occur in the Grenville series which once, no doubt, were ash-beds. Layers of it also occasionally occur from a similar cause in the limestone, and crystals of it have been deposited by water in the veins passing through the limestones and schists. Dr. Johnston-Lavis has described in the July number of the Geological Magazine for 1895 the aqueous deposition at ordinary temperature of crystals of pyroxene and hornblende, in cavities and crevices of bones included in an ash-bed of recent date, and in presence of calcite, apatite, and fluoride of calcium, as in the Grenville series. This is a modern instance analogous to that suggested above. Hence all these minerals filling the cavities and canals of Eozoon may have been deposited by water at ordinary temperatures, and have no connection with the alteration to which the beds have been subsequently subjected.
I may add here that a Tertiary glauconite from the Calcaire Grossier of Paris analysed by Berthier[33] is essentially a serpentine composed of silicate of iron and magnesia, that Loganite as analysed by Hunt contains thirty-one per cent, of magnesia, and that Hoskins has shown[34] that modern glauconites often contain large proportions of magnesia and equivalent bases.
[33] Beudant, Mineralogie, xi. 178.
[34] Geological Magazine, July, 1895.
It is also to be observed that independently of volcanic debris the reports of the Challenger expedition show that in the deep seas the decay of organic matter causes an alkaline condition of the sediments leading to the formation of alkaline silicates, while the presence of decaying volcanic dust furnishes the basis, whether of iron, alumina, or magnesia, necessary for the making up of glauconite. I have also suggested that the assimilation by Protozoa making calcareous skeletons, of the matter of Diatoms or humble plants having soluble silica in their organization or of silicious Protozoa, and sponge germs, must set free much soluble silica as a rejected or excrementitious matter which may contribute to the same result.
It is much more likely that the serpentine of the Laurentian limestones was produced in these ways than that it resulted from the hydration of magnesian minerals after the rock was consolidated. In the former case it would be in the most favourable conditions for mineralizing organisms as glauconites do in the modern seas. In the latter it would cause disturbances and changes of volume of which we have no evidence.
We thus find that the chemistry of the modern seas and that relating to the preservation of fossils of various ages by silicious infiltrations lends great probability to the belief that serpentine played this role in the oldest seas, though it would seem that dolomite was more suitable to the filling of the extremities of the minute tubes and their finer terminations.[35]
[35] I have shown also that in the limestone containing Eozoon we find layers holding concretions of serpentine alternating with others holding crystals of dolomite, as if there were at some times conditions favourable to the deposition of silicate of magnesia, and at others to that of the carbonate.
Fig. 43.—Stromatocerium rugosum, Hall, Ordovician.
(3) Our third question leads to the inquiry in what modern or ancient marine animals we can find structures akin to those of our supposed Laurentian fossil. The first analogy which suggested itself to Sir W. Logan, and a very natural one, was that to the so-called layer-corals (Figs. [43] to [45]) that abound in the Silurian, Ordovician, and Cambrian rocks, and which though undoubtedly fossil animals, have proved very difficult to interpret or to assign to any known group. At first vaguely associated with the true corals, they were subsequently regarded as probably of more simple character, and as gigantic Protozoa; and later strong reasons have been assigned for giving them an intermediate place, as allied to those curious communities of humble animals possessing simple stomachs and prehensile tentacles (Hydroids) which form some of the simpler corals (Millepores, etc.), and the crusts (Hydractiniæ) which cover dead shells and other bodies in the sea. When examined microscopically, however, they differ very much among themselves, and it may be that some of them were Hydroids and some Protozoa.
Fig. 44.—Structures of Stromatopora.
(a) Portion of oblique section, (b) Wall with pores, and coated with crystals of quartz, (c) Thickened portion of wall with canals, (d) Laminæ and pillars.
Fig. 45.—Tubular Structure of Cœnostroma, Silurian.
The oldest that we at present know, and consequently the nearest in time to Eozoon, impress us rather with the latter affinity. They are the fossils of the genus Cryptozoon of Hall ([Fig. 7][36]), which form great masses filling certain beds of Upper Cambrian age, and which, when sliced and studied microscopically, are found to consist of concentric thin laminæ filled in between with a porous mass of calcareous matter penetrated by an infinity of tortuous tubes. Forms of this kind have been traced downward into pre-Cambrian beds in Colorado, and as we shall find in New Brunswick, into the Upper Laurentian itself.
