The nebular hypothesis.It was long held that the earth began as a vaporous, shining sphere, formed by the gathering together of the material of a gaseous ring which had been detached from a cooling and shrinking nebula. Such a vaporous sphere would condense to a liquid fiery globe, whose surface would become cold and solid, while the interior would long remain intensely hot because of the slow conductivity of the crust. Under these conditions the primeval atmosphere of the earth must have contained in vapor the water now belonging to the earth’s crust and surface. It also held all the oxygen since locked up in rocks by their oxidation, and all the carbon dioxide which has since been laid away in limestones, besides that corresponding to the carbon of carbonaceous deposits, such as peat, coal, and petroleum. On this hypothesis the original atmosphere was dense, dark, and noxious, and enormously heavier than the atmosphere at present.The accretion hypothesis.On the other hand, it has been recently suggested that the earth may have grown to its present size by the gradual accretion of meteoritic masses. Such cold, stony bodies might have come together at so slow a rate that the heat caused by their impact would not raise sensibly the temperature of the growing planet. Thus the surface of the earth may never have been hot and luminous; but as the loose aggregation of stony masses grew larger and was more and more compressed by its own gravitation, the heat thus generatedraised the interior to high temperatures, while from time to time molten rock was intruded among the loose, cold meteoritic masses of the crust and outpoured upon the surface.It is supposed that the meteorites of which the earth was built brought to it, as meteorites do now, various gases shut up within their pores. As the heat of the interior increased, these gases transpired to the surface and formed the primitive atmosphere and hydrosphere. The atmosphere has therefore grown slowly from the smallest beginnings. Gases emitted from the interior in volcanic eruptions and in other ways have ever added to it, and are adding to it now. On the other hand, the atmosphere has constantly suffered loss, as it has been robbed of oxygen by the oxidation of rocks in weathering, and of carbon dioxide in the making of limestones and carbonaceous deposits.
The nebular hypothesis.It was long held that the earth began as a vaporous, shining sphere, formed by the gathering together of the material of a gaseous ring which had been detached from a cooling and shrinking nebula. Such a vaporous sphere would condense to a liquid fiery globe, whose surface would become cold and solid, while the interior would long remain intensely hot because of the slow conductivity of the crust. Under these conditions the primeval atmosphere of the earth must have contained in vapor the water now belonging to the earth’s crust and surface. It also held all the oxygen since locked up in rocks by their oxidation, and all the carbon dioxide which has since been laid away in limestones, besides that corresponding to the carbon of carbonaceous deposits, such as peat, coal, and petroleum. On this hypothesis the original atmosphere was dense, dark, and noxious, and enormously heavier than the atmosphere at present.
The accretion hypothesis.On the other hand, it has been recently suggested that the earth may have grown to its present size by the gradual accretion of meteoritic masses. Such cold, stony bodies might have come together at so slow a rate that the heat caused by their impact would not raise sensibly the temperature of the growing planet. Thus the surface of the earth may never have been hot and luminous; but as the loose aggregation of stony masses grew larger and was more and more compressed by its own gravitation, the heat thus generatedraised the interior to high temperatures, while from time to time molten rock was intruded among the loose, cold meteoritic masses of the crust and outpoured upon the surface.
It is supposed that the meteorites of which the earth was built brought to it, as meteorites do now, various gases shut up within their pores. As the heat of the interior increased, these gases transpired to the surface and formed the primitive atmosphere and hydrosphere. The atmosphere has therefore grown slowly from the smallest beginnings. Gases emitted from the interior in volcanic eruptions and in other ways have ever added to it, and are adding to it now. On the other hand, the atmosphere has constantly suffered loss, as it has been robbed of oxygen by the oxidation of rocks in weathering, and of carbon dioxide in the making of limestones and carbonaceous deposits.
While all hypotheses of the earth’s beginnings are as yet unproved speculations, they serve to bring to mind one of the chief lessons which geology has to teach,—that the duration of the earth in time, like the extension of the universe in space, is vastly beyond the power of the human mind to realize. Behind the history recorded in the rocks, which stretches back for many million years, lies the long unrecorded history of the beginnings of the planet; and still farther in the abysses of the past are dimly seen the cycles of the evolution of the solar system and of the nebula which gave it birth.
We pass now from the dim realm of speculation to the earliest era of the recorded history of the earth, where some certain facts may be observed and some sure inferences from them may be drawn.
