The cycle of erosion now in progress and recorded in the layers of stratified rock being spread beneath the sea in continental deltas has therefore been preceded by many similar cycles. Again and again movements of the crust have brought to an end one cycle-- sometimes when only well under way, and sometimes when drawing toward its close--and have begun another. Again and again they have added to the land areas which before were sea, with all their deposition records of earlier cycles, or have lowered areas of land beneath the sea to receive new sediments.
THE AGE OF THE EARTH. The thickness of the stratified rocks now exposed upon the eroded surface of the continents is very great.
In the Appalachian region the strata are seven or eight miles thick, and still greater thicknesses have been measured in several other mountain ranges. The aggregate thickness of all the formations of the stratified rocks of the earth"s crust, giving to each formation its maximum thickness wherever found, amounts to not less than forty miles. Knowing how slowly sediments acc.u.mulate upon the sea floor, we must believe that the successive cycles which the earth has seen stretch back into a past almost inconceivably remote, and measure tens of millions and perhaps even hundreds of millions of years.
HOW THE FORMATIONS ARE CORRELATED AND THE GEOLOGICAL RECORD MADE UP. Arranged in the order of their succession, the formations of the earth"s crust would const.i.tute a connected record in which the geological history of the planet may be read, and therefore known as the GEOLOGICAL RECORD. But to arrange the formations in their natural order is not an easy task. A complete set of the volumes of the record is to be found in no single region. Their leaves and chapters are scattered over the land surface of the globe. In one area certain chapters may be found, though perhaps with many missing leaves, and with intervening chapters wanting, and these absent parts perhaps can be supplied only after long search through many other regions.
Adjacent strata in any region are arranged according to the LAW OF SUPERPOSITION, i.e. any stratum is younger than that on which it was deposited, just as in a pile of paper, any sheet was laid later than that on which it rests. Where rocks have been disturbed, their original att.i.tude must be determined before the law can be applied. Nor can the law of superposition be used in identifying and comparing the strata of different regions where the formations cannot be traced continuously from one region to the other.
The formations of different regions are arranged in their true order by the LAW OF INCLUDED ORGANISMS; i.e. formations, however widely separated, which contain a similar a.s.semblage of fossils are equivalent and belong to the same division of geological time.
The correlation of formations by means of fossils may be explained by the formations now being deposited about the north Atlantic.
Lithologically they are extremely various. On the continental shelf of North America limestones of different kinds are forming off Florida, and sandstones and shales from Georgia northward.
Separated from them by the deep Atlantic oozes are other sedimentary deposits now acc.u.mulating along the west coast of Europe. If now all these offsh.o.r.e formations were raised to open air, how could they be correlated? Surely not by lithological likeness, for in this respect they would be quite diverse. All would be similar, however, in the fossils which they contain. Some fossil species would be identical in all these formations and others would be closely allied. Making all due allowance for differences in species due to local differences in climate and other physical causes, it would still be plain that plants and animals so similar lived at the same period of time, and that the formations in which their remains were imbedded were contemporaneous in a broad way. The presence of the bones of whales and other marine mammals would prove that the strata were laid after the appearance of mammals upon earth, and imbedded relics of man would give a still closer approximation to their age. In the same way we correlate the earlier geological formations.
For example, in 1902 there were collected the first fossils ever found on the antarctic continent. Among the dozen specimens obtained were some fossil ammonites (a family of chambered sh.e.l.ls) of genera which are found on other continents in certain formations cla.s.sified as the Cretaceous system, and which occur neither above these formations nor below them. On the basis of these few fossils we may be confident that the strata in which they were found in the antarctic region were laid in the same period of geologic time as were the Cretaceous rocks of the United States and Canada.
THE RECORD AS A TIME SCALE. By means of the law of included organisms and the law of superposition the formations of different countries and continents are correlated and arranged in their natural order. When the geological record is thus obtained it may be used as a universal time scale for geological history.
Geological time is separated into divisions corresponding to the times during which the successive formations were laid. The largest a.s.semblages of formations are known as groups, while the corresponding divisions of time are known as eras. Groups are subdivided into systems, and systems into series. Series are divided into stages and substages,--subdivisions which do not concern us in this brief treatise. The corresponding divisions of time are given in the following table.
STRATA TIME Group Era System Period Series Epoch
The geologist is now prepared to read the physical history--the geographical development--of any country or of any continent by means of its formations, when he has given each formation its true place in the geological record as a time scale.
The following chart exhibits the main divisions of the record, the name given to each being given also to the corresponding time division. Thus we speak of the CAMBRIAN SYSTEM, meaning a certain succession of formations which are cla.s.sified together because of broad resemblances in their included organisms; and of the CAMBRIAN PERIOD, meaning the time during which these rocks were deposited.
