Chapter IX (3)

Hence, in order to construct a complete system of Geological Chronology it is necessary to collect together these odd volumes, as they may be called, of the Great Geological Calendar, and to assign to each one its proper place in the series. This difficult and complicated task is accomplished chiefly by the aid of Fossil Remains. We have already shown that the Fossil Remains which are found embedded in each group of strata, represent the organic life of the period during which that group of strata was in progress of formation. Moreover, we have seen that each period was marked by the existence of an animal and vegetable creation peculiar to itself. If, therefore, we find that the Fossils of two different districts exhibit the same general character, we may conclude that the beds in which they are preserved were deposited about the same age, and consequently belong to the same Geological Period. Whereas, on the other hand, if, within certain limits, we discover two groups of strata, each of which has a collection of Fossils totally different from the other, it is a proof that these two groups were _not_ deposited in the same age, and must, consequently, be referred to different Epochs of the Geological Calendar. Let us now see in what manner the practical Geologist proceeds to apply these general principles.

He takes first some one country, say England, and in that country he selects some one particular district to begin with. Here he examines a number of different sections, and makes himself familiar with all the strata of the neighborhood, and with the order in which they lie. Let us suppose that he finds three different groups spread out one above another, and let us call these groups A, B, and C; A being the lowest, B immediately above A, and C above B. The chronological order of these strata will be, therefore, A, B, C. He will study next the Fossil Remains which he finds embedded in each group. For convenience we may designate the Fossils of A by the letter a, those of B by b, and those of C by c. Now, according to the principles above explained, these three collections of Fossils will be specifically distinct from one another, each collection being characteristic of one particular set of strata. Our Geologist next goes into a neighboring district, and there examines a number of sections as before. Let us suppose that he encounters again the groups A and B. He may, perhaps, have been able to trace the beds from one district to the other, by observations made upon his line of route: or it may be that the nature of the country has rendered such observations impossible; or the observations may have been so imperfect that from _them_ he could arrive at no certain conclusion regarding the identity of the strata. But, at all events, if the new district yield an abundant supply of Fossils, he cannot long be at a loss. He will recognize the group A by the Fossils a, and the group B by the Fossils b. An important fact, however, soon attracts his attention. Group C has entirely disappeared, and is not to be found in this district; while between A and B there is a new group of rocks that he has not seen before, with a collection of Fossils different from a, b, and c. We will call this new group X, and its Fossils x. It is clear that the formation of X must have intervened between the formation of A and B; and the chronological order now stands A, X, B, C. In like manner another district may disclose a fourth group of strata, say Y, intervening between B and C. The chronological order will then stand A, X, B, Y, C. And thus the Geologist pursues his explorations until he has gone through the whole country, and arranged the principal groups of strata according to the order of time in which they were deposited.

In this way the whole of England has been minutely explored during the last half century. The task was first undertaken by William Smith, who is justly called the Father of English Geology. After multiplied researches, extending over a space of many years, during which he travelled the whole country on foot, this eminent man published in 1815 his Geological Map of England and Wales with part of Scotland; a work which is described by Sir Charles Lyell as “a lasting monument of original talent and extraordinary perseverance.” Hundreds followed in the same course, exploring every day new districts, and, by the new facts which they brought to light, supplying what was wanting in the work of Smith, correcting what was faulty, and confirming what was true; until at length, in our day, it may be said that the Stratified Rocks of England are almost as well known and as completely mapped out as are its counties and its towns, its rivers, lakes, and mountains.

Meanwhile, Geologists were not idle in other parts of the world. Germany, France, Italy, even many districts of America and Australia, have been diligently explored according to the same principles as England. And by a comparison of the observations made, the Chronological order of strata over a considerable part of the Earth, but more particularly of Europe, has been now pretty fairly ascertained. This order we have attempted to set forth in an intelligible and sensible form by means of the table here annexed.