The Archean
The oldest known sedimentary strata, wherever they are exposed by uplift and erosion, are found to be involved with a mass of crystalline rocks which possesses the same characteristics in all parts of the world. It consists of foliated rocks, gneisses, and schists of various kinds, which have been cut with dikes and other intrusions of molten rock, and have beenbroken, crumpled, and crushed, and left in interlocking masses so confused that their true arrangement can usually be made out only with the greatest difficulty if at all. The condition of this body of crystalline rocks is due to the fact that they have suffered not only from the faultings, foldings, and igneous intrusions of their time, but necessarily, also, from those of all later geological ages.
At present three leading theories are held as to the origin of these basal crystalline rocks.
1. They are considered by perhaps the majority of the geologists who have studied them most carefully to be igneous rocks intruded in a molten state among the sedimentary rocks involved with them. In many localities this relation is proved by the phenomena of contact (p. 268); but for the most part the deformations which the rocks have since suffered again and again have been sufficient to destroy such evidence if it ever existed.
2. An older view regards them as profoundly altered sedimentary strata, the most ancient of the earth.
3. According to a third theory they represent portions of the earth’s original crust; not, indeed, its original surface, but deeper portions uncovered by erosion and afterwards mantled with sedimentary deposits. All these theories agree that the present foliated condition of these rocks is due to the intense metamorphism which they have suffered.
It is to this body of crystalline rocks and the stratified rocks involved with it, which form a very small proportion of its mass, that the termArchean(Greek,archē, beginning) is applied by many geologists.
Proterozoic Era: The Algonkian Group
In some regions there rests unconformably on the Archean an immense body of stratified rocks, thousands and in places even scores of thousands of feet thick, known as theAlgonkian.Great unconformities divide it into well-defined systems, but as only the scantiest traces of fossils appear here and there among its strata, it is as yet impossible to correlate the formations of different regions and to give them names of more than local application. We will describe the Algonkian rocks of two typical areas.
The Grand Canyon of the Colorado.We have already studied a very ancient peneplain whose edge is exposed to view deep on the walls of the Colorado Canyon (nn´, Fig. 207). The formation of flat-lying sandstone which covers this buried land surface is proved by its fossils to belong to the Cambrian,—the earliest period of the Paleozoic era. The tilted rocks (b, Fig. 207). on whose upturned edges the Cambrian sandstone rests are far older, for the physical break which separates them from it records a time interval during which they were upheaved to mountainous ridges and worn down to a low plain. They are therefore classified as Algonkian. They comprise two immense series. The upper is more than five thousand feet thick and consists of shales and sandstones with some limestones. Separated from it by an unconformity which does not appear inFigure 207, the lower division, seven thousand feet thick, consists chiefly of massive reddish sandstones with seven or more sheets of lava interbedded. The lowest member is a basal conglomerate composed of pebbles derived from the erosion of the dark crumpled schists beneath,—schists which are supposed to be Archean. As shown inFigure 207, a strong unconformity (nm´, Fig. 207) parts the schists and the Algonkian. The floor on which the Algonkian rests is remarkably even, and here again is proved an interval of incalculable length, during which an ancient land mass of Archean rocks was baseleveled before it received the cover of the sediments of the later age.
The Lake Superior region.In eastern Canada an area of pre- Cambrian rocks, Archean and Algonkian, estimated at two million square miles, stretches from the Great Lakes and theSt. Lawrence River northward to the confines of the continent, inclosing Hudson Bay in the arms of a giganticU. This immense area, which we have already studied as the Laurentian peneplain (p. 89), extends southward across the Canadian border into northern Minnesota, Wisconsin, and Michigan. The rocks of this area are known to be pre-Cambrian; for the Cambrian strata, wherever found, lie unconformably upon them.