Group and Era System and Period Series and Epoch
Quaternary-----Recent Cenozoic------PleistoceneTertiary-------PlioceneMioceneEoceneCretaceous Mesozoic------Jura.s.sicTria.s.sic
PermianCarboniferous--PennsylvanianMississippian Paleozoic-----DevonianSilurianOrdovicianCambrian
Algonkian Archean
FOSSILS AND WHAT THEY TEACH
The geological formations contain a record still more important than that of the geographical development of the continents; the fossils imbedded in the rocks of each formation tell of the kinds of animals and plants which inhabited the earth at that time, and from these fossils we are therefore able to construct the history of life upon the earth.
FOSSILS. These remains of organisms are found in the strata in all degrees of perfection, from trails and tracks and fragmentary impressions, to perfectly preserved sh.e.l.ls, wood, bones, and complete skeletons. As a rule, it is only the hard parts of animals and plants which have left any traces in the rocks.
Sometimes the original hard substance is preserved, but more often it has been replaced by some less soluble material. Petrifaction, as this process of slow replacement is called, is often carried on in the most exquisite detail. When wood, for example, is undergoing petrifaction, the woody tissue may be replaced, particle by particle, by silica in solution through the action of underground waters, even the microscopic structures of the wood being perfectly reproduced. In sh.e.l.ls originally made of ARAGONITE, a crystalline form of carbonate of lime, that mineral is usually replaced by CALCITE, a more stable form of the same substance. The most common petrifying materials are calcite, silica, and pyrite.
Often the organic substance has neither been preserved nor replaced, but the FORM has been retained by means of molds and casts. Permanent impressions, or molds, may be made in sediments not only by the hard parts of organisms, but also by such soft and perishable parts as the leaves of plants, and, in the rarest instances, by the skin of animals and the feathers of birds. In fine-grained limestones even the imprints of jellyfish have been retained.
The different kinds of molds and casts may be ill.u.s.trated by means of a clam sh.e.l.l and some moist clay, the latter representing the sediments in which the remains of animals and plants are entombed.
Imbedding the sh.e.l.l in the clay and allowing the clay to harden, we have a MOLD OF THE EXTERIOR of the sh.e.l.l, as is seen on cutting the clay matrix in two and removing the sh.e.l.l from it. Filling this mold with clay of different color, we obtain a CAST OF THE EXTERIOR, which represents accurately the original form and surface markings of the sh.e.l.l. In nature, sh.e.l.ls and other relics of animals or plants are often removed by being dissolved by percolating waters, and the molds are either filled with sediments or with minerals deposited from solution.
Where the fossil is hollow, a CAST OF THE INTERIOR is made in the same way. Interior casts of sh.e.l.ls reproduce any markings on the inside of the valves, and casts of the interior of the skulls of ancient vertebrates show the form and size of their brains.
IMPERFECTION OF THE LIFE RECORD. At the present time only the smallest fraction of the life on earth ever gets entombed in rocks now forming. In the forest great fallen tree trunks, as well as dead leaves, decay, and only add a little to the layer of dark vegetable mold from which they grew. The bones of land animals are, for the most part, left unburied on the surface and are soon destroyed by chemical agencies. Even where, as in the swamps of river, flood plains and in other bogs, there are preserved the remains of plants, and sometimes insects, together with the bones of some animal drowned or mired, in most cases these swamp and bog deposits are sooner or later destroyed by the shifting channels of the stream or by the general erosion of the land.
In the sea the conditions for preservation are more favorable than on land; yet even here the proportion of animals and plants whose hard parts are fossilized is very small compared with those which either totally decay before they are buried in slowly acc.u.mulating sediments or are ground to powder by waves and currents.
We may infer that during each period of the past, as at the present, only a very insignificant fraction of the innumerable organisms of sea and land escaped destruction and left in continental and oceanic deposits permanent records of their existence. Scanty as these original life records must have been, they have been largely destroyed by metamorphism of the rocks in which they were imbedded, by solution in underground waters, and by the vast denudation under which the sediments of earlier periods have been eroded to furnish materials for the sedimentary records of later times. Moreover, very much of what has escaped destruction still remains undiscovered. The immense bulk of the stratified rocks is buried and inaccessible, and the records of the past which it contains can never be known. Comparatively few outcrops have been thoroughly searched for fossils. Although new species are constantly being discovered, each discovery may be considered as the outcome of a series of happy accidents,--that the remains of individuals of this particular species happened to be imbedded and fossilized, that they happened to escape destruction during long ages, and that they happened to be exposed and found.
SOME INFERENCES FROM THE RECORDS OF THE HISTORY OF LIFE UPON THE PLANET. Meager as are these records, they set forth plainly some important truths which we will now briefly mention.
