About this time Dr. Hunt happened to mention to me, in connection with a paper on the mineralization of fossils which he was preparing, that he proposed to notice the mode of preservation of certain fossil woods and other things with which I was familiar, and that he would show me the paper in proof, in order that I might give him any suggestions that occurred to me. On reading it, I observed, among other things, that he alluded to the supposed Laurentian fossils, under the impression that the organic part was represented by the serpentine or loganite, and that the calcareous matter was the filling of the chambers. I took exception to this, stating that though in the slices I had examined no structure was apparent, still my impression was that the calcareous matter was the fossil, and the serpentine or loganite the filling. He said—"In that case, would it not be well to re-examine the specimens, and try to discover which view is correct?" He mentioned, at the same time, that Sir William had recently shown him some new and beautiful specimens collected by Mr. Lowe, one of the explorers on the staff of the Survey, from a third locality, at Grenville, on the Ottawa. It was supposed that these might throw further light on the subject; and accordingly Dr. Hunt suggested to Sir William to have additional slices of these new specimens made by Mr. Weston, of the Survey, whose skill as a preparer of these and other fossils has often done good service to science. A few days thereafter some slices were sent to me, and were at once put under the microscope. I was delighted to find in one of the first specimens examined a beautiful group of tubuli penetrating one of the calcite layers. Here was evidence, not only that the calcite layers represented the true skeleton of the fossil, but also of its affinities with the foraminifera, whose tubulated supplemental skeleton, as described and figured by Dr. Carpenter, and represented in specimens in my collection, presented by him, was apparently of the same type with that preserved in the canals of these ancient fossils. Fig. 7 is an accurate representation of the group of canals first detected by me.

Feeling that the discovery was most important, but that it would be met with determined scepticism by a great many geologists, I was not content with examining the typical specimens of Eozoon, but had slices prepared of every variety of Laurentian limestone, of altered limestones from the Primordial and Silurian, and of serpentine marbles of all the varieties furnished by our collections. They were examined with ordinary and polarized light, and with every variety of illumination. They were also examined as decalcified specimens, after the carbonate of lime had been removed by acids. An extensive series of notes and camera tracings were made of all the appearances observed; and of some of the more important structures beautiful drawings were executed by the late Mr. H. S. Smith, the then palæontological draughtsman of the Survey. The result of the whole investigation was a firm conviction that the structure was organic and foraminiferal, and that it could be distinguished from any merely mineral or crystalline forms occurring in these or other limestones.

At this stage of the matter, and after exhibiting to Sir William all the characteristic appearances, in comparison with such concretionary, dendritic and crystalline structures as most resembled them, and also with the structure of recent and fossil Foraminifera, I suggested that the further prosecution of the matter should be handed over to Mr. Billings, as palæontologist of the Survey. I was engaged in other researches, not connected with the Survey or with this particular department, and I knew that no little labour must be devoted to the work and to its publication, and that some controversy might be expected. Mr. Billings, however, with his characteristic caution and modesty, declined. His hands were full of other work. He had not given any special attention to the microscopic appearances of Foraminifera or of mineral substances. It was finally arranged that I should prepare a description of the fossil, which Sir William would take to London, along with the more important specimens, and a detailed list stating all the structures observed in each. Sir William was to submit the manuscript and specimens to Dr. Carpenter, or, failing him, to Prof. T. Rupert Jones, in the hope that these eminent authorities would confirm my conclusions, and bring forward new facts which I might have overlooked or been ignorant of. Sir William saw both gentlemen, who gave their testimony in favour of the organic and foraminiferal character of the specimens; and Dr. Carpenter, in particular, gave much attention to the subject, and worked out more in detail many of the finer structures, besides contributing valuable suggestions as to the probable affinities of the supposed fossil.

Dr. Carpenter thus contributed in a very important manner to the perfecting of the investigations begun in Canada, and on him fell the greater part of their illustration and defence,[51] in so far as Great Britain is concerned.