In the Woodcut are represented the strata hitherto examined by Geologists, laid out one above another, according to the order of time in which they are supposed to have been produced. The whole series is divided into a number of Formations, the names of which are given in the first column, together with an approximate estimate of their thickness, in feet. These Formations are distinguished from each other in the drawing by a difference of shading. Each of them, according to Geological theory, is believed to have come into existence by the accumulation of solid matter at the bottom of the sea; and the Period of time occupied in its production is usually designated by the same name as the Formation itself. Thus we read of the Carboniferous Formation and the Carboniferous Period: by the former phrase is meant certain groups of strata contemporaneously deposited over various parts of the Earth’s surface; and by the latter, the Period of time during which these groups of strata were spread out. In like manner, when we hear of the Carboniferous Fauna and Flora, we are to understand the animal and vegetable life that flourished during the Carboniferous Period. And again, when Geologists talk of the Cretaceous sea, and tell us that it rolled over a great part of what is now called Europe, they mean to speak of that sea on the bottom of which the Cretaceous rocks were deposited.

CHRONOLOGICALLY ARRANGED.]

Most of the Formations comprise various groups of strata; and these groups are made up of different varieties of rocks, which are again divided into layers or beds of varying thickness. Even in these beds themselves we can often distinguish an indefinite number of laminæ or plates, scarcely thicker than a sheet of paper, which correspond to the periodical depositions of matter by which the rock was originally formed. These numerous subdivisions may be conveniently illustrated from the Carboniferous Formation. It is divided into two leading groups of strata; the Mountain Limestone below, the Coal Measures above. The upper group is the larger as well as the more important. It attains a maximum thickness in South Wales of 12,000 feet; and consists of numerous strata of Sandstone and Shale, with thin seams of Coal occasionally interposed. In one remarkable instance a hundred distinct layers of Coal, varying in thickness from six inches to ten feet, have been counted in one Coal-field, each resting on a bed of Shale, called in mining phraseology the Underclay. This Shale itself naturally divides into an indefinite number of thin plates, just like the stratum of mud accumulated by the annual inundations of the river Nile, and constituting the present soil of Egypt.

We have not attempted to represent in our Woodcut these various divisions and subdivisions of Stratified Rocks. But the names of some important and well-known groups we have had engraved, to impress more vividly on the mind the place to which they are to be referred in the Geological Calendar. Thus the reader may see at a glance the respective ages of the Coal and the Chalk; of the Lias, in which are preserved the remains of extinct gigantic reptiles, and the Glacial Drift, in which the elephant, the rhinoceros, and the hippopotamus are found entombed; of the Mountain Limestone, which is often nothing else than vast beds of Coral uplifted from beneath the waters of the ocean, and the Oolite, which includes the Portland quarries, where the petrified stems of ancient forest trees are found standing erect in the solid rock.

As the series of Stratified Rocks is divided by Geologists into a certain number or systems or Formations, so these are again grouped into still larger classes, called Primary, Secondary, and Tertiary; that is to say, first, second, and third, in the order of formation. These larger classes correspond to the Great Epochs or Ages of Geological time, each comprising within itself many distinct Periods. The Primary rocks are also called Palæozoic--παλαιὁν, ancient, and ξῶον, an organic being--because they contain the oldest forms of organic life: in like manner the term Mesozoic--μεσον, middle, and ξῶον--is applied to the Secondary strata, inasmuch as they contain the middle or intermediate forms of organic life: and the name Kainozoic--χαινὁν, new, and ξῶον--is given to the Tertiary, which contain the newest forms of organic life.

The term Post-Tertiary has recently been adopted to designate those superficial deposits which are subsequent to the Tertiary Age. They are divided into two groups; the Recent, which corresponds with the period of history, and the Post-Pliocene which precedes it. Some writers seem to think that these deposits, being so very insignificant and so very modern when compared with the long series of Stratified Rocks, are not truly Geological. But this, we should say, is a mistaken view of the question. It seems to us that even the minute layer of mud that is deposited every day at the mouth of the Ganges or the Mississippi, is linked on to the long chain of events which have brought the Crust of the Earth into its present condition; and, therefore, truly belongs to the science of Geology, and is deserving of its proper place in Geological classification.

