Popular Science Monthly/Volume 10/January 1877/The Earlier Forms of Life
|THE EARLIER FORMS OF LIFE.|
STATE GEOLOGIST OF NEW HAMPSHIRE.
THE surmises and discoveries of the past twenty years have established the fact of the existence of life throughout the entire series of stratified rocks known to geologists, both sedimentary and metamorphic. When Principal Dawson published his description of the Eozoön in the Laurentian foundations, he was led to suggest the adoption of the term Eozoic in place of Azoic for all the ages older than Paleozoic, since, if life existed in the oldest formation, it must have continued to flourish in the following æons, even though evidences of its presence had not then been accumulated. Time has approved of this sagacious anticipation, and we can now maintain with serene confidence that the four great Eozoic periods—Laurentian, Labrador, Atlantic or Montalban, and Huronian—were all enlivened by the existence of both vegetable and animal life. And Eozoic life had its peculiar characteristics just as much as the Silurian or Carboniferous. This life has appeared in consonance with the general principles of evolution as announced by our most learned sages. The earliest organic forms were the simplest in their structural relations; and they flourished through untold ages. The world was no longer young when the organic scheme permitted the growth of Cambrian trilobites and mollusca. More than half of geological time had passed away during the reign of protozoans and fungi. This suggests the enunciation of a general principle, in perfect agreement with the doctrines of evolution: the simpler the predominating forms of life the longer the period. In the beginning of Nature's operations, time was the element of which lavish use was made. It has grown more valuable as the ages have passed on, and the perfected type of organic development in our period grudges the loss of a single moment of it.
The evidences of the earlier forms of life naturally divide selves into two groups, or those relating to plants and animals, which we will consider in their natural order:
Evidences of Plant-Life.—An interesting evidence of the existence of vegetation in Eozoic times is derived from the presence of iron-ores, an argument first set forth by Sterry Hunt. The ores are first formed in the hydrated condition, and then lose their water by metamorphic agencies, becoming specular and magnetic, or the state in which the Laurentian irons are now known. Ores of iron are conceived to have been formed under similar conditions in all ages. At the present day they accumulate in swamps and low grounds in the condition of the hydrated peroxide (ferric), or bog-ore, oftentimes in company with manganese. The presence of organic vegetable matter is requisite in order to extract the iron from the rock or soil and effect its deposition. The metal present in slight amount in the soil is the insoluble ferric oxide, or the familiar condition of iron-rust. Water charged with soluble vegetable infusions, like that in swamps too full of the disagreeable extract of leaves, etc., to be palatable, has the power of dissolving ferric oxide. The process consists in the removal of a part of the oxygen by the vegetable compound, or deoxidation, when the compound becomes changed into the readily-soluble oxide. But this is not a stable compound in the presence of our atmosphere. The rejected oxygen is brought back again, and in its recombination takes water with it, producing the hydrated ferric oxide, which, being insoluble, is precipitated, and covers the ground on the bottom of the pool. On visiting almost any swamp at the present day, this reddish-brown coating of hydrated iron-rust may be seen abundantly. Where streams of water cause the swampy water to flow to lower regions, the iron compound is also conveyed in suspension, and in the course of a few years a thick deposit of ore is accumulated. Our New England ancestors used these beds for the manufacture of their pig-iron in localities where only the name now exists for the village, such as the Tamworth or Gilmanton Iron-Works. All tradition of the manufacture there has disappeared. The Katahdin Company, in Maine, however, and some others, still derive their ore-supplies from this bog-compound.
Our theory supposes that the principal iron-ores in every age of the world had their origin in this way. There is no other agent save this organic extract which produces iron-ore on a large scale at the present day; hence it is rational to explain the origin of ancient ferruginous beds in the same way. If we examine the formations in order, we find the very ores themselves obviously thus accumulated: 1. There are the early Tertiary limonite beds of Western New England, New Jersey, and Pennsylvania, still scarcely removed from the bog form, with the accompanying clays. 2. There are the older Carboniferous nodules and the celebrated Clinton hematites, differing from the limonites only by the absence of water. 3. The specular Huronian ores of Lake Superior have the same composition as the Silurian deposits. Lastly, the Laurentian magnetites constitute the other extreme of the ferruginous series. Both the water and a part of the oxygen have disappeared, leaving a compound richer in metal, and therefore more highly prized by the smelter. The application of a gentle, continuous heat is adequate to explain the change of the limonites into hematites and magnetites.
