Chapter 15

Fig. 276.Hypothetical Map of the Eastern United States in Ordovician TimeShaded areas, probable sea; broken lines, approximate shore lines

The shales of the Utica and Hudson show that the waters of the sea now became clouded with mud washed in from land. Either the land was gradually uplifted, or perhaps there had arrived one of those periodic crises which, as we may imagine, have taken place whenever the crust of the shrinking earth has slowly given way over its great depressions, and the ocean has withdrawn its waters into deepening abysses. The land was thus left relatively higher and bordered with new coastal plains.The epicontinental sea was shoaled and narrowed, and muds were washed in from the adjacent lands.

The Taconic deformation.The Ordovician was closed by a deformation whose extent and severity are not yet known. From the St. Lawrence River to New York Bay, along the northwestern and western border of New England, lies a belt of Cambrian-Ordovician rocks more than a mile in total thickness, which accumulated during the long ages of those periods in a gradually subsiding trough between the Adirondacks and a pre-Cambrian range lying west of the Connecticut River. But since their deposition these ancient sediments have been crumpled and crushed, broken with great faults, and extensively metamorphosed. The limestones have recrystallized into marbles, among them the famous marbles of Vermont; the Cambrian sandstones have become quartzites, and the Hudson shale has been changed to a schist exposed on Manhattan Island and northward.

In part these changes occurred at the close of the Ordovician, for in several places beds of Silurian age rest unconformably on the upturned Ordovician strata; but recent investigations have made it probable that the crustal movements recurred at later times, and it was perhaps in the Devonian and at the close of the Carboniferous that the greater part of the deformation and metamorphism was accomplished. As a result of these movements,— perhaps several times repeated,—a great mountain range was upridged, which has been long since leveled by erosion, but whose roots are now visible in the Taconic Mountains of western New England.

The Cincinnati anticline.Over an oval area in Ohio, Indiana, and Kentucky, whose longer axis extends from north to south through Cincinnati, the Ordovician strata rise in a very low, broad swell, called the Cincinnati anticline. The Silurian and Devonian strata thin out as they approach this area and seem never to have deposited upon it. We may regard it, therefore, as an island upwarped from the sea at the close of the Ordovician or shortly after.

Petroleum and natural gas.These valuable illuminants and fuels are considered here because, although they are found in traces in older strata, it is in the Ordovician that they occur for the first time in large quantities. They range throughout later formations down to the most recent.

The oil horizons of California and Texas are Tertiary; those of Colorado, Cretaceous; those of West Virginia, Carboniferous; those of Pennsylvania, Kentucky, and Canada, Devonian; and the large field of Ohio and Indiana belongs to the Ordovician and higher systems.

Petroleum and natural gas, wherever found, have probably originated from the decay of organic matter when buried in sedimentary deposits, just as at present in swampy places the hydrogen and carbon of decaying vegetation combine to form marsh gas. The light and heat of these hydrocarbons we may think of, therefore, as a gift to the civilized life of our race from the humble organisms, both animal and vegetable, of the remote past, whose remains were entombed in the sediments of the Ordovician and later geological ages.

Fig. 277.Diagram Illustrating the Conditions of Accumulation of Oil and Gasa, source;b, reservoir;c, cover. What would be the result of boring to the reservoir rock atd? atd´? atd´´?

Petroleum is very widely disseminated throughout the stratified rocks. Certain limestones are visibly greasy with it, and others give off its characteristic fetid odor when struck with a hammer. Many shales are bituminous, and some are so highly charged that small flakes may be lighted like tapers, and several gallons of oil to the ton may be obtained by distillation.

But oil and gas are found in paying quantities only when certain conditions meet:

1. Asourcebelow, usually a bituminous shale, from whose organic matter they have been derived by slow change.

2. Areservoirabove, in which they have gathered. This is either a porous sandstone or a porous or creviced limestone.

3. Oil and gas are lighter than water, and are usually under pressure owing to artesian water. Hence, in order to hold them from escaping to the surface, the reservoir must have the shape of ananticline,dome, orlens.

4. It must also have animpervious cover, usually a shale. In these reservoirs gas is under a pressure which is often enormous, reaching in extreme cases as high as a thousand five hundred pounds to the square inch. When tapped it rushes out with a deafening roar, sometimes flinging the heavy drill high in air. In accounting for this pressure we must remember that the gas has been compressed within the pores of the reservoir rock by artesian water, and in some cases also by its own expansive force. It is not uncommon for artesian water to rise in wells after the exhaustion of gas and oil.

Life of the Ordovician

During the ages of the Ordovician, life made great advances. Types already present branched widely into new genera and species, and new and higher types appeared.

Sponges continued from the Cambrian. Graptolites now reached their climax.

Fig. 278.Stromatopora

Stromatopora—colonies of minute hydrozoans allied to corals—grew in places on the sea floor, secreting stony masses composed of thin, close, concentric layers, connected by vertical rods. The Stromatopora are among the chief limestone builders of the Silurian and Devonian periods.

Fig. 279.Crinoid, a Jurassic Species

Coralsdeveloped along several distinct lines, like modern corals they secreted a calcareous framework, in whose outer portions the polyps lived. In the Ordovician, corals were represented chiefly by the family of theChætetes, all species of which are long since extinct. The description of other types of corals will be given under the Silurian, where they first became abundant.