We may here observe that the names of the great Geological Epochs are descriptive names; that is to say, the obvious meaning of the words corresponds to the character of the strata they are used to represent. Primary, Secondary, Tertiary, mean First, Second, and Third, in the order of formation: Palæozoic, Mesozoic, and Kainozoic, signify that the strata so called are characterized by Ancient, Middle, and Modern, forms of organic life. But it is very often quite otherwise with the names of the several Formations: and this is a point of no small importance to the student of Geology. These names must be regarded simply _as names_ employed to designate the strata formed in each successive period, and not exactly to describe their character. They generally had their origin in some accidental circumstance, or were derived from some particular locality; and afterward, being perpetuated, gradually came to receive a much more extended application than that which the words themselves would seem to suggest. Thus, for instance, the Cretaceous Formation is so called from the remarkable stratum of white chalk (creta) which was deposited during that period over a great part of Europe; but it would be a mistake to suppose that the whole Formation is made up of chalk. On the contrary, in different localities it is composed of very different materials; near Dresden, for example, it is a gray quartzose sandstone, and in many parts of the Alps it is hard compact limestone.[84] Again, the Devonian Formation derives its name from Devonshire, where the rocks of the Devonian period were first minutely examined; but we must not therefore infer that this Formation is peculiar to the county of Devon; it is to be found in many other parts of England, also in Ireland, and on the continent of Europe. So, too, another Formation has received the name of Carboniferous, which literally means Coal-bearing (carbo fero) because of the beds of Coal which are sometimes associated with its strata; yet this Formation is often found quite destitute of Coal over a very extensive area.

In looking over our Table of strata the reader must have noticed that the successive spaces in the Woodcut are not proportioned to the actual thickness of the successive Formations for which they stand. The Secondary and Tertiary Rocks taken together are scarcely one-third as thick, in reality, as the Primary; yet they occupy an equal space in the engraving: and, more remarkable still, the Cretaceous system is allowed double the space of the Laurentian, though less than half as thick. This circumstance calls for a passing word of explanation. In the early annals of a country there is generally a great scarcity of authentic records; and, from a simple dearth of facts, the history of a whole century is compressed, not unfrequently, into a few pages: whereas, in later times, when documentary evidence begins to accumulate, the historians will often spread out the events of two or three years over several chapters. Something of the same kind takes place in Geology. The Fossil Remains, from which, as from authentic documents, the Geologist chiefly derives his information regarding the history of the Earth’s Crust, are scanty in the earlier Formations, and abundant in the more recent. And thus it happens that the older Geological Periods, notwithstanding the vast thickness of the rocks by which they are represented, do not occupy a very prominent position in the annals of Geology, and are compressed into a comparatively insignificant space in its Tables. Nevertheless, the immense depth of the earliest Stratified Rocks must be taken into account in any attempt to estimate the comparative duration of the several Geological Periods. We have, therefore, set down, under the name of each Formation, an approximate estimate of its actual thickness, taken chiefly from the works of Doctor Haughton and Sir Charles Lyell.

Before bringing this chapter to an end we would observe that the system of classification we have here endeavored to explain does not pretend to be final and complete. It is, on the contrary, little more than a temporary expedient to render intelligible the results at which Geologists have hitherto arrived; and is liable to manifold modifications in proportion as their acquaintance with the records they have undertaken to interpret becomes more extensive and more minute. All that they now contend for is this: that the successive Formations represent successive Periods of time, which followed one another in the order here set forth, and during which the Earth was peopled with certain species of Plants and Animals, for the most part peculiar to their respective eras.[85]

_CHAPTER XIII._

GEOLOGICAL CHRONOLOGY--REMARKS ON THE SUCCESSION OF ORGANIC LIFE.

