Chapter 13

Which will be more strongly altered, the rocks about a closed dike in which lava began to cool as soon as it filled the fissure, or the rocks about a dike which opened on the surface and through which the molten rock flowed for some time?Taking into consideration the part played by heated waters, which will produce the most far-reaching metamorphism, dikes which cut across the bedding planes or intrusive sheets which are thrust between the strata?

Taking into consideration the part played by heated waters, which will produce the most far-reaching metamorphism, dikes which cut across the bedding planes or intrusive sheets which are thrust between the strata?

Regional metamorphism.Metamorphic rocks occur wide-spread in many regions, often hundreds of square miles in area, where such extensive changes cannot be accounted for by igneous intrusions. Such are the dissected cores of lofty mountains, as the Alps, and the worn-down bases of ancient ranges, as in New England, large areas in the Piedmont Belt, and the Laurentian peneplain.

In these regions the rocks have yielded to immense pressure. They have been folded, crumpled, and mashed, and even their minute grains, as one may see with a microscope, have often been puckered, broken, and crushed to powder. It is to these mechanical movements and strains which the rocks have suffered in every part that we may attribute their metamorphism, and the degree to which they have been changed is in direct proportion to the degree to which they have been deformed and mashed.

Other factors, however, have played important parts. Rock crushing develops heat, and allows a freer circulation of heated waters and vapors. Thus chemical reactions are greatly quickened; minerals are dissolved and redeposited in new positions, or their chemical constituents may recombine in new minerals, entirely changing the nature of the rock, as when, for example, feldspar recrystallizes as quartz and mica.

Early stages of metamorphism are seen inslate. Pressure has hardened the marine muds, the arkose (p. 186), or the volcanic ash from which slates are derived, and has caused them to cleave by the rearrangement of their particles.Under somewhat greater pressure, slate becomesphyllite, a clay slate whose cleavage surfaces are lustrous with flat-lying mica flakes. The same pressure which has caused the rock to cleave has set free some of its mineral constituents along the cleavage planes to crystallize there as mica.

Under somewhat greater pressure, slate becomesphyllite, a clay slate whose cleavage surfaces are lustrous with flat-lying mica flakes. The same pressure which has caused the rock to cleave has set free some of its mineral constituents along the cleavage planes to crystallize there as mica.

Fig. 255.A Foliated Rock

Foliation.Under still stronger pressure the whole structure of the rock is altered. The minerals of which it is composed, and the new minerals which develop by heat and pressure, arrange themselves along planes of cleavage or of shear in rudely parallel leaves, orfolia. Of this structure, calledfoliation, we may distinguish two types,—a coarser feldspathic type, and a fine type in which other minerals than feldspar predominate.

Gneissis the general name under which are comprised coarsely foliated rocks banded with irregular layers of feldspar and other minerals. The gneisses appear to be due in many cases to the crushing and shearing of deep-seated igneous rocks, such as granite and gabbro.

The crystalline schists, representing the finer types of foliation, consist of thin, parallel, crystalline leaves, which are often remarkably crumpled. These folia can be distinguished from the laminae of sedimentary rocks by their lenticular form andlack of continuity, and especially by the fact that they consist of platy, crystalline grains, and not of particles rounded by wear.

Mica schist, the most common of schists, and in fact of all metamorphic rocks, is composed of mica and quartz in alternating wavy folia. All gradations between it and phyllite may be traced, and in many cases we may prove it due to the metamorphism of slates and shales. It is widespread in New England and along the eastern side of the Appalachians.Talc schistconsists of quartz andtalc, a light-colored magnesian mineral of greasy feel, and so soft that it can be scratched with the thumb nail.Hornblende schist, resulting in many cases from the foliation of basic igneous rocks, is made of folia of hornblende alternating with bands of quartz and feldspar. Hornblende schist is common over large areas in the Lake Superior region.Quartz schistis produced from quartzite by the development of fine folia of mica along planes of shear. All gradations may be found between it and unfoliated quartzite on the one hand and mica schist on the other.Under the resistless pressure of crustal movements almost any rocks, sandstones, shales, lavas of all kinds, granites, diorites, and gabbros may be metamorphosed into schists by crushing and shearing. Limestones, however, are metamorphosed by pressure intomarble, the grains of carbonate of lime recrystallizing freely to interlocking crystals of calcite.

Hornblende schist, resulting in many cases from the foliation of basic igneous rocks, is made of folia of hornblende alternating with bands of quartz and feldspar. Hornblende schist is common over large areas in the Lake Superior region.