Fig. 262.Ideal Section in the Lake Superior Region
Fig. 262.Ideal Section in the Lake Superior Region
The general relations of the formations of that portion of the area which lies about Lake Superior are shown inFigure 262. Great unconformities,UU´separate the Algonkian both from the Archean and from the Cambrian, and divide it into three distinct systems, —theLower Huronian, theUpper Huronian, and theKeweenawan. The Lower and the Upper Huronian consist in the main of old sea muds and sands and limy oozes now changed to gneisses, schists, marbles, quartzites, slates, and other metamorphic rocks. The Keweenawan is composed of immense piles of lava, such as those of Iceland, overlain by bedded sandstones. What remains of these rock systems after the denudation of all later geologic ages is enormous. The Lower Huronian is more than a mile thick, the Upper Huronian more than two miles thick, while the Keweenawan exceeds nine miles in thickness. The vast length of Algonkian time is shown by the thickness of its marine deposits and by the cycles of erosion which it includes. InFigure 262the student may read an outline of the history of the Lake Superior region, the deformations which it suffered, their relative severity, the timeswhen they occurred, and the erosion cycles marked by the successive unconformities.
Other pre-Cambrian areas in North America.Pre-Cambrian rocks are exposed in various parts of the continent, usually by the erosion of mountain ranges in which their strata were infolded. Large areas occur in the maritime provinces of Canada. The core of the Green Mountains of Vermont is pre-Cambrian, and rocks of these systems occur in scattered patches in western Massachusetts. Here belong also the oldest rocks of the Highlands of the Hudson and of New Jersey. The Adirondack region, an outlier of the Laurentian region, exposes pre-Cambrian rocks, which have been metamorphosed and tilted by the intrusion of a great boss of igneous rock out of which the central peaks are carved. The core of the Blue Ridge and probably much of the Piedmont Belt are of this age. In the Black Hills the irruption of an immense mass of granite has caused or accompanied the upheaval of pre-Cambrian strata and metamorphosed them by heat and pressure into gneisses, schists, quartzites, and slates. In most of these mountainous regions the lowest strata are profoundly changed by metamorphism, and they can be assigned to the pre-Cambrian only where they are clearly overlain unconformably by formations proved to be Cambrian by their fossils. In the Belt Mountains of Montana, however, the Cambrian is underlain by Algonkian sediments twelve thousand feet thick, and but little altered.
Mineral wealth of the pre-Cambrian rocks.The pre-Cambrian rocks are of very great economic importance, because of their extensive metamorphism and the enormous masses of igneous rock which they involve. In many parts of the country they are the source of supply of granite, gneiss, marble, slate, and other such building materials. Still more valuable are the stores of iron and copper and other metals which they contain.
At the present time the pre-Cambrian region about Lake Superior leads the world in the production of iron ore, itsoutput for 1903 being more than five sevenths of the entire output of the whole United States, and exceeding that of any foreign country. The ore bodies consist chiefly of the red oxide of iron (hematite) and occur in troughs of the strata, underlain by some impervious rock. A theory held by many refers the ultimate source of the iron to the igneous rocks of the Archean. When these rocks were upheaved and subjected to weathering, their iron compounds were decomposed. Their iron was leached out and carried away to be laid in the Algonkian water bodies in beds of iron carbonate and other iron compounds. During the later ages, after the Algonkian strata had been uplifted to form part of the continent, a second concentration has taken place. Descending underground waters charged with oxygen have decomposed the iron carbonate and deposited the iron, in the form of iron oxide, in troughs of the strata where their downward progress was arrested by impervious floors.
The pre-Cambrian rocks of the eastern United States also are rich in iron. In certain districts, as in the Highlands of New Jersey, the black oxide of iron (magnetite) is so abundant in beds and disseminated grains that the ordinary surveyor’s compass is useless.
The pre-Cambrian copper mines of the Lake Superior region are among the richest on the globe. In the igneous rocks copper, next to iron, is the most common of all the useful metals, and it was especially abundant in the Keweenawan lavas. After the Keweenawan was uplifted to form land, percolating waters leached out much of the copper diffused in the lava sheets and deposited it within steam blebs as amygdules of native copper, in cracks and fissures, and especially as a cement, or matrix, in the interbedded gravels which formed the chief aquifers of the region. The famous Calumet and Hecla mine follows down the dip of the strata to the depth of nearly a mile and works such an ancient conglomerate whose matrix is pure copper.
Fig. 263.Successive Stages in the Development of the Ovum to the Gastrula Stage
Fig. 263.Successive Stages in the Development of the Ovum to the Gastrula Stage
The appearance of life.Sometime during the dim ages preceding the Cambrian, whether in the Archean or in the Algonkian we know not, occurred one of the most important events in the history of the earth. Life appeared for the first time upon the planet. Geology has no evidence whatever to offer as to whence or how life came. All analogies lead us to believe that its appearance must have been sudden. Its earliest forms are unknown, but analogy suggests that as every living creature has developed from a single cell, so the earliest organisms upon the globe—the germs from which all later life is supposed to have been evolved—were tiny, unicellular masses of protoplasm, resembling the amoeba of to-day in the simplicity of their structure.