_Summary of the history of stratified rocks--Striking
characteristics of certain formations--Human remains found
only in superficial deposits--Gradual transition from the
organic life of one period to that of the next--Evidence
in favor of this opinion--Advance from lower to higher
types of organic life as we ascend from the older to the
more recent formations--Economic value of geological
chronology--Illustration--Search for coal--The practical man
at fault--The geologist comes to his aid, and saves him from
useless expense._

With this sketch of Geological Chronology before us, we can now more fully realize to our minds the story we are told about the formation of the Earth’s Crust. In the earliest age to which Geologists can trace back the history of the Aqueous Rocks--for they do not profess to trace it back to the beginning--this Globe of ours was, as it is now, partly covered with water, and partly dry land. The formation of stratified rocks went on in that age, as it is still going on, chiefly over those areas that were under water--not indeed throughout the entire extent of such areas, but over certain portions of them to which mineral matter happened to be carried by the action of natural causes. And the Earth was peopled then as now, though with animals and plants very different from those by which we are surrounded at the present day. Some of these happened to escape destruction, and to be embedded in the deposits of that far distant age, and have thus been preserved even to our time. And these strata with their Fossils are the same that we now group together under the title of the Laurentian Formation: which being the oldest group of stratified rocks we can recognize in the depths of the Earth’s Crust, occupies the lowest position in our table of Chronology. Ages rolled on; and the Crust of the Earth was moved from within by some giant force, the bed of the ocean was lifted up in one place, islands and continents were submerged in another, and so the outlines of land and water were changed. With this change the old forms of life passed away; a new creation came in; and the Laurentian period gave place to the Cambrian. But the order of nature was still the same as before. The deposition of stratified rocks still continued, though the areas of deposition were, in many cases, shifted from one locality to another. And the organic life that flourished in the Cambrian times left its memorials behind it buried in the Cambrian rocks. Then that age, too, came to an end, and gave place in its turn to the Silurian: and this was, again, followed by the Devonian. Thus one period succeeded to another in the order set forth in our table; and every part of the globe was, in the course of ages, more than once submerged, and covered with the deposits of more than one age, and enriched with the Organic Remains of more than one creation.

As we advance upward in the series of Formations we soon perceive that the Fossil Remains, which, in the earlier groups were scanty enough, become profusely abundant, until even the unpractised eye cannot fail to mark the peculiar character of each successive period;--the exuberant vegetation of the Carboniferous, with its luxuriant herbage and its tangled forests, its huge pines, its tall tree-ferns, and its stately araucarias: the enormous creeping monsters of the Jurassic, the ichthyosaurs, the megalosaurs, the iguanodons, which filled its seas, or crowded its plains, or haunted its rivers; and higher up in the scale, the colossal quadrupeds of the Miocene and the Pliocene, the mammoths, the mastodons, the megatheriums, which begin to approximate more closely to the organic types of our own age. But amidst these various forms of life, the eye looks in vain for any relic of human kind. No bone of man, no trace of human intelligence, is to be found in any bed of rock that belongs to the Primary, Secondary, or Tertiary Formations. It is only when we have passed all these, and come to the latest formation of the whole series, nay, it is only in the uppermost beds of this Formation, that we meet, for the first time, with human bones, and the works of human art.

Thus it appears pretty plain, even from the testimony of Geology, that man was the last work of the creation; and that, if the world is old, the human race is comparatively young. These broken and imperfect records, which have been so curiously preserved in the Crust of the Earth, carry us back to an antiquity which may not be measured by years and centuries, and then set before us, as in a palpable form, how the tender herbage appeared, and the fruit-tree yielding fruit according to its kind; and how the Earth was afterward peopled with great creeping things, and winged fowl, and the cattle, and the beasts of the field; and then, at length, they disclose to us how, last of all, man appeared, to whom all these things seem to tend, and who was to have dominion over the fish of the sea, and the fowl of the air, and every living thing that moveth upon the earth. We do not mean to dwell just now upon this view of the history of creation so clearly displayed in the records of Geology. But we shall return to it hereafter when we come in the sequel to consider how admirably the genuine truths of this science fit in with the inspired narrative of Moses.