The immediate result was a composite paper in the Proceedings of the Geological Society, by Sir W. E. Logan, Dr. Carpenter, Dr. Hunt, and myself, in which the geology, palæontology and mineralogy of Eozoon Canadense and its containing rocks were first given to the world.[52] It cannot be wondered at that when geologists and palæontologists were thus required to believe in the existence of organic remains in rocks regarded as altogether Azoic and hopelessly barren of fossils, and to carry back the dawn of life as far before those Primordial rocks, which were supposed to contain its first traces, as these are before the middle period of the earth's life history, some hesitation should be felt. Further, the accurate appreciation of the evidence for such a fossil as Eozoon required an amount of knowledge of minerals, of the more humble types of animals, and of the conditions of mineralization of organic remains, possessed by few even of professional geologists. Thus Eozoon has met with some scepticism and not a little opposition,—though the latter has been weaker than we might have expected when we consider the startling character of the facts adduced, and has mostly come from men imperfectly informed.

But what is Eozoon, if really of animal origin? The shortest answer to this question is, that this ancient fossil is supposed to be the skeleton of a creature belonging to that simple and humbly organized group of animals which are known by the name Protozoa. If we take as a familiar example of these the gelatinous and microscopic creature found in stagnant ponds, and known as the Amœba[53] (Fig. 8), it will form a convenient starting-point. Viewed under a low power, it appears as a little patch of jelly, irregular in form, and constantly changing its aspect as it moves, by the extension of parts of its body into finger-like processes or pseudopods which serve as extempore limbs. When moving on the surface of a slip of glass under the microscope, it seems, as it were, to flow along rather than creep, and its body appears to be of a semi-fluid consistency. It may be taken as an example of the least complex forms of animal life known to us, and is often spoken of by naturalists as if it were merely a little particle of living and scarcely organized jelly or protoplasm. When minutely examined, however, it will not be found so simple as it at first sight appears. Its outer layer is clear and transparent, and more dense than the inner mass, which seems granular. It has at one end a curious vesicle which can be seen gradually to expand and become filled with a clear drop of liquid, and then suddenly to contract and expel the contained fluid through a series of pores in the adjacent part of the outer wall. This is the so-called pulsating vesicle, and is an organ both of circulation and excretion. In another part of the body may be seen the nucleus, which is a little cell capable, at certain times, of producing by its division new individuals. Food, when taken in through the wall of the body, forms little pellets, which become surrounded by a digestive liquid exuded from the enclosing mass into rounded cavities or extemporised stomachs. Minute granules are seen to circulate in the gelatinous interior, and may be substitutes for blood-cells, and the outer layer of the body is capable of protrusion in any direction into long processes, which are very mobile, and used for locomotion and prehension. Further, this creature, though destitute of most of the parts which we are accustomed to regard as proper to animals, seems to exercise volition, and to show the same appetites and passions with animals of higher type. I have watched one of these animalcules endeavouring to swallow a one-celled plant as long as its own body; evidently hungry and eager to devour the tempting morsel, it stretched itself to its full extent, trying to envelope the object of its desire. It failed again and again; but renewed the attempt, until at length, convinced of its hopelessness, it flung itself away as if in disappointment, and made off in search of something more manageable. With the Amœba are found other types of equally simple Protozoa, but somewhat differently organized. One of these, Actinophrys (Fig. 9), has the body globular and unchanging in form, the outer wall of greater thickness; the pulsating vesicle like a blister on the surface, and the pseudopods long and thread-like. Its habits are similar to those of the Amœba, and I introduce it to show the variations of form and structure possible even among these simple creatures.

The Amœba and Actinophrys are fresh-water animals, and are destitute of any shell or covering. But in the sea there exist swarms of similar creatures, equally simple in organization, but gifted with the power of secreting around their soft bodies beautiful little shells or crusts of carbonate of lime, having one orifice, and often in addition multitudes of microscopic pores through which the soft gelatinous matter can ooze, and form outside finger-like or thread-like extensions for collecting food. In some cases the shell consists of a single cavity only, but in most, after one cell is completed, others are added, forming a series of cells or chambers communicating with each other, and often arranged spirally or otherwise in most beautiful and symmetrical forms. Some of these creatures, usually named Foraminifera, are locomotive, others sessile and attached. Most of them are microscopic, but some grow by multiplication of chambers till they are a quarter of an inch or more in breadth.

