Читать книгу Life's Dawn on Earth - Sir John William Dawson - Страница 3
ОглавлениеLIST OF ILLUSTRATIONS.
FULL PAGE ILLUSTRATIONS.
| TO FACE PAGE | ||
| I. | Cape Trinity, from a Photograph (Frontispiece) | |
| II. | Map of the Laurentian Region on the River Ottawa | 7 |
| III. | Weathered Specimen of Eozoon, from a Photograph | 35 |
| IV. | Restoration of Eozoon | 59 |
| V. | Nature-print of Eozoon | 93 |
| VI. | Canals of Eozoon, Magnified, from Photographs | 127 |
| VII. | Nature-print of Large Laminated Specimen | 169 |
| VIII. | Eozoon With Chrysotile, etc. | 207 |
WOODCUTS.
| FIG. | PAGE |
| 1. General Section | 9 |
| 2. Laurentian Hills | 11 |
| 3. Section of Laurentian | 13 |
| 4. Laurentian Map | 16 |
| 5. Section at St. Pierre | 22 |
| 6. Sketch of Rocks at St. Pierre | 22 |
| 7. Eozoon from Burgess | 36 |
| 8, 9. Eozoon from Calumet | 39 |
| 10. Canals of Eozoon | 41 |
| 11. Nummuline Wall | 43 |
| 12. Amœba | 60 |
| 13. Actinophrys | 60 |
| 14. Entosolenia | 62 |
| 15. Biloculina | 62 |
| 16. Polystomella | 62 |
| 17. Polymorphina | 63 |
| 18. Archæospherinæ | 67 |
| 19. Nummulites | 73 |
| 20. Calcarina | 73 |
| 21. Foraminiferal Rock-builders | 75 |
| 21a. Casts of Cells of Eozoon | 92 |
| 22. Modes of Mineralization | 96 |
| 23. Silurian Organic Limestone | 98 |
| 24. Wall of Eozoon Penetrated with Canals | 98 |
| 25. Crinoid Infiltrated with Silicate | 103 |
| 26. Shell Infiltrated with Silicate | 104 |
| 27. Diagram of Proper Wall, etc. | 106 |
| 28, 29. Casts of Canals | 107 |
| 30. Eozoon from Tudor | 111 |
| 31. Acervuline Variety of Eozoon | 135 |
| 32, 33, 34. Archæospherinæ | 137, 138 |
| 35. Annelid Burrows | 140 |
| 36. Archæospherinæ | 148 |
| 37. Eozoon Bavaricum | 149 |
| 38, 39, 40. Archæocyathus | 152, 153 |
| 41. Archæocyathus (Structure of) | 154 |
| 42. Stromatopora | 157 |
| 43. Stromatopora (Structure of) | 158 |
| 44. Caunopora | 159 |
| 45. Cœnostroma | 160 |
| 46. Receptaculites | 162 |
| 47, 48. Receptaculites (Structure of) | 163 |
| 49. Laminæ of Eozoon | 176 |
THE DAWN OF LIFE.
CHAPTER I.
INTRODUCTORY.
Every one has heard of, or ought to have heard of, Eozoon Canadense, the Canadian Dawn-animal, the sole fossil of the ancient Laurentian rocks of North America, the earliest known representative on our planet of those wondrous powers of animal life which culminate and unite themselves with the spirit-world in man himself. Yet few even of those to whom the name is familiar, know how much it implies, and how strange and wonderful is the story which can be evoked from this first-born of old ocean.
No one probably believes that animal life has been an eternal succession of like forms of being. We are familiar with the idea that in some way it was introduced; and most men now know, either from the testimony of Genesis or geology, or of both, that the lower forms of animal life were introduced first, and that these first living creatures had their birth in the waters, which are still the prolific mother of living things innumerable. Further, there is a general impression that it would be the most appropriate way that the great procession of animal existence should commence with the humblest types known to us, and should march on in successive bands of gradually increasing dignity and power, till man himself brings up the rear.