It may here, very naturally, be asked, if the records of Geology give us any information as to the manner in which each period of animal and vegetable life was brought to an end? Did the old organic forms gradually die out, and the new gradually come in to take their places? or were the one suddenly extinguished and the others as suddenly produced? This question has been a subject of controversy among Geologists themselves; and therefore it is somewhat outside our scope, since we propose to exhibit only that more general outline of Geological theory which is accepted by all. Nevertheless, as it is a question that must needs occur to the mind of every reader, it seems to call for a few words of explanation as we pass along. In the early days of Geology, it was commonly held that each great period was brought to an end by a sudden and violent convulsion of Nature. The Crust of the Earth was burst open in many places all at once; the bottom of the ocean was upheaved with a tremendous shock; the waters, driven from their accustomed bed, rushed with furious impetuosity over islands and continents; and the whole existing creation perished in a universal deluge. Then succeeded an interval of chaotic confusion, and when at length the waters subsided, and dry land again appeared, a new age in the history of the Globe was ushered in, and the Earth was again peopled by a new creation.

But this old theory has gradually given way as the Stratified Rocks have been more and more fully examined, and at the present day it is almost universally abandoned. Geologists have observed that the same species of Fossil Remains which prevail in the upper beds of one Formation, are met with also in the lower beds of the next, though in less numbers and mixed up with new species; and that, as we ascend higher and higher into the later Formation, the old species gradually become more and more scarce, while the new gradually become more and more numerous; until at length the characteristic forms of one age have disappeared altogether, and those of the succeeding age have attained their full development.

For this important fact, which was brought to light within the last half century, we are mainly indebted to the unwearied researches and great ability of Sir Charles Lyell. Speaking of the Formations of the Tertiary Epoch, to which, as is well known, he has principally devoted himself, this distinguished writer thus sums up the result of his long investigation:--“In passing from the older to the newer members of the Tertiary system we meet with many chasms, but none which separate entirely, by a broad line of demarkation, one state of the organic world from another. There are no signs of an abrupt termination of one fauna and flora, and the starting into life of new and wholly distinct forms. Although we are far from being able to demonstrate geologically an insensible transition from the Eocene to the Miocene, or even from the latter to the recent fauna, yet the more we enlarge and perfect our general survey, the more nearly do we approximate to such a continuous series, and the more gradually are we conducted from times when many of the genera and nearly all the species were extinct, to those in which scarcely a single species flourished which we do not know to exist at present.”[86] Hence he concludes, and his conclusion is now the common doctrine of Geologists, that the extinction and creation of species has been “the result of a slow and gradual change in the organic world.”[87]

It was long argued against this view, that we often meet, especially in the Primary and Secondary Formations, two groups of strata in immediate contact, in which there is a perfectly sudden transition from one set of Fossil Remains to another altogether different. Each group contains a countless variety of species, and yet there is not a single species common to the two. Does it not appear that in such a case the organic life of one period was suddenly destroyed, and that of the next as suddenly introduced? Not so; there is one link wanting in the argument. It must be shown that these two strata which are now in _immediate contact_ were originally deposited in _immediate succession_. But this it is impossible to prove: nay, it must needs be very often false. We have before observed that the areas of deposition were limited in every age, and were ever shifting from one locality to another. Therefore it must have been a frequent occurrence that, after one bed of rock was formed, the process of deposition ceased altogether in that locality, and did not begin again for many ages. Thus a long lapse of time often intervened between the deposition of two strata, which were laid out one immediately above the other. Furthermore, we have also seen that whole groups of strata may in any age be swept away by Denudation; and then the rocks which are next deposited in that locality, will be in immediate contact with strata indefinitely more ancient than themselves. From these considerations it is plain that two groups of strata which are now found in juxtaposition, may have been deposited in two Geological ages widely remote from each other. And consequently a sudden transition from the Organic Life of one group to the Organic Life of the other affords no proof of a sudden transition from the Organic Life of one Geological Period to the Organic Life of that which next succeeded. We may observe, however, that the recent researches, which have contributed so much to fill up the interstices of the Geological Calendar, have conduced in no small degree to fill up likewise some of the more remarkable gaps or chasms in the succession of Organic Life. It is, therefore, not unreasonable to suppose that, as our knowledge of the Earth’s Crust becomes more and more minute, the sudden breaks in the continuity of the scale will be still further diminished and the successive stages of gradual transition will be made more clearly apparent.