Quartz schistis produced from quartzite by the development of fine folia of mica along planes of shear. All gradations may be found between it and unfoliated quartzite on the one hand and mica schist on the other.

Under the resistless pressure of crustal movements almost any rocks, sandstones, shales, lavas of all kinds, granites, diorites, and gabbros may be metamorphosed into schists by crushing and shearing. Limestones, however, are metamorphosed by pressure intomarble, the grains of carbonate of lime recrystallizing freely to interlocking crystals of calcite.

These few examples must suffice of the great class of metamorphic rocks. As we have seen, they owe their origin to the alteration of both of the other classes of rocks—the sedimentary and the igneous—by heat and pressure, assisted usually by the presence of water. The fact of change is seen in their hardness arid cementation, their more or less complete recrystallization, and their foliation; but the change is often so complete that no trace of their original structure and mineral composition remains to tell whether the rocks from which they were derived were sedimentary or igneous, or to what variety of either of these classes they belonged.

Fig. 256.Contorted Gneiss, the Ottawa River, Canada

Fig. 257.Quartz Veins in Slate

In many cases, however, the early history of a metamorphic rock can be deciphered. Fossils not wholly obliterated may prove it originally water-laid. Schists may contain rolled-out pebbles, showing their derivation from a conglomerate. Dikes of igneous rocks may be followed into a region where they have been foliated by pressure. The most thoroughly metamorphosed rocks may sometimes be traced out into unaltered sedimentary or igneous rocks, or among them may be found patches of little change where their history maybe read.

Metamorphism is most common among rocks of the earlier geological ages, and most rare among rocks of recent formation. No doubt it is now in progress where deep-buried sediments are invaded by heat either from intrusive igneous masses or from the earth’s interior, or are suffering slow deformation under the thrust of mountain-making forces.

Suggest how rocks now in process of metamorphism may sometimes be exposed to view. Why do metamorphic rocks appear on the surface to-day?

Mineral Veins

In regions of folded and broken rocks fissures are frequently found to be filled with sheets of crystalline minerals deposited from solution by underground water, and fissures thus filled are known asmineral veins. Much of the importance of mineral veins is due to the fact that they are often metalliferous,carrying valuable native metals and metallic ores disseminated in fine particles, in strings, and sometimes in large masses in the midst of the valueless nonmetallic minerals which make up what is known as thevein stone.

The most common vein stones arequartzandcalcite.fluorite(calcium fluoride), a mineral harder than calcite and crystallizing in cubes of various colors, andbarite(barium sulphate), a heavy white mineral, are abundant in many veins.

Fig. 258.Placer Deposits in Californiag, gold-bearing gravels in present river beds;g´, ancient gold-bearing river gravels;a,a, lava flows capping table mountains;s, slate. Draw a diagram showing by dotted lines conditions before the lava flows occurred. What changes have since taken place?

The gold-bearing quartz veins of California traverse the metamorphic slates of the Sierra Nevada Mountains. Below the zone of solution (p. 45) these veins consist of a vein stone of quartz mingled with pyrite (p. 13), the latter containing threads and grains of native gold. But to the depth of about fifty feet from the surface the pyrite of the vein has been dissolved, leaving a rusty, cellular quartz with grains of the insoluble gold scattered through it.Theplacer depositsof California and other regions are gold- bearing deposits of gravel and sand in river beds. The heavy gold is apt to be found mostly near or upon the solid rock, and its grains, like those of the sand, are always rounded. How the gold came in the placers we may leave the pupil to suggest.

Theplacer depositsof California and other regions are gold- bearing deposits of gravel and sand in river beds. The heavy gold is apt to be found mostly near or upon the solid rock, and its grains, like those of the sand, are always rounded. How the gold came in the placers we may leave the pupil to suggest.

Copper is found in a number of ores, and also in the native metal. Below the zone of surface changes the ore of a copper vein is often a double sulphide of iron and copper calledchalcopyrite, a mineral softer than pyrite—it can easily be scratched with a knife—and deeper yellow in color. For several score of feet below the ground the vein may consist of rusty quartz from which the metallic ores have been dissolved; but at the base of the zone of solution we may find exceedingly rich deposits of copper ores,— copper sulphides, red and black copper oxides, and green and blue copper carbonates, whichhave clearly been brought down in solution from the leached upper portion of the vein.