Do we know the first animal? Can we name it, explain its structure, and state its relations to its successors? Can we do this by inference from the succeeding types of being; and if so, do our anticipations agree with any actual reality disinterred from the earth’s crust? If we could do this, either by inference or actual discovery, how strange it would be to know that we had before us even the remains of the first creature that could feel or will, and could place itself in vital relation with the great powers of inanimate nature. If we believe in a Creator, we shall feel it a solemn thing to have access to the first creature into which He breathed the breath of life. If we hold that all things have been evolved from collision of dead forces, then the first molecules of matter which took upon themselves the responsibility of living, and, aiming at the enjoyment of happiness, subjected themselves to the dread alternatives of pain and mortality, must surely evoke from us that filial reverence which we owe to the authors of our own being, if they do not involuntarily draw forth even a superstitious adoration. The veneration of the old Egyptian for his sacred animals would be a comparatively reasonable idolatry, if we could imagine any of these animals to have been the first that emerged from the domain of dead matter, and the first link in a reproductive chain of being that produced all the population of the world. Independently of any such hypotheses, all students of nature must regard with surpassing interest the first bright streaks of light that break on the long reign of primeval night and death, and presage the busy day of teeming animal existence.
No wonder then that geologists have long and earnestly groped in the rocky archives of the earth in search of some record of this patriarch of the animal kingdom. But after long and patient research, there still remained a large residuum of the oldest rocks, destitute of all traces of living beings, and designated by the hopeless name “Azoic,”—the formations destitute of remains of life, the stony records of a lifeless world. So the matter remained till the Laurentian rocks of Canada, lying at the base of these old Azoic formations, afforded forms believed to be of organic origin. The discovery was hailed with enthusiasm by those who had been prepared by previous study to receive it. It was regarded with feeble and not very intelligent faith by many more, and was met with half-concealed or open scepticism by others. It produced a copious crop of descriptive and controversial literature, but for the most part technical, and confined to scientific transactions and periodicals, read by very few except specialists. Thus, few even of geological and biological students have clear ideas of the real nature and mode of occurrence of these ancient organisms, and of their relations to better known forms of life; while the crudest and most inaccurate ideas have been current in lectures and popular books, and even in text-books, although to the minds of those really acquainted with the facts, all the disputed points have long ago been satisfactorily settled, and the true nature and affinities of Eozoon are distinctly and satisfactorily understood.
This state of things has long ceased to be desirable in the interests of science, since the settlement of the questions raised is in the highest degree important to the history of life. We cannot, it is true, affirm that Eozoon is in reality the long sought prototype of animal existence; but it is for us at present the last organic foothold, on which we can poise ourselves, that we may look back into the abyss of the infinite past, and forward to the long and varied progress of life in geological time. Its consideration, therefore, is certain, if properly entered into, to be fruitful of interesting and valuable thought, and to form the best possible introduction to the history of life in connection with geology.
It is for these reasons, and because I have been connected with this great discovery from the first, and have for the last ten years given to it an amount of labour and attention far greater than could be adequately represented by short and technical papers, that I have planned the present work. In it I propose to give a popular, yet as far as possible accurate, account of all that is known of the Dawn-animal of the Laurentian rocks of Canada. This will include, firstly: a descriptive notice of the Laurentian formation itself. Secondly: a history of the steps which led to the discovery and proper interpretation of this ancient fossil. Thirdly: the description of Eozoon, and the explanation of the manner in which its remains have been preserved. Fourthly: inquiries as to forms of animal life, its contemporaries and immediate successors, or allied to it by zoological affinity. Fifthly: the objections which have been urged against its organic nature. And sixthly: the summing up of the lessons in science which it is fitted to teach. On these points, while I shall endeavour to state the substance of all that has been previously published, I shall bring forward many new facts illustrative of points hitherto more or less obscure, and shall endeavour so to picture these in themselves and their relations, as to give distinct and vivid impressions to the reader.