This subject has been very happily illustrated by Sir Charles Lyell:--“To make still more clear the supposed working of this machinery [for the deposition of Stratified Rocks and the preservation of Organic Remains], I shall compare it to a somewhat analogous case that might be imagined to occur in the history of human affairs. Let the mortality of the population of a large country represent the successive extinction of species, and the birth of new individuals, the introduction of new species. While these fluctuations are gradually taking place everywhere, suppose commissioners to be appointed to visit each province of the country in succession, taking an exact account of the number, names, and individual peculiarities of all the inhabitants, and leaving in each district a register containing a record of this information. If, after the completion of one census, another is immediately made on the same plan, and then another, there will, at last, be a series of statistical documents in each province. When these belonging to any one province are arranged in chronological order, the contents of such as stand next to each other will differ according to the length of time between the taking of each census. If, for example, there are sixty provinces, and all the registers are made in a single year, and renewed annually, the number of births and deaths will be so small in proportion to the whole of the inhabitants, during the interval between the compiling of two consecutive documents, that the individuals described in such documents will be nearly identical; whereas, if the survey of each of the sixty provinces occupies all the commissioners for a whole year, so that they are unable to revisit the same place until the expiration of sixty years, there will then be an almost entire discordance between the persons enumerated in two consecutive registers in the same province.

“But I must remind the reader that the case above proposed has no pretentions to be regarded as an exact parallel to the Geological phenomena which I desire to illustrate; for the commissioners are supposed to visit the different provinces in rotation; whereas the commemorating processes by which organic remains become fossilized, although they are always shifting from one area to the other, are yet very irregular in their movements. They may abandon and revisit many spaces again and again, before they once approach another district; and besides this source of irregularity, it may often happen that, while the depositing process is suspended, Denudation may take place, which may be compared to the occasional destruction by fire or other causes of some of the statistical documents before mentioned. It is evident that where such accidents occur, the want of continuity in the series may become indefinitely great, and that the monuments which follow next in succession will by no means be equi-distant from each other in point of time.

“If this train of reasoning be admitted, the occasional distinctness of the fossil remains, in formations immediately in contact, would be a necessary consequence of the existing laws of sedimentary deposition and subterranean movement, accompanied by a constant mortality and renovation or species.”[88]

There is another and a very striking fact in the succession of ancient organic life, which claims from us a moment’s notice. As we proceed upward through the series of Stratified Rocks, from the oldest to the newest, we find a gradual advance in the types of animal organization therein preserved, from the humbler and more simple forms of structure to those of a higher and more perfect character. That form of organization is regarded among Zoologists as the more perfect in which there is “a greater number of organs specially devoted to particular functions.” Now all the forms of animal life with which we are acquainted, may be reduced to two great divisions, the Vertebrate and the Invertebrate,--the former having a _vertebral_ or spinal column, the latter having none: and it is agreed in conformity with the notion set forth above, that the Vertebrate animals as a class exhibit a more perfect organization than the Invertebrate. Again, among the Vertebrate themselves there is a gradation; the Reptiles are ranked higher than the Fish, the Birds higher than the Reptiles, and the Mammalia higher again than the Birds.

All this we learn from Zoologists, who have pursued their investigations without any reference whatever to the science of Geology. It is, therefore, not a little remarkable that we should discover this very order and gradation of animal life in the successive groups of Stratified Rocks. All the Remains hitherto discovered in the earliest Geological Formations belong to Invertebrate animals, while the Vertebrate, which appear for the first time in the latter part of the Silurian Period, are, from that age on, more and more fully developed down to the present day, and now constitute, if not the most numerous, at least the most important part of the animal creation. Moreover, it is to be observed that the Vertebrate animals do not all make their appearance at once, but come in successively according to the same scale of organic perfection,--the Fish appearing first, then the Reptiles, then the Birds, and lastly the Mammalia. Even among the Mammalia a well-defined order of progressive succession has been observed, which finally culminates in the appearance of Man, the last created and the most perfect of animals.

SHOWING THE FIRST APPEARANCE ON THE EARTH OF THE VARIOUS FORMS OF ANIMAL LIFE.]