Such lowly forms were destitute of any hard parts and could leave no evidence of their existence in the record of the rocks. And of their supposed descendants we find so few traces in the pre- Cambrian strata that the first steps in organic evolution must be supplied from such analogies in embryology as the following. The fertilized ovum, the cell with which each animal begins its life, grows and multiplies by cell division, and develops into a hollow globe of cells called theblastosphere. This stage is succeeded by the stage of thegastrula,—an ovoid or cup-shaped body with a double wall of cells inclosing a body cavity, and with an opening, the primitive mouth. Each of these early embryological stages is represented by living animals,—the undivided cell by theprotozoa, the blastosphere bysome rare forms, and the gastrula in the essential structure of thecœlenterates,—the subkingdom to which the fresh-water hydra and the corals belong. All forms of animal life, from the cœlenterates to the mammals, follow the same path in their embryological development as far as the gastrula stage, but here their paths widely diverge, those of each subkingdom going their own separate ways.
We may infer, therefore, that during the pre-Cambrian periods organic evolution followed the lines thus dimly traced. The earliest one-celled protozoa were probably succeeded by many- celled animals of the type of the blastosphere, and these by gastrula-like organisms. From the gastrula type the higher sub- divisions of animal life probably diverged, as separate branches from a common trunk. Much or all of this vast differentiation was accomplished before the opening of the next era; for all the subkingdoms are represented in the Cambrian except the vertebrates.
Evidences of pre-Cambrian life.An indirect evidence of life during the pre-Cambrian periods is found in the abundant and varied fauna of the next period; for, if the theory of evolution is correct, the differentiation of the Cambrian fauna was a long process which might well have required for its accomplishment a large part of pre-Cambrian time.
Other indirect evidences are the pre-Cambrian limestones, iron ores, and graphite deposits, since such minerals and rocks have been formed in later times by the help of organisms. If the carbonate of lime of the Algonkian limestones and marbles was extracted from sea water by organisms, as is done at present by corals, mollusks, and other humble animals and plants, the life of those ancient seas must have been abundant. Graphite, a soft black mineral composed of carbon and used in the manufacture of lead pencils and as a lubricant, occurs widely in the metamorphic pre-Cambrian rocks. It is known to be produced in some cases by the metamorphism of coal, which itselfis formed of decomposed vegetal tissues. Seams of graphite may therefore represent accumulations of vegetal matter such as seaweed. But limestone, iron ores, and graphite can be produced by chemical processes, and their presence in the pre-Cambrian makes it only probable, and not certain, that life existed at that time.
Pre-Cambrian fossils.Very rarely has any clear trace of an organism been found in the most ancient chapters of the geological record, so many of their leaves have been destroyed and so far have their pages been defaced. Omitting structures whose organic nature has been questioned, there are left to mention a tiny seashell of one of the most lowly types,—aDiscinafrom the pre-Cambrian rocks of the Colorado Canyon,—and from the pre-Cambrian rocks of Montana trails of annelid worms and casts of their burrows in ancient beaches, and fragments of the tests of crustaceans. These diverse forms indicate that before the Algonkian had closed, life was abundant and had widely differentiated. We may expect that other forms will be discovered as the rocks are closely searched.
Pre-Cambrian geography.Our knowledge is far too meager to warrant an attempt to draw the varying outlines of sea and land during the Archean and Algonkian eras. Pre-Cambrian time probably was longer than all later geological time down to the present, as we may infer from the vast thicknesses of its rocks and the unconformities which part them. We know that during its long periods land masses again and again rose from the sea, were worn low, and were submerged and covered with the waste of other lands. But the formations of separated regions cannot be correlated because of the absence of fossils, and nothing more can be made out than the detached chapters of local histories, such as the outline given of the district about Lake Superior.