For the benefit of those who may not have access to the original memoirs, or may not have time to consult them, I shall append to the several chapters some of the technical details. These may be omitted by the general reader; but will serve to make the work more complete and useful as a book of reference.
The only preparation necessary for the unscientific reader of this work, will be some little knowledge of the division of geological time into successive ages, as represented by the diagram of formations appended to this chapter, and more full explanations may be obtained by consulting any of the numerous elementary manuals on geology, or “The Story of the Earth and Man,” by the writer of the present work.
TABULAR VIEW OF THE EARTH’S GEOLOGICAL HISTORY.
| Animal Kingdom. | Geological Periods. | Vegetable Kingdom. | ||
| Age of Man. Age of Mammals. | CENOZOIC, OR NEOZOIC, OR NEOZOIC, OR TERTIARY | Modern. Post-Pliocene, or Pleistocene. Pliocene. Miocene. Eocene. | Age of Angiosperms and Palms. | |
| Age of Reptiles. | MESOZOIC | Cretaceous. Jurassic. Triassic. | Age of Cycads and Pines. | |
| Age of Amphibians and Fishes. Age of Mollusks, Corals, and Crustaceans. | PALÆOZOIC | Permian. Carboniferous. Erian, or Devonian. Upper Silurian. Lower Silurian, or Siluro-Cambrian. Cambrian or Primordial. | Age of Acrogens and Gymnosperms. Age of Algæ. | |
| Age of Protozoa, and dawn of Animal Life. | EOZOIC | Huronian. Upper Laurentian. Lower Laurentian. | Beginning of Age of Algæ. |
Plate II.
MAP SHEWING THE DISTRIBUTION OF THE LAURENTIAN LIMESTONES HOLDING EOZOON IN THE COUNTIES OF OTTAWA & ARGENTEUIL.
Drawn by M. R. Barlow
Stanford’s Geog. Estabt. Charing Cross, London.
Reprinted with additions from the Report of the Geology of Canada, by Sir W. Logan, F.R.S., 1863.
Click on map to view larger sized image.
CHAPTER II.
THE LAURENTIAN ROCKS.
As we descend in depth and time into the earth’s crust, after passing through nearly all the vast series of strata constituting the monuments of geological history, we at length reach the Eozoic or Laurentian rocks, deepest and oldest of all the formations known to the geologist, and more thoroughly altered or metamorphosed by heat and heated moisture than any others. These rocks, at one time known as Azoic, being supposed destitute of all remains of living things, but now more properly Eozoic, are those in which the first bright streaks of the dawn of life make their appearance.[A]
[A] Dana has recently proposed the term “Archæan,” on the ground that some of these rocks are as yet unfossiliferous but as the oldest known part of them contains fossils, there seems no need for this new name.
The name Laurentian, given originally to the Canadian development of these rocks by Sir William Logan, but now applied to them throughout the world, is derived from a range of hills lying north of the St. Lawrence valley, which the old French geographers named the Laurentides. In these hills the harder rocks of this old formation rise to considerable heights, and form the highlands separating the St. Lawrence valley from the great plain fronting on Hudson’s Bay and the Arctic Sea. At first sight it may seem strange that rocks so ancient should anywhere appear at the surface, especially on the tops of hills; but this is a necessary result of the mode of formation of our continents. The most ancient sediments deposited in the sea were those first elevated into land, and first altered and hardened by heat. Upheaved in the folding of the earth’s crust into high and rugged ridges, they have either remained uncovered with newer sediments, or have had such as were deposited on them washed away; and being of a hard and resisting nature, they have remained comparatively unworn when rocks much more modern have been swept off by denuding agencies.