Origin of mineral veins.Both vein stones and ores have been deposited slowly from solution in water, much as crystals of salt are deposited on the sides of a jar of saturated brine. In our study of underground water we learned that it is everywhere circulating through the permeable rocks of the crust, descending to profound depths under the action of gravity and again driven to the surface by hydrostatic pressure. Now fissures, wherever they occur, form the trunk channels of the underground circulation. Water descends from the surface along these rifts; it moves laterally from either side to the fissure plane, just as ground water seeps through the surrounding rocks from every direction to a well; and it ascends through these natural water ways as in an artesian well, whenever they intersect an aquifer in which water is under hydrostatic pressure.

The waters which deposit vein stones and ores are commonly hot, and in many cases they have derived their heat from intrusions of igneous rock still uncooled within the crust. The solvent power of the water is thus greatly increased, and it takes up into solution various substances from the igneous and sedimentary rocks which it traverses. For various reasons these substances stances are deposited in the vein as ores and vein stones. On rising through the fissure the water cools and loses pressure, and its capacity to hold minerals in solution is therefore lessened. Besides, as different currents meet in the fissure, some ascending, some descending, and some coming in from the sides, the chemical reaction of these various weak solutions upon one another and upon the walls of the vein precipitates the minerals of vein stuffs and ores.

As an illustration of the method of vein deposits we may cite the case of a wooden box pipe used in the Comstock mines, Nevada, to carry the hot water of the mine from one level to another, which in ten years was lined with calcium carbonate more than half an inch thick.The Steamboat Springs, Nevada, furnish examples of mineral veins in process of formation. The steaming water rises through fissures in volcanic rocks and is now depositing in the rifts a vein stone of quartz, with metallic ores of iron, mercury, lead, and other metals.

The Steamboat Springs, Nevada, furnish examples of mineral veins in process of formation. The steaming water rises through fissures in volcanic rocks and is now depositing in the rifts a vein stone of quartz, with metallic ores of iron, mercury, lead, and other metals.

Fig. 259.Reconcentration of Ores in Mineral VeinsA, original vein;B, same after reconcentration;v, mineral vein;s, surface of ground (dotted line, former surfaces of the ground);sp, spring;o, vein leached of ores by descending waters in zone of solution;m, rich ore deposits reconcentrated from above;n, unchanged portion of vein

Reconcentration.Near the base of the zone of solution veins are often stored with exceptionally large and valuable ore deposits. This local enrichment of the vein is due to the reconcentration of its metalliferous ores. As the surface of the land is slowly lowered by weathering and running water, the zone of solution is lowered at an equal rate and encroaches constantly on the zone of cementation. The minerals of veins are therefore constantly being dissolved along their upper portions and carried down the fissures by ground water to lower levels, where they are redeposited.

Many of the richest ore deposits are thus due to successive concentrations: the ores were leached originally from the rocksto a large extent by laterally seeping waters; they were concentrated in the ore deposits of the vein chiefly by ascending currents; they have been reconcentrated by descending waters in the way just mentioned.

The original source of the metals.It is to the igneous rocks that we may look for the original source of the metals of veins. Lavas contain minute percentages of various metallic compounds, and no doubt this was the case also with the igneous rocks which formed the original earth crust. By the erosion of the igneous rocks the metals have been distributed among sedimentary strata, and even the sea has taken into solution an appreciable amount of gold and other metals, but in this widely diffused condition they are wholly useless to man. The concentration which has made them available is due to the interaction of many agencies. Earth movements fracturing deeply the rocks of the crust, the intrusion of heated masses, the circulation of underground waters, have all cooperated in the concentration of the metals of mineral veins.

While fissure veins are the most important of mineral veins, the latter term is applied also to any water way which has been filled by similar deposits from solution. Thus in soluble rocks, such as limestones, joints enlarged by percolating water are sometimes filled with metalliferous deposits, as, for example, the lead and zinc deposits of the upper Mississippi valley. Even a porous aquifer may be made the seat of mineral deposits, as in the case of some copper-bearing and silver-bearing sandstones of New Mexico.