The pre-Cambrian rocks show no evidence of any forces then at work upon the earth except the forces which are at workupon it now. The most ancient sediments known are so like the sediments now being laid that we may infer that they were formed under conditions essentially similar to those of the present time. There is no proof that the sands of the pre-Cambrian sandstones were swept by any more powerful waves and currents than are offshore sands to-day, or that the muds of the pre-Cambrian shales settled to the sea floor in less quiet water than such muds settle in at present. The pre-Cambrian lands were, no doubt, worn by wind and weather, beaten by rain, and furrowed by streams as now, and, as now, they fronted the ocean with beaches on which waves dashed and along which tidal currents ran.
Perhaps the chief difference between the pre-Cambrian and the present was the absence of life upon the land. So far as we have any knowledge, no forests covered the mountain sides, no verdure carpeted the plains, and no animals lived on the ground or in the air. It is permitted to think of the most ancient lands as deserts of barren rock and rock waste swept by rains and trenched by powerful streams. We may therefore suppose that the processes of their destruction went on more rapidly than at present.
CHAPTER XVI
THE CAMBRIAN
The Paleozoic era.The second volume of the geological record, called the Paleozoic (Greek,palaios, ancient;zoē, life), has come down to us far less mutilated and defaced than has the first volume, which contains the traces of the most ancient life of the globe. Fossils are far more abundant in the Paleozoic than in the earlier strata, while the sediments in which they were entombed have suffered far less from metamorphism and other causes, and have been less widely buried from view, than the strata of the pre-Cambrian groups. By means of their fossils we can correlate the formations of widely separated regions from the beginning of the Paleozoic on, and can therefore trace some outline of the history of the continents.
Paleozoic time, although shorter than the pre-Cambrian as measured by the thickness of the strata, must still be reckoned in millions of years. During this vast reach of time the changes in organisms were very great. It is according to the successive stages in the advance of life that the Paleozoic formations are arranged in five systems,—theCambrian, theOrdovician, theSilurian, theDevonian, and theCarboniferous. On the same basis the first three systems are grouped together as the older Paleozoic, because they alike are characterized by the dominance of the invertebrates; while the last two systems are united in the later Paleozoic, and are characterized, the one by the dominance of fishes, and the other by the appearance of amphibians and reptiles.
Each of these systems is world-wide in its distribution, and may be recognized on any continent by its own peculiar fauna.The names first given them in Great Britain have therefore come into general use, while their subdivisions, which often cannot be correlated in different countries and different regions, are usually given local names.
The first three systems were named from the fact that their strata are well displayed in Wales. The Cambrian carries the Roman name of Wales, and the Ordovician and Silurian the names of tribes of ancient Britons which inhabited the same country. The Devonian is named from the English county Devon, where its rocks were early studied. The Carboniferous was so called from the large amount of coal which it was found to contain in Great Britain and continental Europe.
The Cambrian
Distribution of strata.The Cambrian rocks outcrop in narrow belts about the pre-Cambrian areas of eastern Canada and the Lake Superior region, the Adirondacks and the Green Mountains. Strips of Cambrian formations occupy troughs in the pre-Cambrian rocks of New England and the maritime provinces of Canada; a long belt borders on the west the crystalline rocks of the Blue Ridge; and on the opposite side of the continent the Cambrian reappears in the mountains of the Great Basin and the Canadian Rockies. In the Mississippi valley it is exposed in small districts where uplift has permitted the stripping off of younger rocks. Although the areas of outcrop are small, we may infer that Cambrian rocks were widely deposited over the continent of North America.
Physical geography.The Cambrian system of North America comprises three distinct series, theLower Cambrian, theMiddle Cambrian, and theUpper Cambrian, each of which is characterized by its own peculiar fauna. In sketching the outlines of the continent as it was at the beginning of the Paleozoic, it must be remembered that wherever the Lower Cambrian formations now are found was certainly then sea bottom, andwherever the Lower Cambrian are wanting, and the next formations rest directly on pre-Cambrian rocks, was probably then land.
Fig. 264.Hypothetical Map of Eastern North America at the Beginning of Cambrian TimeUnshaded areas, probable land
Fig. 264.Hypothetical Map of Eastern North America at the Beginning of Cambrian TimeUnshaded areas, probable land
Early Cambrian geography.In this way we know that at the opening of the Cambrian two long, narrow mediterranean seas stretched from north to south across the continent. The eastern sea extended from the Gulf of St. Lawrence down the Champlain-Hudson valley and thence along the western base of the Blue Ridge south at least to Alabama. The western sea stretched from the Canadian Rockies over the Great Basin and at least as far south as the Grand Canyon of the Colorado in Arizona.