The process of change may be seen in the manufacture of common bricks, or the purification of quartz for the production of glass. The blue clay becomes red when burned, because it parts with its water of composition; and likewise the small percentage of hematite in the quartz becomes magnetic on the application of heat, and, after pulverization, has the iron removed by magnets, so that the silica-flour may be perfectly pure, and not impart a green tint to the glass. It is not maintained that the native limonites have been converted into magnetites in precisely the way in which the same results have been accomplished artificially; but the manipulation of the manufactured products shows that the metamorphosis is a feasible process, and by no means of difficult accomplishment in Nature.
In a review of a report by the author, in which this theory of iron ore origin is elucidated, Prof. Dana objects to its value, because "carbonic acid, which does now some of the work of iron-transportation, may have done far more then," on account of its presence in the atmosphere in great abundance. No doubt exists as to the assistance afforded by carbonic acid in this work, but this fact only confirms the truth of our argument, since no chemist will allow that carbonic acid can remove the iron-rust from the soil without the help of some deoxidating agent, such as vegetation. The chemical change for which we require the presence of vegetation is the same, whether carbonic acid be involved or not. Indeed, an excellent authority for the form in which this change is effected is the professor's own treatise on mineralogy, where he says, "The iron is transported in solution as a, protoxide carbonate in carbonated waters, a sulphate, or as a salt of an organic acid." Each of these methods requires the presence of a deoxidating agent like vegetation; and nothing better has yet been suggested. The iron-ores produced by volcanic ejections are of very limited amount, and mingled with too much dead rock to be capable of utilization. Nor does the suggestion of the decomposition of pyrites by atmospheric agents to form limonite necessitate the origin of all iron ores in that way.
Accepting the validity of the argument, it follows that vegetation must have been extremely abundant in the Laurentian and Huronian ages on account of the presence in them of enormous deposits of iron ores, as on Lake Superior, in the Adirondacks, Missouri, etc. Some of the beds are hundreds of feet in thickness.
The presence of graphite, or plumbago, in the Eozoic rocks is by many regarded as a still stronger argument for the former existence of vegetation. As graphite is nearly pure carbon, it is easy to believe that it has accumulated from the remains of plants. Greater changes have been effected in its mass through metamorphism than in the alterations of the ore-beds. No traces of vegetable structure have yet been detected in graphite, so that no evidence as to the nature of the earliest plants can be afforded from morphology.
Nature of the Eozoic Flora.—What species of vegetation can we imagine to have existed in these early periods? Possibly we may derive a hint as to its nature from the general course of plant-development in later ages, and assume that there has been a correspondence between the order in which the different classes have appeared and their successive stages of complexity of structure. The simpler forms should appear first; or, reversing the statement, if we find a succession of all the higher forms of growth in later times, it is reasonable to expect in the still earlier periods larger developments of the inferior cryptogams, such as now play a comparatively insignificant part in the economy of Nature. Their easy decomposition would prevent the preservation of their specific shapes as fossils.
To particularize, we have among the lower orders of terrestrial vegetation the lichens, by some thought to be the parent of the fungi and algæ, since they can be resolved into two different plants, a fungus parasitic upon an alga; the mushrooms, puff-balls, mildews, blight, or fungi; the hepaticæ, and mosses. Of aquatic vegetation there are the numerous protophytes, the diatoms, with their siliceous shells; the desmids, the coccoliths, with their lenticular calcareous disks; the nullipores and corallines, making calcareous incrustations; and the great family of Algæ, simple, branched, and confluent. These afford us abundant material from which we may reconstruct the Eozoic meadows, forests, and submarine carpets.