[36] See Figs. [7] and [7a], pp. [37], [38]; also [Fig. 8] and Microscopic slice, [Fig. 59], at end.
They present, however, structural differences from Eozoon, which rather conforms to the arrangements found in some Protozoa of smaller size, and which, under the name of Foraminifera, have abounded in all geological periods, and are excessively abundant in the modern ocean. They may be defined as animals composed of a soft and apparently homogeneous animal jelly known as protoplasm or sarcode. When carefully examined, however, it is found to have a granular texture and to be divisible into two layers, an outer and an inner, while it possesses a little hollow vessel capable of expanding and absorbing the liquid matter of the enclosing protoplasm, and of contracting so as to expel its contents. This seems to be the only organ of circulation and excretion. There are, however, small cells or reproductive bodies in the interior, varying in number, size, and development in different forms. The most remarkable property of these creatures is that of stretching out from the surface of the body threads or projections of the protoplasm,[37] often of considerable length, and which serve at once as organs of locomotion and prehension.
[37] Known as Pseudopodia.
Amœba. |
Actinophrys. |
| From original sketches. | |
Biloculina. A many-chambered Foraminifer. |
Polystomella. A spiral Foraminifer. |
Fig. 46.—Recent Protozoa. | |
These creatures are in some respects the simplest of animals, yet in other respects they present strange complexities. This is more especially evident in their tests or coverings, made for the most part of limestone or calcium carbonate, but sometimes of grains of fine sand cemented together. These coverings are always perforated with at least one orifice for the emission of the thread-like processes or pseudopods, and often with a vast number of small pores for the same purpose. Sometimes the test or shell is smooth, sometimes beautifully sculptured externally. Sometimes it consists of a single chamber like a ball or vase. More often, as the animals increase in size, they form additional chambers, and the body thus becomes divided into lobes connected with each other by necks passing through orifices in the partitions. The chambers are arranged in rows or in spirals, and in other ways, giving a vast variety of forms, often presenting the most beautiful patterns executed in the purest white marble, and the ornamental parts constitute thickenings of the walls giving greater strength, and are penetrated with microscopic canals communicating with the soft substance of the animal.
These creatures abound in all parts of the ocean, from the surface to the greatest depths. The Foraminifera have also existed from the earliest geological times, and in all the long ages of the earth's history seem to have retained the same structures and even ornamentation; so that species from very old geological formations are often scarcely distinguishable from those now living, and must have played precisely the same parts in the system of nature. One of these functions is that of accumulating great thicknesses of calcareous matter in the sea-bottom.
The manner in which such accumulation takes place we learn from what is now going on in the ocean, more especially from the result of the recent deep-sea dredging expeditions. The Foraminifera are vastly numerous, both near the surface and at the bottom of the sea, and multiply rapidly; and as successive generations die, their shells accumulate on the ocean bed, or are swept by currents into banks, and thus in process of time constitute thick beds of white chalky material, which may eventually be hardened into limestone. This process is now depositing a great thickness of white ooze in the bottom of the ocean; and in times past it has produced such vast thicknesses of calcareous matter as the chalk and the nummulitic limestone of Europe and the orbitoidal limestone of America. The chalk, which alone attains a maximum thickness of 1,000 feet, and, according to Lyell, can be traced across Europe for 1,100 geographical miles, may be said to be entirely composed of shells of Foraminifera imbedded in a paste of still more minute calcareous bodies, the Coccoliths, which are probably products of marine vegetable life, if not of some animal organism still simpler than the Foraminifera.
There are, however, some sessile examples of these animals which attain to larger dimensions than the free and locomotive forms. As an example of these we may take the Polytrema, which forms little hard red lumps on West Indian corals. Such a creature, beginning life as a little round spot of protoplasm, almost invisible, and protected with a little dome of carbonate of lime for the extension of its pseudopods as it grows in size, adds chamber to chamber in successive tiers till it assumes an appreciable size, all the chambers communicating with each other, while the outer ones are perforated with pores for extension of the pseudopods. In one form (Carpenteria) the same end is secured by leaving an open space in the middle of the conical mass like the crater of a small volcano. It is with these larger and sessile forms that we must compare Eozoon, though some of its minute structures rather resemble those of some smaller types.