Between these mediterraneans lay a great central land which included the pre-CambrianU-shaped area of the Laurentian peneplain, and probably extended southward to the latitude of New Orleans. To the east lay a land which we may designate asAppalachia, whose western shore line was drawn along the site of the present Blue Ridge, but whose other limits are quite unknown. The land of Appalachia must have been large, for it furnished a great amount of waste during the entire Paleozoic era, and its eastern coast may possibly have lain evenbeyond the edge of the present continental shelf. On the western side of the continent a narrow land occupied the site of the Sierra Nevada Mountains.
Thus, even at the beginning of the Paleozoic, the continental plateau of North America had already been left by crustal movements in relief above the abysses of the great oceans on either side. The mediterraneans which lay upon it were shallow, as their sediments prove. They wereepicontinental seas; that is, they restedupon(Greek,epi) the submerged portion of the continental plateau. We have no proof that the deep ocean ever occupied any part of where North America now is.
The Middle and Upper Cambrian strata are found together with the Lower Cambrian over the area of both the eastern and the western mediterraneans, so that here the sea continued during the entire period. The sediments throughout are those of shoal water. Coarse cross-bedded sandstones record the action of strong shifting currents which spread coarse waste near shore and winnowed it of finer stuff. Frequent ripple marks on the bedding planes of the strata prove that the loose sands of the sea floor were near enough to the surface to be agitated by waves and tidal currents. Sun cracks show that often the outgoing tide exposed large muddy flats to the drying action of the sun. The fossils, also, of the strata are of kinds related to those which now live in shallow waters near the shore.
The sediments which gathered in the mediterranean seas were very thick, reaching in places the enormous depth of ten thousand feet. Hence the bottoms of these seas were sinking troughs, ever filling with waste from the adjacent land as fast as they subsided.
Late Cambrian geography.The formations of the Middle and Upper Cambrian are found resting unconformably on the pre-Cambrian rocks from New York westward into Minnesota and at various points in the interior, as in Missouri and inTexas. Hence after earlier Cambrian time the central land subsided, with much the same effect as if the Mississippi valley were now to lower gradually, and the Gulf of Mexico to spread northward until it entered Lake Superior. The Cambrian seas transgressed the central land and strewed far and wide behind their advancing beaches the sediments of the later Cambrian upon an eroded surface of pre-Cambrian rocks.
The succession of the Cambrian formations in North America records many minor oscillations and varying conditions of physical geography; yet on the whole it tells of widening seas and lowering lands. Basal conglomerates and coarse sandstones which must have been laid near shore are succeeded by shaly sandstones, sandy shales, and shales. Toward the top of the series heavy beds of limestone, extending from the Blue Ridge to Missouri, speak of clear water, and either of more distant shores or of neighboring lands which were worn or sunk so low that for the most part their waste was carried to the sea in solution.
In brief, the Cambrian was a period of submergence. It began with the larger part of North America emerged as great land masses. It closed with most of the interior of the continental plateau covered with a shallow sea.
The Life of the Cambrian Period
It is now for the first time that we find preserved in the offshore deposits of the Cambrian seas enough remains of animal life to be properly called a fauna. Doubtless these remains are only the most fragmentary representation of the life of the time, for the Cambrian rocks are very old and have been widely metamorphosed. Yet the five hundred and more species already discovered embrace all the leading types of invertebrate life, and are so varied that we must believe that their lines of descent stretch far back into the pre-Cambrian past.
Plants.No remains of plants have been found in Cambrian strata, except some doubtful markings, as of seaweed.
Fig. 265.Sponge Spicules as seen in Flint under the Microscope
Fig. 265.Sponge Spicules as seen in Flint under the Microscope
Sponges.The sponges, the lowest of the multicellular animals, were represented by several orders. Their fossils are recognized by the siliceous spicules, which, as in modern sponges, either were scattered through a mass of horny fibers or were connected in a flinty framework.
Cœlenterates.This subkingdom includes two classes of interest to the geologist,—theHydrozoa, such as the fresh-water hydra and the jellyfish, and thecorals. Both classes existed in the Cambrian.