The present system of plants seems to have originated in the Cretaceous period. The older Mesozoic gives us the cycads and tree ferns, like those of the Asiatic tropics. The Paleozoic formations furnish a unique assemblage of combined cryptogamous and phenogamic nature of types not now existing. Granting that the two divisions of the plant kingdom are of equal importance in the line of development, we find ourselves in Silurian times only half-way back to the starting-point. If the Cambrian should furnish us with representations of the mosses and lichens, we might expect in Eozoic times some of these, together with the protophytes, etc., in order to complete the systematic and orderly development of the plant kingdom in time. Furthermore, the later ages have afforded gigantic representations of the higher orders. Why, then, should not the Eozoic land have had its forests of mushrooms and arborescent lichens; its swamps of diatoms, confervæ, the charæ and desmids, and enormous aquatic growth of algæ, coccoliths, nullipores, and corallines? If we grant that the parasitic fungi could not exist for want of their proper organic food of higher organization, there are still enough forms remaining to take their place, and thus afford us a symmetrical development of all the phases of vegetable growth in the enormous periods when the simplest organic structures ruled the world.
Evidences of Animal Life.—It has been argued by high authority that the existence of carbonate and phosphate of lime suggests the presence of animal life in the Laurentian seas, because at the present day these mineral substances are principally derived from organic secretions. The graphite may also have been partly of animal derivation. There is as much carbon in the Laurentian as in the Paleozoic Carboniferous. But these indications need not be dwelt upon, since recent discoveries have brought to light the actual relics of protozoans preserved in stones of Laurentian age. These are so convincing that the discussion of probabilities derived from rocks of supposed organic origin need not be dwelt upon. The organism has the name Eozoön Canadense, the dawn-animal, inhabiting the Canadian district.
Several names are connected with the discovery of this Eozoön from Ontario and elsewhere. Dr. Wilson, of Perth, sent specimens of it many years since to Sir William E. Logan, Director of Canadian Geological Survey, in which Dr. Sterry Hunt found a new hydrous silicate, which he called Loganite. In 1858 J. McMullen brought specimens which reminded Logan of the Stromatopora of the Silurian. They were examined by various scientists, and in 1865 a composite paper upon the geology, paleontology, and mineralogy of the fossil appeared in the journal of the Geological Society of London, prepared by Messrs. Logan, Dawson, Carpenter, and Hunt. Soon after Vennor discovered other specimens in the Montalban of Tudor, Ontario; Gümbel recognized it in both the Laurentian and Huronian in Bavaria; Bicknell and Burbank discovered it in Laurentian limestones at Newbury and Chelmsford, Massachusetts; and Edwards described it from the Adirondacks in New York. Scientists have not universally accepted the genuineness of this fossil. I will endeavor to present a brief sketch of the nature of the organism before stating their objections.
This animal structure belongs to the subkingdom Protozoa, a unique and inferior group of organisms. These animals are distinguished by possessing no alimentary cavity, or, if a stomach be present, it is not bounded by any walls. The three divisions, using the classification adopted by Dawson, are: the Rhizopods, Sponges, and Infusoria. The first is the lowest, including all the sarcodous animals whose only external organs are pseudopodia. The rhizopods are divided into the Reticularia or Foraminifera, possessing thread-like and reticulating pseudopodia with granular matter instead of a nucleus, and with calcareous, membranous, or arenaceous skeletons; the Radiolaria and Lobosa, the first being the lowest, and embracing Eozoön. The reticularis, may be still further divided into two sub-orders, Perforata and Imperforata, the first having calcareous skeletons penetrated with pores. This is the higher one, and holds Eozoön. Of the three families Nummulinidæ, Globigerinidæ, and Lagundæ, Eozoön belongs to the first and the highest in rank. It is not strictly, then, the lowest of the animal kingdom, though very near to it. Fig. 1 shows several species of the foraminifera.
a, Orbulina universa; b, Globigerina rubra; c, Chrysalidina gradata; d, Cuneolina pavonia; e, Grammostomum phyllodes; f, Rotalia globosa; g, Flabellina rugosa; h, Frondicularia annularis; i, Nummulites nummularia.