All the creatures referred to above, notwithstanding the differences in their skeletons, resemble each other very closely in their soft parts, and come under the general name of Foraminifera, a name having reference to the openings by which the animal matter within communicates with the water without, for nutrition and respiration. Such creatures may be regarded as the simplest and most ready media for the conversion of vegetable matter into animal tissues, and their functions are almost entirely limited to those of nutrition. Hence it is likely that they will be able to appear in the most gigantic forms under such conditions as afford them the greatest amount of pabulum for the nourishment of their soft parts and for their skeletons. There is reason to believe, for example, that the occurrence, both in the chalk and the deep-sea mud, of immense quantities of the minute oval bodies known as Coccoliths along with Foraminifera, is not accidental. The Coccoliths appear to be grains of calcareous matter formed in minute plants adapted to a deep-sea habitat; and these, along with the vegetable and animal debris constantly being derived from the death of the living things at the surface, and falling to the bottom, afford the material both of sarcode and shell. Now if the Laurentian graphite represents an exuberance of vegetable growth in those old seas proportionate to the great supplies of carbonic acid in the atmosphere and in the waters, and if the Eozoic ocean was even better supplied with carbonate of lime than those Silurian seas whose vast limestones bear testimony to their richness in such material, we can easily imagine that the conditions may have been more favourable to a creature like Eozoon than those of any other period of geological time.
Growing, as Eozoon may be supposed to have done, on the floor of the ocean, and covering wide patches with more or less irregular masses, it must have thrown up from its whole surface its pseudopods to seize whatever floating particles of food the waters carried over it There is also reason to believe, from the outline of certain specimens, that it often grew upward in inverted, conical, or club-shaped forms, and that only the broader patches were penetrated by the tubes or oscula already mentioned, admitting the sea-water deeply into the substance of the masses. In this way its growth might be rapid and continuous; but it does not seem to have possessed the power of growing indefinitely by new and living layers covering those that had died, in the manner of some corals. Its life seems to have had a definite termination, and when that was reached, an entirely new colony had to be commenced. In this it had more affinity with the Foraminifera, as we now know them, than with the corals, though practically it had the same power with the coral polyps of accumulating limestone in the sea-bottom, a power indeed still possessed by its foraminiferal successors. In the case of coral limestones, we know that a large proportion of these consist, not of continuous reefs, but of fragments of coral mixed with other calcareous organisms, spread usually by waves and currents in continuous beds over the sea-bottom. In like manner we find in the limestones containing Eozoon, layers of fragmental matter which shows in places the characteristic structures, and which evidently represents the debris swept from the Eozoon masses and reefs by the action of the waves. With this fragmental matter small rounded organisms to be noticed in the sequel occur; and while they may be distinct animals resembling the smaller modern species, they may also be the fry of Eozoon, or small portions of its acervuline upper surface floated off in a living state, and possibly capable of living independently and of founding new colonies.
Fig. 47.—Slice of Limestone (magnified),
(a) Fragment of Eozoon with canals, (b) Fragments of granular calcite, probably organic, (c) Structureless calcite with cleavage lines (Côte St. Pierre).
It is only by a somewhat wild poetical licence that Eozoon has been represented as a "kind of enormous composite animal stretching from the shores of Labrador to Lake Superior, and thence northward and southward to an unknown distance, and forming masses 1,500 feet in depth." We may discuss by-and-by the question of the composite nature of masses of Eozoon, and we see in the corals evidence of the great size to which composite animals of a higher grade can attain. In the case of Eozoon we must imagine an ocean floor more uniform and level than that now existing. On this the organism would establish itself in spots and patches. These might finally become confluent over large areas, just as massive corals do. As individual masses attained maturity and died, their pores would be filled up with limestone or silicious deposits, and thus could form a solid basis for new generations, and in this way limestone to an indefinite extent might be produced. Further, wherever such masses were high enough to be attacked by the breakers, or where portions of the sea-bottom were elevated, the more fragile parts of the surface would be broken up and scattered widely in beds of fragments over the bottom of the sea, while here and there beds of mud or sand or of volcanic debris would be deposited over the living or dead organic mass, and would form the layers of gneiss and other schistose rocks interstratified with the Laurentian limestone. In this way, in short, Eozoon would perform a function combining that which corals and Foraminifera perform in the modern seas; forming both reef limestones and extensive chalky beds, and probably living both in the shallow and the deeper parts of the ocean. If in connection with this we consider the rapidity with which the soft, simple, and almost structureless sarcode of these Protozoa can be built up, and the probability that they were more abundantly supplied with food, both for nourishing their soft parts and skeletons, than any similar creatures in later times, we can readily understand the great volume and extent of the Laurentian limestones which they aided in producing. I say aided in producing, because I would not desire to commit myself to the doctrine that the Laurentian limestones are wholly of this origin. There may have been other animal limestone-builders than Eozoon, and there may have been limestones formed by plants like the modern Nullipores or by merely mineral deposition.