Fig. 266.Graptolites
Fig. 266.Graptolites
The Hydrozoa were represented not only by jellyfish but also by thegraptolite, which takes its name from a fanciedresemblance of some of its forms to a quill pen. It was a composite animal with a horny framework, the individuals of the colony living in cells strung on one or both sides along a hollow stem, and communicating by means of a common flesh in this central tube. Some graptolites were straight, and some curved or spiral; some were single stemmed, and others consisted of several radial stems united. Graptolites occur but rarely in the Upper Cambrian. In the Ordovician and Silurian they are very plentiful, and at the close of the Silurian they pass out of existence, never to return.
Coralsare very rarely found in the Cambrian, and the description of their primitive types is postponed to later chapters treating of periods when they became more numerous.
Echinoderms.This subkingdom comprises at present such familiar forms as the crinoid, the starfish, and the sea urchin. The structure of echinoderms is radiate. Their integument is hardened with plates or particles of carbonate of lime.
Fig. 267.Cystoids, one showing Two Rudimentary Arms
Fig. 267.Cystoids, one showing Two Rudimentary Arms
Of the free echinoderms, such as the starfish and the sea urchin, the former has been found in the Cambrian rocks of Europe, but neither have so far been discovered in the strata of this period in North America. The stemmed and lower division of the echinoderms was represented by a primitive type, thecystoid, so called from its saclike form, A small globular or ovate “calyx” of calcareous plates, with an aperture at thetop for the mouth, inclosed the body of the animal, and was attached to the sea bottom by a short flexible stalk consisting of disks of carbonate of lime held together by a central ligament.
Arthropods.These segmented animals with “jointed feet,” as their name suggests, may be divided in a general way into water breathers and air breathers. The first-named and lower division comprises the class of theCrustacea,—arthropods protected by a hard exterior skeleton, or “crust,”—of which crabs, crayfish, and lobsters are familiar examples. The higher division, that of the air breathers, includes the following classes: spiders, scorpions, centipedes, and insects.
Fig. 268.TrilobitesA, a Cambrian species;B, a Devonian species showing eyes;C, restoration of an Ordovician species
Fig. 268.TrilobitesA, a Cambrian species;B, a Devonian species showing eyes;C, restoration of an Ordovician species
The trilobite.The aquatic arthropods, the Crustacea, culminated before the air breathers; and while none of the latter are found in the Cambrian, the former were the dominant life of the time in numbers, in size, and in the variety of their forms. The leading crustacean type is thetrilobite, which takes its name from the three lobes into which its shell is divided longitudinally. There are also three cross divisions,—the head shield,the tail shield, and between the two the thorax, consisting of a number of distinct and unconsolidated segments. The head shield carries a pair of large, crescentic, compound eyes, like those of the insect. The eye varies greatly in the number of its lenses, ranging from fourteen in some species to fifteen thousand in others.Figure 268,C, is a restoration of the trilobite, and shows the appendages, which are found preserved only in the rarest cases.
Fig. 269.A Phyllopod
Fig. 269.A Phyllopod
During the long ages of the Cambrian the trilobite varied greatly. Again and again new species and genera appeared, while the older types became extinct. For this reason and because of their abundance, trilobites are used in the classification of the Cambrian system. The Lower Cambrian is characterized by the presence of a trilobitic fauna in which the genus Olenellus is predominant. This, theOlenellus Zone, is one of the most important platforms in the entire geological series; for, the world over, it marks the beginning of Paleozoic time, while all underlying strata are classified as pre-Cambrian. The Middle Cambrian is marked by the genus Paradoxides, and the Upper Cambrian by the genus Olenus. Some of the Cambrian trilobites were giants, measuring as much as two feet long, while others were the smallest of their kind, a fraction of an inch in length.
Another type of crustacean which lived in the Cambrian and whose order is still living is illustrated inFigure 269.
Worms.Trails and burrows of worms have been left on the sea beaches and mud flats of all geological times from the Algonkian to the present.
Fig. 270.A Cambrian Articulate Brachiopod, Orthis
Fig. 270.A Cambrian Articulate Brachiopod, Orthis
Fig. 271.Cambrian Inarticulate BrachiopodsA, Lingulella;B, Discina
Fig. 271.Cambrian Inarticulate BrachiopodsA, Lingulella;B, Discina
Brachiopods.These soft-bodied animals, with bivalve shells and two interior armlike processes which served for breathing, appeared in the Algonkian, and had now become very abundant. The two valves of the brachiopod shell are unequal in size, andin each valve a line drawn from the beak to the base divides the valve into two equal parts (Fig. 270). It may thus be told from the pelecypod mollusk, such as the clam, whose two valves are not far from equal in size, each being divided into unequal parts by a line dropped from the beak (Fig. 272).