But the exposure of the old Laurentian skeleton of mother earth is not confined to the Laurentide Hills, though these have given the formation its name. The same ancient rocks appear in the Adirondack mountains of New York, and in the patches which at lower levels protrude from beneath the newer formations along the American coast from Newfoundland to Maryland. The older gneisses of Norway, Sweden, and the Hebrides, of Bavaria and Bohemia, belong to the same age, and it is not unlikely that similar rocks in many other parts of the old continent will be found to be of as great antiquity. In no part of the world, however, are the Laurentian rocks more extensively distributed or better known than in North America; and to this as the grandest and most instructive development of them, and that which first afforded organic remains, we may more especially devote our attention. Their general relations to the other formations of America may be learned from the rough generalised section (fig. 1); in which the crumpled and contorted Laurentian strata of Canada are seen to underlie unconformably the comparatively flat Silurian beds, which are themselves among the oldest monuments of the geological history of the earth.
Fig. 1. General Section, showing the Relations of the Laurentian and Palæozoic Rocks in Canada. (L.) Laurentian. (1.) Cambrian, or Primordial. (2.) Lower Silurian. (3.) Upper Silurian. (4.) Devonian and Carboniferous.
The Laurentian rocks, associated with another series only a little younger, the Huronian, form a great belt of broken and hilly country, extending from Labrador across the north of Canada to Lake Superior, and thence bending northward to the Arctic Sea. Everywhere on the lower St. Lawrence they appear as ranges of billowy rounded ridges on the north side of the river; and as viewed from the water or the southern shore, especially when sunset deepens their tints to blue and violet, they present a grand and massive appearance, which, in the eye of the geologist, who knows that they have endured the battles and the storms of time longer than any other mountains, invests them with a dignity which their mere elevation would fail to give. (Fig. 2.) In the isolated mass of the Adirondacks, south of the Canadian frontier, they rise to a still greater elevation, and form an imposing mountain group, almost equal in height to their somewhat more modern rivals, the White Mountains, which face them on the opposite side of Lake Champlain.
The grandeur of the old Laurentian ranges is, however, best displayed where they have been cut across by the great transverse gorge of the Saguenay, and where the magnificent precipices, known as Capes Trinity and Eternity, look down from their elevation of 1500 feet on a fiord, which at their base is more than 100 fathoms deep (see frontispiece[** insert link in PP]). The name Eternity applied to such a mass is geologically scarcely a misnomer, for it dates back to the very dawn of geological time, and is of hoar antiquity in comparison with such upstart ranges as the Andes and the Alps.
Fig. 2. Laurentian Hills opposite Kamouraska, Lower St. Lawrence.
The islands in front are Primordial.
On a nearer acquaintance, the Laurentian country appears as a broken and hilly upland and highland district, clad in its pristine state with magnificent forests, but affording few attractions to the agriculturist, except in the valleys, which follow the lines of its softer beds, while it is a favourite region for the angler, the hunter, and the lumberman. Many of the Laurentian townships of Canada are, however, already extensively settled, and the traveller may pass through a succession of more or less cultivated valleys, bounded by rocks or wooded hills and crags, and diversified by running streams and romantic lakes and ponds, constituting a country always picturesque and often beautiful, and rearing a strong and hardy population. To the geologist it presents in the main immensely thick beds of gneiss, and similar metamorphic and crystalline rocks, contorted in the most remarkable manner, so that if they could be flattened out they would serve as a skin much too large for mother earth in her present state, so much has she shrunk and wrinkled since those youthful days when the Laurentian rocks were her outer covering. (Fig. 3.)
The elaborate sections of Sir William Logan show that these old rocks are divisible into two series, the Lower and Upper Laurentian; the latter being the newer of the two, and perhaps separated from the former by a long interval of time; but this Upper Laurentian being probably itself older than the Huronian series, and this again older than all the other stratified rocks. The Lower Laurentian, which attains to a thickness of more than 20,000 feet, consists of stratified granitic rocks or gneisses, of indurated sandstone or quartzite, of mica and hornblende schist, and of crystalline limestones or marbles, and iron ores, the whole interstratified with each other. The Upper Laurentian, which is 10,000 feet thick at least, consists in part of similar rocks, but associated with great beds of triclinic feldspar, especially of that peculiar variety known as labradorite, or Labrador feldspar, and which sometimes by its wonderful iridescent play of colours becomes a beautiful ornamental stone.