The animal part of the rhizopods is a gelatinous body called sarcode, a bit of scarcely organized protoplasm. Food is taken in through the outer wall, and is made into small pellets, which are surrounded by a digestive fluid in extemporized stomachs. Minute granules move about the interior, perhaps the substitute for a circulating fluid; and the outer wall can be moulded into the long processes called pseudopodia, used for locomotion and prehension. When these rhizopods secrete stony matter for a covering, the interior is the same structureless mass; but the shells assume characteristic forms for the different varieties. The Orbulina consists of a single cell with one orifice, but permeated by numerous microscopic pores, through which the protoplasmic material can ooze and form the pseudopodia. In the Globigerina and other genera there are several cells agglutinated together, all communicating with one another. In many species the thin cell-wall is inadequate for the wants of the structure, and an additional growth or "supplementary skeleton" is added, traversed by tubes larger, longer, and more branched, than in the first. In the ocean these minute creatures swarm in astonishing numbers, and their remains accumulate at the bottom, commingled with a paste of still more minute coccoliths and calcareous débris to form the ooze brought up in the sounding lead from the telegraphed plateau. When this calcareous mixture becomes solidified it is the chalk which abounds in the Cretaceous formation of Europe, and makes up the nummulitic and orbitoidal limestones.
Fig. 2 is a close copy of a small slab of Eozoön, showing what are called the laminated, acervuline, and fragmental portions. The diagonal white line represents the course of a vein of calcite. The dark lines and marks correspond to the sarcode or animal matter of the animal, now consisting of serpentine. Calling the base of the
figure the ocean-floor, there may be said to grow upon it the gelatinous sarcode or dark mass. Upon it grew first the delicate calcareous shell, penetrated by the numerous minute orifices or tubuli, larger pores, and occasionally supports of perpendicular plates. Added to this is the supplemental skeleton without the minute tubuli, but traversed by branching canals. This whole skeleton is represented by the white mass next the dark base, consisting of calcite. These two layers or laminæ constitute the. essential part of the structure, and all the numerous layers above are but repetitions of them. Each lamina may cover several inches square of surface at the bottom of the ocean, and perhaps diminish in size as the organism grew upward. In the sketch the layers are seen to grow thinner toward the top, as if the vital energy became exhausted by the demands made upon it, and the supplemental skeleton first disappears. Finally, we have a mass of rounded chambers irregularly piled up near the top, constituting the "acervuline" structure. We may suppose the growth arrested at this stage, and the sending forth of reproductive germs to found new colonies in the adjacent ground. In Fig. 3 we have an enlarged restoration, after Dawson, of a portion of the Eozoön structure, which will enable us to better appreciate the several parts of the organism. The dark, granulated layers at the base and at intervals higher up constitute the chambers, and contain the sarcode or gelatinous animal matter. Immediately above and below each dark layer is the thin calcareous shell penetrated by the minute orifices or tubuli. The white spaces represent the supplementary skeletons traversed by the larger canals. At the summit the sarcode is developed into several pseudopodia or cilia, by means of which food is brought to be assimilated.
In Fig. 4 we have a portion of Eozoön magnified one hundred diameters, drawn by Carpenter. The upper covering (a a) represents the original cell-wall penetrated by the tubuli or pores in great abundance. A bit of this is still more magnified in 2, by the side of the first, seemingly consisting of an upper and lower part. The greater part of the sketch consists of the supplemental or intermediate skeleton, traversed by two kinds of canals (b, c), of much larger size and greater irregularity than the tubulation of the cell-wall.