Fig. 260.Geological Map of the United states and Part of CanadaClick on map to view larger version

PART III

HISTORICAL GEOLOGY

CHAPTER XIV

THE GEOLOGICAL RECORD

What a formation records.We have already learned that each individual body of stratified rock, or formation, constitutes a record of the time when it was laid. The structure and the character of the sediments of each formation tell whether the area was land or sea at the time when they were spread; and if the former, whether the land was river plain, or lake bed, or was covered with wind-blown sands, or by the deposits of an ice sheet. If the sediments are marine, we may know also whether they were laid in shoal water near the shore or in deeper water out at sea, and whether during a period of emergence, or during a period of subsidence when the sea transgressed the land. By the same means each formation records the stage in the cycle of erosion of the land mass from which its sediments were derived (p. 185). An unconformity between two marine formations records the fact that between the periods when they were deposited in the sea the area emerged as land and suffered erosion (p. 227). The attitude and structure of the strata tell also of the foldings and fractures, the deformation and the metamorphism, which they have suffered; and the igneous rocks associated with them as lava flows and igneous intrusions add other details to the story. Each formation is thus a separate local chapter in the geological history of theearth, and its strata are its leaves. It contains an authentic record of the physical conditions—the geography—of the time and place when and where its sediments were laid.

Past cycles of erosion.These chapters in the history of the planet are very numerous, although much of the record has been destroyed in various ways. A succession of different formations is usually seen in any considerable section of the crust, such as a deep canyon or where the edges of upturned strata are exposed to view on the flanks of mountain ranges; and in any extensive area, such as a state of the Union or a province of Canada, the number of formations outcropping on the surface is large.

It is thus learned that our present continent is made up for the most part of old continental deltas. Some, recently emerged as the strata of young coastal plains, are the records of recent cycles of erosion; while others were deposited in the early history of the earth, and in many instances have been crumpled into mountains, which afterwards were leveled to their bases and lowered beneath the sea to receive a cover of later sediments before they were again uplifted to form land.

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 allthe 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 accumulate upon the sea floor (p.184), 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 constitute a connected record in which the geological history of the planet may be read, and therefore known as thegeological 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 thelaw 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 attitude 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 thelaw of included organisms; i.e. formations, however widely separated, which contain a similar assemblage 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 accumulating along the west coast of Europe. If now all these offshore 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 shells) of genera which are found on other continents in certain formations classified 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.

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 shells) of genera which are found on other continents in certain formations classified 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 thetimes during which the successive formations were laid. The largest assemblages 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.

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 theCambrian system, meaning a certain succession of formations which are classified together because of broad resemblances in their included organisms; and of theCambrian period, meaning the time during which these rocks were deposited.

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 shells, 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 hardsubstanceis 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 shells originally made ofaragonite, a crystalline form of carbonate of lime, that mineral is usually replaced bycalcite, a more stable form of the same substance. The most common petrifying materials are calcite, silica, and pyrite (p. 13).Often the organic substance has neither been preserved nor replaced, but theformhas 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.

Often the organic substance has neither been preserved nor replaced, but theformhas 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.

Fig. 261.Section of Cast and Mold of a Shella, shell;b, mold of exterior;c, cast of interior

The different kinds of molds and casts may be illustrated by means of a clam shell and some moist clay, the latter representing the sediments in which the remains of animals and plants are entombed. Imbedding the shell in the clay and allowing the clayto harden, we have amold of the exteriorof the shell, as is seen on cutting the clay matrix in two and removing the shell from it. Filling this mold with clay of different color, we obtain acast of the exterior, which represents accurately the original form and surface markings of the shell. In nature, shells 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, acast of the interioris made in the same way. Interior casts of shells 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.

Where the fossil is hollow, acast of the interioris made in the same way. Interior casts of shells 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 accumulating 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 theirexistence. 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. Hencethe 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 formationdiffer on the whole from those of every other. The assemblage of animals and plants (thefauna-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. Classes 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,—thezoantharia,— the particular characters—which constituted a certainspecies—Facosites niagarensis—of the order are confined to the Niagara series. Itsgenericcharacters appeared in other species earlier in the Silurian and continued through the Devonian. Itsfamilycharacters, 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 seriesclosely 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 noblest 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 noble 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 shellfish, 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 passes 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 passes 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 assumed that land forms have changed in the past under the influence of forces now in operation. Assuming 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 auniversal time scaleand 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 onpage 295is 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 theDark Ages, theAge of Elizabeth, and theAge 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.

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 onpage 294there were shallow-water marine shells identical in species with Mesozoic shells 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.

CHAPTER XV

THE PRE-CAMBRIAN SYSTEMS

The earth’s beginnings.The geological record does not tell us of the beginnings of the earth. The history of the planet, as we have every reason to believe, stretches far back beyond the period of the oldest stratified rocks, and is involved in the history of the solar system and of the nebula,—the cloud of glowing gases or of cosmic dust,—from which the sun and planets are believed to have been derived.

Back to Index Next