I cannot describe such rocks, but their names will tell something to those who have any knowledge of the older crystalline materials of the earth’s crust. To those who have not, I would advise a visit to some cliff on the lower St. Lawrence, or the Hebridean coasts, or the shore of Norway, where the old hard crystalline and gnarled beds present their sharp edges to the ever raging sea, and show their endless alternations of various kinds and colours of strata often diversified with veins and nests of crystalline minerals. He who has seen and studied such a section of Laurentian rock cannot forget it.
Fig. 3. Section from Petite Nation Seigniory to St. Jerome (60 miles). After Sir W. E. Logan.
(a, b.) Upper Laurentian. (c.) Fourth gneiss. (d′.) Third limestone. (d.) Third gneiss. (e′.) Second limestone. (x.) Porphyry. (y.) Granite.
All the constituents of the Laurentian series are in that state known to geologists as metamorphic. They were once sandstones, clays, and limestones, such as the sea now deposits, or such as form the common plebeian rocks of everyday plains and hills and coast sections. Being extremely old, however, they have been buried deep in the bowels of the earth under the newer deposits, and hardened by the action of pressure and of heat and heated water. Whether this heat was part of that originally belonging to the earth when a molten mass, and still existing in its interior after aqueous rocks had begun to form on its surface, or whether it is a mere mechanical effect of the intense compression which these rocks have suffered, may be a disputed question; but the observations of Sorby and of Hunt (the former in connection with the microscopic structure of rocks, and the latter in connection with the chemical conditions of change) show that no very excessive amount of heat would be required. These observations and those of Daubrée indicate that crystallization like that of the Laurentian rocks might take place at a temperature of not over 370° of the centigrade thermometer.
The study of those partial alterations which take place in the vicinity of volcanic and older aqueous masses of rock confirms these conclusions, so that we may be said to know the precise conditions under which sediments may be hardened into crystalline rocks, while the bedded character and the alternations of different layers in the Laurentian rocks, as well as the indications of contemporary marine life which they contain, show that they actually are such altered sediments. (See Note D.)
It is interesting to notice here that the Laurentian rocks thus interpreted show that the oldest known portions of our continents were formed in the waters. They are oceanic sediments deposited perhaps when there was no dry land or very little, and that little unknown to us except in so far as its debris may have entered into the composition of the Laurentian rocks themselves. Thus the earliest condition of the earth known to the geologist is one in which old ocean was already dominant on its surface; and any previous condition when the surface was heated, and the water constituted an abyss of vapours enveloping its surface, or any still earlier condition in which the earth was gaseous or vaporous, is a matter of mere inference, not of actual observation. The formless and void chaos is a deduction of chemical and physical principles, not a fact observed by the geologist. Still we know, from the great dykes and masses of igneous or molten rock which traverse the Laurentian beds, that even at that early period there were deep-seated fires beneath the crust; and it is quite possible that volcanic agencies then manifested themselves, not only with quite as great intensity, but also in the same manner, as at subsequent times. It is thus not unlikely that much of the land undergoing waste in the earlier Laurentian time was of the same nature with recent volcanic ejections, and that it formed groups of islands in an otherwise boundless ocean.
However this may be, the distribution and extent of these pre-Laurentian lands is, and probably ever must be, unknown to us; for it was only after the Laurentian rocks had been deposited, and after the shrinkage of the earth’s crust in subsequent times had bent and contorted them, that the foundations of the continents were laid. The rude sketch map of America given in fig. 4 will show this, and will also show that the old Laurentian mountains mark out the future form of the American continent.