The arrangement and composition of the mineral matter of the Eozoön is quite interesting, and the more remarkable since it has awakened hostile criticism and resulted in illustrating the presence of silicates in organisms in every age of the world. Formerly it was believed that carbonate of lime was the principal mineral found replacing organic substances, thus producing petrifactions. Now we have iron oxide, silica, clay, sand, sulphuret of iron, ores of copper, lead, etc., fluor-spar, heavy spar, phosphate of lime, all unmistakably occupying
a a, Original cell-wall with tubulation; b c Supplementary skeleton, with canals; 2. Portion of a a magnified.
the place of decomposable organic material. And the discussions about Eozoön recall and enforce facts about the employment of silicates by Nature to preserve her structures, especially in foraminiferal forms. In New Jersey there are beds of green-sand of Cretaceous and Tertiary ages full of concretions composed of a silicate of iron and potash called glauconite. Owing to its value as a fertilizer, thousands of tons of it are annually employed by the farmers to enrich their lands. This silicate has replaced modern organic structures of various kinds, but noticeably corals, echinoderms, nummulites, and other rhizopods. The fine tubulation and pores of these microscopic structures have been penetrated by the silicates, so that, when the calcareous parts have been removed by acid, the insoluble glauconite residue shows us the forms of the chambers and cavities. This process of the infiltration of organisms by glauconite was known long before the discovery of Eozoön. It goes on at the present day at the bottoms of the warmer seas, as evidenced in the facts discovered by the numerous deep-sea dredgings recently undertaken in the interests of science. Dr. Hunt suggests that the mineral is developed through chemical reactions in the ooze at the sea-bottom, a combination of dissolved silica with iron put into the ferrous soluble condition by means of organic matter. Hydrous silicates act as mineralizers elsewhere than in the greensand. Crinoidal joints from the Silurian limestones of New Brunswick have been saturated by it, filling all the interstices, and small mollusks from Wales have had their interior permeated by it. There is much variation in the composition of these infiltrating silicates. Some from the calcaire grossier, near Paris, approach serpentine. Others carry magnesia. Those from the Lower Silurian of the Upper Mississippi Valley are like glauconite. In the Eozoön, as described above, serpentine, which is a hydrous silicate of magnesia, replaces the supposed sarcodous or animal part of the structure. It has thus corresponded to the glauconite of the present day filling the canals of the supplementary skeleton, the tubuli of the shell, and replacing the softer animal portions. Pyroxene and Loganite also replace the animal matter in the Canadian Laurentian fossils, and in the Eozoön discovered in the supposed Montalban series of Ontario carbonate of lime is the mineralizer. These last-named specimens were not described till 1867; and, as they exhibit the foraminiferal structure without the presence of any form of silicate, they completely establish the genuineness of the fossil. In Bavaria Gümbel states that chondrodite, hornblende, and scapolite, and perhaps other minerals, should be added to the list of silicates petrifying the Eozoön.
The objections that have been made to the organic character of Eozoön relate chiefly to the close resemblances between mineral and organic replacement, or between pseudomorphs and petrifactions. Other resemblances are to dendritic and concretionary structures. Inasmuch as these structures represent the higher efforts of the mineral kingdom in crystallization and the nearest approach to the inorganic world allowed by animal forms, it is not strange that the two extremes should resemble each other sufficiently to deceive practical observers. The canal system may be almost the very picture of certain dendrites. The latter, however, usually occupy a flat surface like moss-agates; whereas the former branch out in every direction, as appears in Fig. 5, projecting upward and downward, as well as sideways.