The original skeleton or primary cell wall of most of these creatures is seen under the microscope to be perforated with innumerable pores, and is extremely thin. When, however, owing to the increased size of the shell, or other wants of the creature, it is necessary to give strength, this is done by adding new portions of carbonate of lime to the outside, and to these Dr. Carpenter has given the appropriate name of "supplemental skeleton"; and this, when covered by new growths, becomes what he has termed an "intermediate skeleton." The supplemental skeleton is also traversed by tubes, but these are often of larger size than the pores of the cell wall, and of greater length, and branched in a complicated manner. Thus there are microscopic characters by which these curious shells can be distinguished from those of other marine animals; and by applying these characters we learn that multitudes of creatures of this type have existed in former periods of the world's history, and that their shells, accumulated in the bottom of the sea, constitute large portions of many limestones. 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 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 smaller calcareous bodies, the Coccoliths, which are probably products of marine vegetable life, if not of some animal organism still simpler than the Foraminifera.

Lastly, while we have in such modern forms as the masses of Polytrema attached to corals, and the Loftusia of the Eocene and the carboniferous, large fossil foraminiferal species, there is some reason to believe that in the earlier geological ages there existed much larger animals of this grade than are found in our present seas; and that these, always sessile on the bottom, grew by the addition of successive chambers, in the same manner with the smaller species.[54]

Let us, then, examine the structure of Eozoon, taking a typical specimen, as we find it in the limestone of Grenville or Petite Nation. In such specimens the skeleton of the animal is represented by a white crystalline marble, the cavities of the cells by green serpentine, the mode of whose introduction we shall have to consider in the sequel. The lowest layer of serpentine represents the first gelatinous coat of animal matter which grew upon the bottom, and which, if we could have seen it before any shell was formed upon its surface, must have resembled a minute patch of living slime. On this primary layer grew a delicate calcareous shell, perforated by innumerable minute tubuli, and resting on the slimy matter of the animal, though supported also by some perpendicular plates or septa. Upon this again was built up, in order to strengthen it, a thickening or supplemental skeleton, more dense, and destitute of fine tubuli, but traversed by branching canals, through which the soft gelatinous matter could pass for the nourishment of the skeleton itself, and the extension of pseudopods beyond it. (Figs. 11, 12.) So was formed the first layer of Eozoon, which probably was at its beginning only of very small dimensions. On this the process of growth of successive layers of animal sarcode and of calcareous skeleton was repeated again and again, till in some cases even a hundred or more layers were formed (nature-print, Chap. VI.) As the process went on, however, the vitality of the organism became exhausted, probably by the deficient nourishment of the central and lower layers making greater and greater demands on those above, and so the succeeding layers became thinner, and less supplemental skeleton was developed. Finally, toward the top, the regular arrangement in layers was abandoned, and the cells became a mass of rounded chambers, irregularly piled up in what Dr. Carpenter has termed an "acervuline" manner, and with very thin walls unprotected by supplemental skeleton. Then the growth was arrested, and possibly these upper layers gave off reproductive germs, fitted to float or swim away and to establish new colonies. We may have such reproductive germs in certain curious globular bodies, like loose cells, found in connection with Eozoon in many of the Laurentian limestones.[55] At St. Pierre, on the Ottawa, these bodies occur on the surface of layers of the limestone in vast numbers, as if they had been growing separately on the bottom, or had been drifted over it by currents. They may have served as reproductive buds or germs to establish new colonies of the species. Such was the general mode of growth of Eozoon, and we may now consider more in detail some questions as to its gigantic size, its precise mode of nutrition, the arrangement of its parts, its relations to more modern forms, and the effects of its growth in the Laurentian seas.

With respect to the size of Eozoon, this was rivalled by some succeeding animals of the same humble type in later geological ages; and, as a whole, foraminiferal animals have been diminishing in size in the lapse of geological time. This is indeed a fact of so frequent occurrence that it may almost be regarded as a law of the introduction of new forms of life, that they assume in their early history gigantic dimensions, and are afterwards continued by less magnificent species. The relations of this to external conditions, in the case of higher animals, are often complex and difficult to understand; but in organisms so low as Eozoon and its allies, they lie more on the surface. 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 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 débris constantly being derived from the death of the living things at the surface, 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 salts 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 did, 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 conical or club-shaped forms, and that the broader patches were penetrated by large pits or oscula, 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 show in places the characteristic structures, and which evidently represent the débris swept from the Eozoic masses and reefs by the action of the waves. It is with this fragmental matter that the small rounded organisms already referred to most frequently occur; and while they may be distinct animals, 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.