This remarkable succession of animal life in the history of the Earth’s Crust will be more readily understood by means of the annexed Table. The remains of Invertebrate animals have been traced as far back as the Lower Laurentian Rocks. The Vertebrate first become manifest in the Ludlow beds of the Upper Silurian; where they are represented by the bones of Fish, the lowest class belonging to the Province of Vertebrates. Next in order come the Reptiles: the oldest known Reptile having been found in the Coal Measures of Saarbrück between Strasburg and Treves. The skeletons of Birds are rare in the Stratified Rocks. It is supposed that their powers of flight have in all ages secured them, to great a extent, from being carried away by floods, like other land animals, and buried in the sedimentary deposits of rivers and estuaries. Nevertheless their presence in the ancient world is frequently attested by their footsteps, impressed originally on the sandy beach, and still preserved now that the soft sand has been converted into solid rock. Such traces have been discovered in great abundance on the New Red Sandstone of the Connecticut River in America; and afford the earliest evidence we possess in the records of Geology regarding the existence of the feathered tribe. This group of strata belongs to the lower Trias. In the higher beds of the same Formation we meet with the first relic of ancient Mammals. It was found near Stuttgardt, in 1847, and belongs to the more imperfect form of Mammalian life, the Non-Placental. Similar remains have been since discovered in the Upper Trias of Somersetshire. The Placental, or more perfect form of animal life in the same class, first appears in the Eocene Formation: and the bones of Man, the highest of the Placental, are found for the first time in the upper deposits of the Post-Tertiary Age.

Let it be remembered that we are here but stating the facts which have been hitherto brought to light by the researches of Geologists. It may be, it is indeed most probable, that new discoveries will lead to numerous modifications in our Table. There is no reason to suppose that Geologists have yet exhumed the earliest remains of Vertebrates or Invertebrates preserved in the Crust of the Earth: that Fish may not hereafter be traced back beyond the Silurian, or Reptiles beyond the Carboniferous Period: that Birds may not be found among the Primary Rocks, and Placentals among the Secondary. But in a science which depends mainly upon observation, it is better to register the facts we have than to speculate idly about those we have not. And, having registered them, we cannot fail to be struck with the succession of animal life on the Earth, to which they seem to point. It is certainly deserving of notice that, as far as the Organic Remains hitherto discovered may be taken as a guide, Invertebrates and Vertebrates, Fish, Reptiles, Birds, and Mammals, Non-Placentals and Placentals, follow one another in the ascending series of Geological Formations exactly in the same order as they follow one another in the ascending scale of Zoological Classification.

* * * * *

And so Geologists go on ever searching out new phenomena, and grouping them together into classes, until from particular facts they lead us to general truths. Then starting with these general truths as the groundwork of their science, they proceed to sketch out the Natural History of our Globe from the remotest ages of the past down to the present time. They first study the stratified deposits of each succeeding age, and analyze the Fossil Remains embedded therein; afterward they make their inferences, and they compile their history. They describe the forms, the character, the habits, of the plants and animals that flourished of old in this world of ours; they tell us where the deep sea rolled its waves in each succeeding age, and where the dry land appeared; they point out the Deltas of its ancient rivers, they measure the breadth of its Estuaries, they trace the course of its Glaciers, they mark the outlines of its Mountain chains. But with these and such like speculations we are not here concerned. Many of them are open to controversy, and not a few are at this moment warmly disputed among Geologists themselves: besides, whether true or false, they do not in any way affect the relations between Geology and Revealed Religion. We shall be quite content, and it is all that our present scope demands, if we have made intelligible the general theory of Geological Chronology, and the kind of evidence on which it rests.

Before taking leave of this subject, however, we will venture to offer what seems to us an interesting illustration of the principles we have been explaining in the last two chapters;--one that will help to confirm the conclusions for which we have been contending, and that will also bring home to many minds the practical advantage to be derived from a thorough knowledge and just application of Geological science. Perhaps, too, it may help to revive the flagging attention of our readers; for the subject of our illustration is _Coal, and the way to find it_. In this age of manufactories and steam-engines,--when the atmosphere of great towns is heavy with smoke, and the quiet solitude of the country is so rudely disturbed by the shrieking of the railway-whistle and the snorting of the sooty locomotive,--this black, dirty mineral has acquired a value and importance, which may succeed in rousing even the practical money-making man to pay some heed to the lessons of science.