Organisms are preserved because of the more or less complete substitution of mineral for animal matter. Pieces of coal or wood that have been deposited in clay may be washed out, but the small pores and interstices will be seen to be filled with the matrix. When the burial has been in a solution capable of precipitating solid matter, the wood will be found more or less changed according to the nature of the solution and its capacity for alteration. Some specimens become nearly pure agate in consequence of the gradual substitution, particle by particle, of the organic matter by silica. Fig. 6 shows different stages of petrifaction in coniferous wood: a is a small fragment where the pores have been filled with silica, assuming a somewhat rhomboidal appearance, and the black parts represent the woody substance, still intact; in b the vegetable matter is wanting, having rotted away, and only silica remains, the rhomboidal pieces being the only remnants of the original structure. The Eozoön has not been so completely fossilized as in the example of coniferous wood. The cell-wall and supplementary skeleton still retain much of the original lime, while the animal part has been entirely replaced by serpentine or some other mineral. Subsequent pressure or desiocation has produced cracks in the mass, which have been filled with an asbestus-like mineral of silky lustre, and this has sometimes been confounded with some part of the animal by objectors. a l
|Fig. 5.—Canals of Eozoön, highly magnified.||Fig. 6.—Coniferous Wood, illustrating Fossilization.|
|a, Partially mineralized, the white spaces being silica, the black vegetable matter; b. Vegetable matter removed by decomposition, leaving outline of the forms of the original pores.|
Few special subjects have been so carefully studied as the genuineness of Eozoön. The treatises of Logan, Dawson, Carpenter, and Hunt, admirably set forth every possible phase of geological position, intimate zoological structure and affinity, mineral character both original and derived, and the conditions of origination. The elaborate papers of the objectors, Messrs. King, Rowney, Carter, Burbank, and others, show what the weaker positions are, and have enabled the advocates to satisfactorily fortify the less defensible points of their arguments. Every new discovery seems to aid the defenders, while the philosophy of evolution is in harmony with the existence of a long Eozoöic age where the predominant life is scarcely elevated above the working of crystalline forces.
Huronian Life.—Gümbel has described a species of Eozoön from the supposed Huronian rocks of Bavaria. In this country Billings has mentioned the occurrence of an Aspidella and Arenicolites from a series of Newfoundland rocks called "Intermediate," most probably of this age. The Aspidella bears some resemblance to the limpet-shell or Patella, while it may have been some variety of crustacean. The Arenicolites is a petrified worm-burrow.
But the specimens of greatest interest are those brought to light the present year by Mr. George W. Hawes from the Huronian of New Hampshire. The rocks have been carefully studied stratigraphically and lithologically, so that their place in the column is well understood, and the fossil is so allied to the Eozoön as to abundantly confirm all that has been held for it by its warmest advocates.
As a matter of convenience Mr. Hawes proposes to call the group of rocks affording these organisms greenstones, in allusion to their color. They have not been melted like a certain class of traps once called by this name, but have been metamorphosed somewhat; they embrace most of the chloritic and talcose schists, or, technically, "all basic metamorphic rocks whose predominant coloring ingredient is either hornblende, pyroxene, or chlorite." Those of special interest to us now are varieties of diorite and diabase, the first consisting mainly of hornblende and feldspar, the second adding labradorite to the constituents of the first-named rock. These rocks by many authors are regarded as of igneous origin.
The method of examination employed in determining the composition of these greenstones is of some interest. A bit of the specimen is carefully ground to the thinnest dimensions possible, so that it can be examined optically under the microscope. With common and polarized light it is possible to understand the nature of the minutest minerals present, as well as the cavities contained in them. The study of rocks in this way has been prosecuted so energetically of late, that it is common to speak of the sub-sciences micro-lithology, micro-petrology, etc., and the appearances of every mineral are now well understood by those skilled in observation, so that the conclusions are often more reliable than those obtained by ultimate chemical analysis. Mr. Hawes combines in his studies the use of the microscope and chemical analysis, so that whatever cannot be determined in the one will be ascertained by the other method.
He was accordingly gratified to recognize in one of his rock-sections the fragment of a rhizopod. The structure has some resemblance to the acaleph Chætetes, but on account of the minuteness of the layers it should be classed with the rhizopods, reminding one very much of the Stromatopora. Figs. 7 and 8 illustrate these organisms magnified thirty-five diameters, thus making the breadth of the cells only 1/280 of an inch. The smaller figure is probably a section of the
same rhizopod, cut in a different direction. The rock holding these fossiliferous bits is diabase, a variety common between Connecticut Lake and Bellows Falls, both in New Hampshire and Vermont.