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, it is true, readily believe in the composite nature of masses or 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 débris 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 care to commit myself to the doctrine that the Laurentian limestones are wholly of this origin. There may have been other 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; 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. 11. To the Nummulites it also conforms in its tendency to form a supplemental or intermediate skeleton with canals, though the canals themselves in the arrangement more nearly resemble Calcarina, which is represented in Fig. 12. In its superposition of many layers, and in its tendency to a heaped up or acervuline irregular growth it resembles Polytrema and Tinoporus, forms of a different group in so far as shell-structure is concerned. It may thus be regarded as a composite type, combining peculiarities now observed in two groups, or it may be regarded as representing one of these in another 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 allied to both Nummulites and Rotalines; and Mr. Brady has discovered a true Nummulite in the Lower Carboniferous of Belgium. I have myself found small Foraminifera in the Silurian and Cambro-Silurian of Canada. This group being now brought down to the Palæozoic, we may hope to trace it to the Primordial, and thus to bring it still nearer to Eozoon in time.

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 débris. 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 characterised 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 hundred 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. (See Note appended.)

All this seems sufficiently reasonable, more especially since no mineralogist has yet succeeded in giving a feasible inorganic explanation of the combination of canals, laminæ, tubulation and varied mineral character existing in Eozoon. But then comes the strange fact of its apparent isolation without companions in highly crystalline rocks, and with apparently no immediate successors. This has staggered many, and it certainly, if taken thus baldly, seems in some degree improbable. Recent discoveries, however, are removing this reproach from Eozoon. The Laurentian rocks have yielded other varieties, or perhaps species of the genus, those which I have described as variety Acervulina, and variety Minor, and still another form, more like a Stromatopora, which I have provisionally named E. latior, from the breadth and uniformity of its plates.[56] There are also in the Laurentian limestone cylindrical bodies apparently originally tubular, and with the sides showing radiating markings in the manner of corals, or of the curious Cambrian Archæocyathus. Matthew, a most careful observer, has found in the Laurentian limestone of New Brunswick certain laminated bodies of cylindrical form, constituting great reefs in the limestone, and in the slates linear flat objects resembling Algæ or Graptolites, and spicular structures resembling those of sponges.[57] Britton has also described from the Laurentian limestone of New Jersey certain ribbon-like objects of graphite which he regards as vegetable, and names Archæophyton Newberryii.[58] Should these objects prove to be organic, Eozoon will no longer be alone. Besides this the peculiar bodies named Cryptozoum by Hall, and which are intermediate in structure between Eozoon and Loftusia, have now been found as low as the Lower Cambrian.[59] Barrois has also recently announced the discovery of forms which he regards as akin to the modern Radiolaria, creatures of a little higher grade than the Foraminifera, in the "Archæan" rocks of Brittany.[60] Thus Eozoon is no longer isolated, but has companions of the same great age with itself. The progress of discovery is also daily carrying the life of the Cambrian to lower beds, and thus nearer to the Laurentian. It is not unlikely that in a few years a pre-Cambrian fauna will force itself on the attention of the most sceptical geologists.

References:—"Life's Dawn on Earth," London, 1875. (Now out of print.) "Specimens of Eozoon Canadense in the Peter Red path Museum, Montreal," 1888. (This memoir contains reference to previous papers.)

Appended Notes.

1. Stromatoporæ. It has been usual of late to regard these as allies of the modern Millepores and Hydractineæ; but careful study of large series of specimens has convinced me that some species, notably the Stromatocerium of the Cambro-Silurian and the cryptozoum of the Cambrian, cannot be so referred. I hope to establish this in the future, if time permit.

2. Modern Foraminifera. The discovery by Brady and Lister of reproductive chamberlets at the margin of the modern orbitolites, tends to connect this with Eozoon. The gigantic foraminiferal species discovered by Agassiz at the Gallipagos, has points of affinity with Eozoon; and its arenaceous nature does not affect this, as we know sandy species in this group closely allied to others that are calcareous.

WHAT MAY BE LEARNED FROM EOZOON.

DEDICATED TO THE MEMORY OF

DR. WILLIAM B. CARPENTER,

Who, among his many Services to Science,
devoted much Time and Labour to the Investigation
of Eozoon,
and by his Knowledge of Foraminifera
and unrivalled Power of unravelling difficult
Structures
did much to Render it intelligible.