Its relations to modern animals of its type have been very clearly defined by Dr. Carpenter. In the structure of its proper wall and its fine parallel perforations, it resembles the Nummulites and their allies (Figs. [48], [49]); and the organism may therefore be regarded as an aberrant member of the Nummuline group, which affords some of the largest and most widely distributed of the fossil Foraminifera. This resemblance may be seen in [Fig. 48].
Fig. 48.—Section of a Nummulite, from Eocene Limestone of Syria.
Showing chambers, tubuli, and canals. Compare this and Fig. 49 with Figs. [28] and [29].
Fig. 49.—Portion of Shell of Calcarina.
Magnified, after Carpenter, (a) Cells. (b) Original cell-wall with tubuli. (c) Supplementary skeleton with canals.
To the Nummulites it also conforms in its tendency to form a supplemental or intermediate skeleton with canals, though the canals themselves in their arrangement more nearly resemble Calcarina, which is represented in [Fig. 49]. In its superposition of many layers, and in its tendency to a heaped-up or acervuline irregular growth it resembles Carpenteria, Polytrema and Tinoporus, forms of a different group in so far as shell-structure is concerned. The large and curious sandy Foraminifer from the Pacific dredged by Alexander Agassiz, and named by Goës, Neusina Agassizi, may also be mentioned as presenting some points of resemblance.[38] It may thus be regarded as a composite type, combining peculiarities now observed in two groups, or it may be regarded as a representative in the Nummuline series of Polytrema and Tinoporus in the Rotaline series. At the time when Dr. Carpenter stated these affinities, it might be objected that Foraminifera of these families are in the main found in the Modern and Tertiary periods. Dr. Carpenter has since shown that the curious oval Foraminifer called Fusulina, found in the coal formation, is in like manner allied to both Nummulites and Rotalines; and still more recently Mr. Brady has discovered a true Nummulite in the Lower Carboniferous of Belgium. This group being now fairly brought down to the Palæozoic, we may hope finally to trace it back to the Primordial, and thus to bring it still nearer to Eozoon in time.
[38] Bulletin Mus. Comp. Zoology, vol. xxiii., No. 5, Dec., 1892.
Though Eozoon was probably not the only animal of the Laurentian seas, yet it was in all likelihood the most conspicuous and important as a collector of calcareous matter, filling the same place afterwards occupied by the reef-building corals. Though probably less efficient than these as a constructor of solid limestones, from its less permanent and continuous growth, it formed wide floors and patches on the sea-bottom, and when these were broken up vast quantities of limestone were formed from their debris. It must also be borne in mind that Eozoon was not everywhere infiltrated with serpentine or other silicious minerals; quantities of its substance were merely filled with carbonate of lime, resembling the chamber-wall so closely that it is nearly impossible to make out the difference, and thus is likely to pass altogether unobserved by collectors, and to baffle even the microscopist. Although therefore the layers which contain well-characterized Eozoon are few and far between, there is reason to believe that in the composition of the limestones of the Laurentian it bore no small part; and as these limestones are some of them several hundreds of feet in thickness, and extend over vast areas, Eozoon may be supposed to have been as efficient a world-builder as the Stromatoporæ of the Silurian and Devonian, the Globigerinæ and their allies in the chalk, or the Nummulites and Miliolites in the Eocene. It is a remarkable illustration of the constancy of natural causes and of the persistence of animal types, that these humble Protozoans, which began to secrete calcareous matter in the Laurentian period, have been continuing their work in the ocean through all the geological ages, and are still busy in accumulating those chalky muds with which recent dredging operations in the deep sea have made us so familiar.
Fig. 50.—Figures of Archæospherinæ.
(1) Specimen with tubulated wall. (2 to 5) Casts in serpentine, Côte St. Pierre and Long Lake
CONTEMPORARIES OF EOZOON