Brachiopods include two orders. In the most primitive order—that of theinarticulatebrachiopods—the two valves are held together only by muscles of the animal, and the shell is horny or is composed of phosphate of lime. TheDiscina, which began in the Algonkian, is of this type, as is also theLingulellaof the Cambrian (Fig. 271). Both of these genera have lived on during the millions of years of geological time since their introduction, handing down from generation to generation with hardly any change to their descendants now living off our shores the characters impressed upon them at the beginning.
The more highly organizedarticulatebrachiopods have valves of carbonate of lime more securely joined by a hinge with teeth and sockets (Fig. 270). In the Cambrian the inarticulates predominate, though the articulates grow common toward the end of the period.
Fig. 272.A Cambrian Pelecypod
Fig. 272.A Cambrian Pelecypod
Mollusks.The three chief classes of mollusks—thepelecypods(represented by the oyster and clam of to-day), thegastropods(represented now by snails, conches, and periwinkles), and thecephalopods(such as the nautilus, cuttlefish, and squids)—were all represented in the Cambrian, although very sparingly.
Pteropods, a suborder of the gastropods, appeared in this age. Their papery shells of carbonate of lime are found in great numbers from this time on.
Fig. 273.Gastropods
Fig. 273.Gastropods
Fig. 274.Cambrian Pteropods
Fig. 274.Cambrian Pteropods
Cephalopods, the most highly organized of the mollusks, started into existence, so far as the record shows, toward, the end of the Cambrian, with the long extinctOrthoceras(straighthorn) and the allied genera of its family. The Orthoceras had a long, straight, and tapering shell, divided by cross partitions into chambers. The animal lived in the “body chamber” at the larger end, and walled off the other chambers from it in succession during the growth of the shell. A central tube,thesiphuncle(s, Fig. 275,B), passed through from the body chamber to the closed tip of the cone.
Fig. 275.OrthocerasA, fossil;B, restoration
Fig. 275.OrthocerasA, fossil;B, restoration
The seashells, both brachiopods and mollusks, are in some respects the most important to the geologist of all fossils. They have been so numerous, so widely distributed, and so well preserved because of their durable shells and their station in growing sediments, that better than any other group of organisms they can be used to correlate the strata of different regions and to mark by their slow changes the advance of geological time.
Climate.The life of Cambrian times in different countries contains no suggestion of any marked climatic zones, and as in later periods a warm climate probably reached to the polar regions.
CHAPTER XVII
THE ORDOVICIAN[2]AND SILURIAN
[2] Often known as the Lower Silurian.
The Ordovician
In North America the Ordovician rocks lie conformably on the Cambrian. The two periods, therefore, were not parted by any deformation, either of mountain making or of continental uplift. The general submergence which marked the Cambrian continued into the succeeding period with little interruption.
Subdivisions and distribution of strata.The Ordovician series, as they have been made out in New York, are given for reference in the following table, with the rocks of which they are chiefly composed:
These marine formations of the Ordovician outcrop about the Cambrian and pre-Cambrian areas, and, as borings show, extend far and wide over the interior of the continent beneath more recent strata. The Ordovician sea stretched from Appalachia across the Mississippi valley. It seems to have extended to California, although broken probably by several mountainous islands in the west.
Physical geography.The physical history of the period is recorded in the succession of its formations. The sandstones of the Upper Cambrian, as we have learned, tell of a transgressingsea which gradually came to occupy the Mississippi valley and the interior of North America. The limestones of the early and middle Ordovician show that now the shore had become remote and the lands had become more low. The waters now had cleared. Colonies of brachiopods and other lime-secreting animals occupied the sea bottom, and their débris mantled it with sheets of limy ooze. The sandy limestones of the Calciferous record the transition stage from the Cambrian when some sand was still brought in from shore. The highly fossiliferous limestones of the Trenton tell of clear water and abundant life. We need not regard this epicontinental sea as deep. No abysmal deposits have been found, and the limestones of the period are those which would be laid in clear, warm water of moderate depth like that of modern coral seas.