Coal might have been produced in any Geological Period; and in point of fact, beds of coal have been discovered in many different Formations. But in England, and in Western Europe generally, it has been found by long experience that the Coal-beds of the Carboniferous Formation are more abundant, and of better quality, than those of any other. Indeed the beds of Coal that occur in other Formations are so thin, and of such inferior quality, that they cannot be worked with profit. It is therefore of the highest importance in the search for Coal, before going to the enormous expense of sinking deep shafts, to discover whether or no the rocks in which the search is to be made belong to the Carboniferous Period. In this matter the more _practical man_ is often seriously at fault. Coal-bearing strata generally consist pretty largely of dark-colored clay, black shales, and similar deposits. This is a fact which, as it strikes the eye, is perfectly familiar to all who are engaged in the working of Coal mines. Hence it happens, not unfrequently, that the practical man, when he meets with strata of this kind, is apt at once to infer that Coal is near at hand. The Geologist, on the contrary, knows well that such strata are not peculiar to the Carboniferous rocks, but are often found in other Formations in which there is no Coal at all, or at least no Coal that will repay the expense of working; and therefore he will pronounce it most rash to undertake costly works on the strength of these appearances. He has learned, however, that there are certain species of animals and plants which are found in the Carboniferous rocks and in them alone; he will search for these in the strata which it is proposed to explore, and by their presence or their absence he will know whether the strata in question belong to the Carboniferous Formation or not.

Again, it will often happen that, in the midst of an extensive region well known to abound in Coal, the rocks which appear at the surface in one particular locality, are not wholly devoid of Coal, but exhibit no resemblance either in mineral character or in Fossil Remains to the Coal-bearing strata. A question then arises of the highest practical importance. May it be that the Coal-bearing strata are spread out beneath this uppermost bed of rocks? and is it worth the expense to sink a shaft through the one in the hope of reaching the other? The practical miner has no very clear or certain principles to help him in the solution of this problem; and thus it has often happened that thousands upon thousands of pounds have been expended in sinking shafts to look for Coal, where, as it afterward proved, there was not the slightest chance of finding it. Now, though Geology cannot tell if we shall succeed in finding Coal beneath these rocks, it _can_ tell if there is a _good chance_ of succeeding. It can tell whether there is a reasonable hope, by penetrating into the Crust of the Earth at this particular spot, of reaching the Carboniferous Formation; and if we can reach the Carboniferous Formation in the midst of a Coal district, it is very likely we shall meet with beds of Coal.

His first object will be to ascertain what is the Formation to which the superficial rocks belong. If it be a Formation earlier in date than the Carboniferous,--the Silurian, for instance, or the Devonian,--he knows that it would be simply waste of money to look for Coal beneath them; because the Carboniferous rocks cannot possibly be found underneath the rocks of an earlier age. And so the Geologist can tell beforehand what the mere practical man would find out only when he had spent his money. If, on the other hand, the rocks which appear at the surface belong to a period later than the Carboniferous, the Geologist will not always conclude that it is expedient to sink a shaft in search of Coal. For though the Carboniferous rocks may, in this case, be underneath, they may be so far down in the Crust of the Earth that we should have no chance of ever reaching them. Suppose, for example, that the strata which appear at the surface belong to the Cretaceous Formation. He knows from his Chronological table that the Carboniferous age is separated from the Cretaceous by three intermediate Periods,--the Permian, the Triassic, the Jurassic. Therefore, when he finds the Cretaceous rocks at the surface in any locality, it is quite possible, though of course not certain, that before the Carboniferous Formation could be reached it would be necessary to bore through thousands of feet of Jurassic, Triassic, and Permian rocks. And even then there would be no certainty of meeting with the Coal-bearing strata. Perhaps they were never deposited over this area of the earth’s surface; or, if deposited, perhaps they were subsequently swept away by Denudation. Hence our Geologist would reasonably conclude that, the probable expense of the search being so enormous, and the chance of success so remote, it would be much wiser not to make the attempt.