Since the naming of Stromatopora by Goldfuss fifty years since, naturalists have separated the acaleph structures from the true corals, but this genus is generally regarded as different from either of them. Prof. Hall described it as a polyp-coral in his "Paleontology of New York," but would not so regard it now. The most common form of it, as figured by him, is herewith presented (Fig. 9), from the Niagara limestone of Lockport, occurring in masses one or two feet in diameter. It is a protozoan coral, assisting in the work of reef-building, however, as much as the polyp-structures. By way of comparison we add a figure of a bryozoan mollusk (Lichenalia concentrica), from the same formation and locality with the Stromatopora (Fig. 10). The relations of our new specimens are rather with the first of these forms, and will probably be described hereafter as species of Stromatopora.
It is an interesting fact that these "layer corals" have impressed the minds of all students of the Eozoön by their resemblances to the dawn-animal. Logan speaks of it in his first remarks upon them, referring more particularly to their weathered outcrops and somewhat concentric structure; while Dawson sees much in their internal organization suggestive of a fitness for foraminiferal requirement. The layer seems better arranged for sheltering a gelatinous body, throwing out pseudopodia reaching after food, than for accommodating the
sponge animals, subsisting through the passage of currents of water. The canal system, with the supplemental skeleton, is wanting in this genus, but appears in the allied forms of the Devonian.
A very important feature of the greenstone fossils is their mineral composition. They are composed of silicates, very probably of feldspar. Mr. Hawes has not been able yet to satisfy himself fully as to the nature of the silicate, because of the smallness of the particles obtained. A drop of acid placed upon one of the specimens exhibited a slight effervescence, indicating the traces of carbonate of lime—perhaps part of the original foraminifer before its fracture and dispersion in the mud. He suggests that the presence of these lime-structures afforded the material for the manufacture of so much labradorite in the diabases containing the fossils.
I cannot forbear alluding to the interesting confirmation of the genuineness of Eozoön afforded by the discovery of these Huronian fossils in New Hampshire: 1. Eozoön sprung upon us with affinities rather remote from existing forms, but the Stromatopora has been known for fifty years as a veritable organism. 2. The latter has the same silicated condition with the former; hence we cannot set aside Eozoön merely because the supposed animal parts have been infiltrated
by a silicate. A well-known organism is proved to be silicated; hence all presumption against the existence of the same mineral condition in a related animal is removed. 3. Stromatopora is zoologically allied to Eozoön. 4. It appears in a subsequent period, showing a natural order of development. 5. Stratigraphical and petrographical studies prove the greenstones to satisfactorily belong to the true Huronian formation, and thus make the sequence of life free from ambiguity.
Eozoöic Geography.—Such vast periods are necessarily involved in those early stages of the earth's growth, that we cannot portray Eozoic scenery as a whole; while an artist would find material for only one sketch. At the first we must conceive of an earth with a larger diameter than is now accepted as the standard for the metric system of measures; of a shallow ocean covering the greater portion of the surface, interspersed with numerous islands, scattered every-where without any method of arrangement that we understand. In the areas marked as Eozoic upon our maps, accumulations of strata were going on, of enormous thickness. We cannot recognize now the original land which supported the primeval vegetation, but can conjecture the boundaries of the contiguous oceans. In the latter part of the period the areas of deposition occupied basins situated within the limits of the earlier-formed rocks, being usually the deeper portions of the original oceans. Ridges between the water-basins resulted from the slow elevation of the land, the nuclei of great mountain-ranges, and there were ejections of melted matter, with marvelous alterations of sediments deep down beneath the surface.
Respecting the age as a whole, we may say that the waters were probably somewhat thermal, still simmering from the proximity of the heated interior; the air was thick and moist, partly composed of carbonic-acid gas; the sky was filled with dense clouds, marking the transition of day and night by periods of total darkness and seasons of feeble illumination, not permitting sunshine to cheer the vegetation. The life was characterized by its lowness of grade; the terrestrial plants hardly suitable for the food of air-breathing animals; the marine largely of the lime-secreting varieties and unicellular diatoms. The animals colonized the bottoms of the oceans, building up enormous reefs, but invisible to sight, if any one could have been permitted to look upon the infant world.
- American Journal of Science, iii., vol. ix., p. 223.
- Fifth edition, p. 173.
- American Journal of Science, iii., vol. xii., p. 134.