1. Each series of the stratified rocks, except the very deepest, contains vestiges of life. Hence THE EARTH WAS TENANTED BY LIVING CREATURES FOR AN UNCALCULATED LENGTH OF TIME BEFORE HUMAN HISTORY BEGAN.
2. LIFE ON THE EARTH HAS BEEN EVERCHANGING. The youngest strata hold the remains of existing species of animals and plants and those of species and varieties closely allied to them. Strata somewhat older contain fewer existing species, and in strata of a still earlier, but by no means an ancient epoch, no existing species are to be found; the species of that epoch and of previous epochs have vanished from the living world. During all geological time since life began on earth old species have constantly become extinct and with them the genera and families to which they belong, and other species, genera, and families have replaced them. The fossils of each formation differ on the whole from those of every other. The a.s.semblage of animals and plants (the FAUNA- FLORA) of each epoch differs from that of every other epoch.
In many cases the extinction of a type has been gradual; in other instances apparently abrupt. There is no evidence that any organism once become extinct has ever reappeared. The duration of a species in time, or its "vertical range" through the strata, varies greatly. Some species are limited to a stratum a few feet in thickness; some may range through an entire formation and be found but little modified in still higher beds. A formation may thus often be divided into zones, each characterized by its own peculiar species. As a rule, the simpler organisms have a longer duration as species, though not as individuals, than the more complex.
3. THE LARGER ZOOLOGICAL AND BOTANICAL GROUPINGS SURVIVE LONGER THAN THE SMALLER. Species are so short-lived that a single geological epoch may be marked by several more or less complete extinctions of the species of its fauna-flora and their replacement by other species. A genus continues with new species after all the species with which it began have become extinct.
Families survive genera, and orders families. Cla.s.ses are so long- lived that most of those which are known from the earliest formations are represented by living forms, and no sub-kingdom has ever become extinct.
Thus, to take an example from the stony corals,--the ZOANTHARIA,-- the particular characters--which const.i.tuted a certain SPECIES-- Facosites niagarensis--of the order are confined to the Niagara series. Its GENERIC characters appeared in other species earlier in the Silurian and continued through the Devonian. Its FAMILY characters, represented in different genera and species, range from the Ordovician to the close of the Paleozoic; while the characters which it shares with all its order, the Zoantharia, began in the Cambrian and are found in living species.
4. THE CHANGE IN ORGANISMS HAS BEEN GRADUAL. The fossils of each life zone and of each formation of a conformable series closely resemble, with some explainable exceptions, those of the beds immediately above and below. The animals and plants which tenanted the earth during any geological epoch are so closely related to those of the preceding and the succeeding epochs that we may consider them to be the descendants of the one and the ancestors of the other, thus accounting for the resemblance by heredity. It is therefore believed that the species of animals and plants now living on the earth are the descendants of the species whose remains we find entombed in the rocks, and that the chain of life has been unbroken since its beginning.
5. THE CHANGE IN SPECIES HAS BEEN A GRADUAL DIFFERENTIATION.
Tracing the lines of descent of various animals and plants of the present backward through the divisions of geologic time, we find that these lines of descent converge and unite in simpler and still simpler types. The development of life may be represented by a tree whose trunk is found in the earliest ages and whose branches spread and subdivide to the growing twigs of present species.
6. THE CHANGE IN ORGANISMS THROUGHOUT GEOLOGIC TIME HAS BEEN A PROGRESSIVE CHANGE. In the earliest ages the only animals and plants on the earth were lowly forms, simple and generalized in structure; while succeeding ages have been characterized by the introduction of types more and more specialized and complex, and therefore of higher rank in the scale of being. Thus the Algonkian contains the remains of only the humblest forms of the invertebrates. In the Cambrian, Ordovician, and Silurian the invertebrates were represented in all their subkingdoms by a varied fauna. In the Devonian, fishes--the lowest of the vertebrates--became abundant. Amphibians made their entry on the stage in the Carboniferous, and reptiles came to rule the world in the Mesozoic. Mammals culminated in the Tertiary in strange forms which became more and more like those of the present as the long ages of that era rolled on; and latest of all appeared the n.o.blest product of the creative process, man.
Just as growth is characteristic of the individual life, so gradual, progressive change, or evolution, has characterized the history of life upon the planet. The evolution of the organic kingdom from its primitive germinal forms to the complex and highly organized fauna-flora of to-day may be compared to the growth of some n.o.ble oak as it rises from the acorn, spreading loftier and more widely extended branches as it grows.