The Microscope in Geology—Contributions of the Study of Eozoon to our Knowledge of the Mode of Preservation of Fossils—Its Teaching Relatively to the Origin of Life and the Laws of its Introduction and Progress

CHAPTER VI.

WHAT MAY BE LEARNED FROM EOZOON.

The microscope has long been a recognised and valued aid of the geological observer, and is perhaps now in danger of being somewhat overrated by enthusiastic specialists. To the present writer its use is no novelty. When, as a very young geologist, collecting fossil plants in the coal fields of Novia Scotia, I obtained access to the then recently published work of Witham on the "Internal Structure of Fossil Vegetables."[61] Fired by the desire to learn something of the structure of the blocks of fossil wood in my collection, I at once procured a microscope of what would now be considered a very imperfect kind, and proceeded to make attempts to slice and examine my specimens, and was filled with joy when these old blackened stems for the first time revealed to me their wonderful structures. At the same time I extended my studies to every minute form of life that could be obtained from the sea or fresh waters. A few years later (in 1841), when a student in Edinburgh, I made the acquaintance of Mr. Sanderson of that city, who had worked for Nicol and Witham in the preparation of specimens, and learnt the modes which he had employed. Since that time I have been accustomed to subject every rock, earth or fossil which came under my notice to microscopic scrutiny, not as a mere specialist in that mode of observation, or with the parade of methods and details now customary, but with the view of obtaining valuable facts bearing on any investigation I might have in hand. It was this habit which induced my old friend, Sir William Logan, in 1858 and subsequent years to ask my aid in the study of the forms believed or suspected to be organic, which had been discovered in the course of his surveys of the Laurentian rocks. In one respect this was unfortunate. It occupied much time, interfered to some extent with other researches, led to unpleasant controversies. But these evils were more than compensated by the insight which the study gave into the fact of the persistence of organic structures in highly crystalline rocks, and to the modes of ascertaining and profiting, by these obscure remains, while it has guided and stimulated enquiry and thought as to the origin and history of life. These benefits entitle the researches and discussions on Eozoon to be regarded as marking a salient point in the history of geological discovery, and it is to these principally that I would attract attention in the present chapter.

Perhaps nothing excites more scepticism as to the animal nature of Eozoon than the prejudice existing among geologists that no organism can be preserved in rocks so highly crystalline 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 and the rocks containing them may undergo the most remarkable and complete mechanical and chemical changes without losing their minute structure, and that limestones, if once fossiliferous, are hardly ever so much altered as to lose all traces 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 may have been, or corals were and are, become imbedded in clay, marl, or other soft sediment, they can be washed out and recovered in a condition similar to that of recent specimens, except that their pores or cells, if open, may be filled with the material of the matrix, or if not so open that they can be thus filled, they may be more or less incrusted with mineral deposits introduced by water percolating the mass, 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 with mineral matter. 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 grey 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 crystallization, as to act differently on light, and so reveal the original structure. These fillings are very curious. Sometimes they are mere earthy or muddy matter which has been washed into the cavities. Sometimes they are transparent and crystalline. Often they are stained with oxide of iron or coaly materials. 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 mineralisation 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 replaced by 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, which are so perfectly replaced by flinty matter that when weathered out of the limestone, or treated with acid till the latter is removed, we find the coral as perfect as when recent. It may be well to present to the eye these different stages of fossilization. I have attempted to do this in Fig. 13, taking a tabulate coral of the genus Favosites for an example, and supposing the material 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 (a). When fossilized the cavities may be merely filled with carbonate of lime, or they may be filled with silica (b, c); or the walls themselves may be replaced by silica, and the cavities may remain filled with carbonate of lime (d); or both the walls and cavities may be represented by or filled with silica or silicates (e). The ordinary specimens of Eozoon are supposed to be 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. I have further to remark that the reason why wood and the cells of corals so readily become silicified is that the organic matter which they contain, becoming oxidized in decay, produces carbon dioxide, which, by its affinity for alkalies, can decompose soluble silicates and thus throw down their silica in an insoluble state. Thus a fragment of decaying wood imbedded in a deposit holding water and alkaline silicates almost necessarily becomes silicified. It is also to be remarked that the ordinary specimens of Eozoon have actually not attained to the extreme degree of mineralization seen in some much more recent silicified woods and corals, inasmuch as the portion believed to have been the original calcareous test has not usually been silicified, but still remains in the state of calcium carbonate.