7. While higher and still higher types have continually been evolved, until man, the highest of all, appeared, THE LOWER AND EARLIER TYPES HAVE GENERALLY PERSISTED. Some which reached their culmination early in the history of the earth have since changed only in slight adjustments to a changing environment. Thus the brachiopods, a type of sh.e.l.lfish, have made no progress since the Paleozoic, and some of their earliest known genera are represented by living forms hardly to be distinguished from their ancient ancestors. The lowest and earliest branches of the tree of life have risen to no higher levels since they reached their climax of development long ago.
8. A strange parallel has been found to exist between the evolution of organisms and the development of the individual. In the embryonic stages of its growth the individual pa.s.ses swiftly through the successive stages through which its ancestors evolved during the millions of years of geologic time. THE DEVELOPMENT OF THE INDIVIDUAL RECAPITULATES THE EVOLUTION OF THE RACE.
The frog is a typical amphibian. As a tadpole it pa.s.ses through a stage identical in several well-known features with the maturity of fishes; as, for example, its aquatic life, the tail by which it swims, and the gills through which it breathes. It is a fair inference that the tadpole stage in the life history of the frog represents a stage in the evolution of its kind,--that the Amphibia are derived from fishlike ancestral forms. This inference is amply confirmed in the geological record; fishes appeared before Amphibia and were connected with them by transitional forms.
THE GREAT LENGTH OF GEOLOGIC TIME INFERRED FROM THE SLOW CHANGE OF SPECIES. Life forms, like land forms, are thus subject to change under the influence of their changing environment and of forces acting from within. How slowly they change may be seen in the apparent stability of existing species. In the lifetime of the observer and even in the recorded history of man, species seem as stable as the mountain and the river. But life forms and land forms are alike variable, both in nature and still more under the shaping hand of man. As man has modified the face of the earth with his great engineering works, so he has produced widely different varieties of many kinds of domesticated plants and animals, such as the varieties of the dog and the horse, the apple and the rose, which may be regarded in some respects as new species in the making. We have a.s.sumed that land forms have changed in the past under the influence of forces now in operation. a.s.suming also that life forms have always changed as they are changing at present, we come to realize something of the immensity of geologic time required for the evolution of life from its earliest lowly forms up to man.
It is because the onward march of life has taken the same general course the world over that we are able to use it as a UNIVERSAL TIME SCALE and divide geologic time into ages and minor subdivisions according to the ruling or characteristic organisms then living on the earth. Thus, since vertebrates appeared, we have in succession the Age of Fishes, the Age of Amphibians, the Age of Reptiles, and the Age of Mammals.
The chart given on page 295 is thus based on the law of superposition and the law of the evolution of organisms. The first law gives the succession of the formations in local areas. The fossils which they contain demonstrate the law of the progressive appearance of organisms, and by means of this law the formations of different countries are correlated and set each in its place in a universal time scale and grouped together according to the affinities of their imbedded organic remains.
GEOLOGIC TIME DIVISIONS COMPARED WITH THOSE OF HUMAN HISTORY. We may compare the division of geologic time into eras, periods, and other divisions according to the dominant life of the time, to the ill-defined ages into which human history is divided according to the dominance of some nation, ruler, or other characteristic feature. Thus we speak of the DARK AGES, the AGE OF ELIZABETH, and the AGE OF ELECTRICITY. These crude divisions would be of much value if, as in the case of geologic time, we had no exact reckoning of human history by years.
And as the course of human history has flowed in an unbroken stream along quiet reaches of slow change and through periods of rapid change and revolution, so with the course of geologic history. Periods of quiescence, in which revolutionary forces are perhaps gathering head, alternate with periods of comparatively rapid change in physical geography and in organisms, when new and higher forms appear which serve to draw the boundary line of new epochs. Nevertheless, geological history is a continuous progress; its periods and epochs shade into one another by imperceptible gradations, and all our subdivisions must needs be vague and more or less arbitrary.
HOW FOSSILS TELL OF THE GEOGRAPHY OF THE PAST. Fossils are used not only as a record of the development of life upon the earth, but also in testimony to the physical geography of past epochs.
They indicate whether in any region the climate was tropical, temperate, or arctic. Since species spread slowly from some center of dispersion where they originate until some barrier limits their migration farther, the occurrence of the same species in rocks of the same system in different countries implies the absence of such barriers at the period. Thus in the collection of antarctic fossils referred to on page 294 there were shallow-water marine sh.e.l.ls identical in species with Mesozoic sh.e.l.ls found in India and in the southern extremity of South America. Since such organisms are not distributed by the currents of the deep sea and cannot migrate along its bottom, we infer a shallow-water connection in Mesozoic times between India, South America, and the antarctic region. Such a shallow-water connection would be offered along the marginal shelf of a continent uniting these now widely separated countries.