Echinoderms.The cystoid reaches its climax, but there appear now two higher types of echinoderms,—the crinoid and the starfish. Thecrinoid, named from its resemblance to the lily, is like the cystoid in many respects, but has a longer stem and supports a crown of plumose arms. Stirring the water with these arms, it creates currents by which particles of food are wafted to its mouth. Crinoids are rare at the present time, but they grew in the greatest profusion in the warm Ordovician seas and for long ages thereafter. In many places the sea floor was beautiful with these graceful, flowerlike forms, as with fields of long-stemmed lilies. Of the higher, free-moving classes of the echinoderms, starfish are more numerous than in the Cambrian, and sea urchins make their appearance in rare archaic forms.

Crustaceans.Trilobites now reach their greatest development and more than eleven hundred species have been described from the rocks of this period. It is interesting to note that in many species the segments of the thorax have now come to be so shaped that they move freely on one another. Unlike their Cambrian ancestors, many of the Ordovician trilobites could roll themselves into balls at the approach of danger. It is in this attitude, taken at theapproach of death, that trilobites are often found in the Ordovician and later rocks. The gigantic crustaceans called theeurypteridswere also present in this period (Fig. 282).

The arthropods had now seized upon the land. Centipedes and insects of a low type, the earliest known land animals, have been discovered in strata of this system.

Fig. 280.An Ordovician StarfishFig. 281.An Ordovician Sea UrchinFig. 282.Eurypterus

Fig. 280.An Ordovician Starfish

Fig. 281.An Ordovician Sea Urchin

Fig. 282.Eurypterus

Fig. 283.A Bryozoan

Bryozoans.No fossils are more common in the limestones of the time than the small branching stems and lacelike mats of the bryozoans,—the skeletons of colonies of a minute animal allied in structure to the brachiopod.

Fig. 284.Ordovician Brachiopods

Fig. 285.A, Cyrtoceras;B, Trochoceras;C, Lituites

Brachiopods.These multiplied greatly, and in places their shells formed thick beds of coquina. They still greatly surpassed the mollusks in numbers.

Cephalopods.Among the mollusks we must note the evolution of the cephalopods. The primitive straight Orthoceras hasnow become abundant. But in addition to this ancestral type there appears a succession of forms more and more curved and closely coiled, as illustrated inFigure 285. The nautilus, which began its course in this period, crawls on the bottom of our present seas.

Fig. 286.Nautilus

Vertebrates.The most important record of the Ordovician is that of the appearance of a new and higher type, with possibilities of development lying hidden in its structure that the mollusk and the insect could never hope to reach. Scales and plates of minute fishes found in the Ordovician rocks near Canon City, Colorado, show that the humblest of the vertebrates had already made its appearance. But it is probable that vertebrates had been on the earth for ages before this in lowly types, which, being destitute of hard parts, would leave no record.

The Silurian

The narrowing of the seas and the emergence of the lands which characterized the closing epoch of the Ordovician in eastern North America continue into the succeeding period of the Silurian. New species appear and many old species now become extinct.

The Appalachian region.Where the Silurian system is most fully developed, from New York southward along the Appalachian Mountains, it comprises four series:

The rocks of these series are shallow-water deposits and reach the total thickness of some five thousand feet. Evidently they were laid over an area which was on the whole gradually subsiding, although with various gentle oscillations which are recorded in the different formations. The coarse sands of the heavy Medina formations record a period of uplift of the oldland of Appalachia, when erosion went on rapidly and coarse waste in abundance was brought down from the hills by swift streams and spread by the waves in wide, sandy flats. As the lands were worn lower the waste became finer, and during an epoch of transition—the Clinton— there were deposited various formations of sandstones, shales, and limestones. The Niagara limestones testify to a long epoch of repose, when low-lying lands sent little waste down to the sea.

The gypsum and salt deposits of the Salina show that toward the close of the Silurian period a slight oscillation brought the sea floor nearer to the surface, and at the north cut off extensive tracts from the interior sea. In these wide lagoons, which now and then regained access to the open sea and obtained new supplies of salt water, beds of salt and gypsum were deposited as the briny waters became concentrated by evaporation under a desert climate. Along with these beds there were also laid shales and impure limestones.

•••••

In New York the “salt pans” of the Salina extended over an area one hundred and fifty miles long from east to west and sixty miles wide, and similar salt marshes occurred as far west as Cleveland, Ohio, andGoderich on Lake Huron. At Ithaca, New York, the series is fifteen hundred feet thick, and is buried beneath an equal thickness of later strata. It includes two hundred and fifty feet of solid salt, in several distinct beds, each sealed within the shales of the series.Would you expect to find ancient beds of rock salt inclosed in beds of pervious sandstone?The salt beds of the Salina are of great value. They are reached by well borings, and their brines are evaporated by solar heat and by boiling. The rock salt is also mined from deep shafts.Similar deposits of salt, formed under like conditions, occur in the rocks of later systems down to the present. The salt beds of Texas are Permian, those of Kansas are Permian, and those of Louisiana are Tertiary.

Would you expect to find ancient beds of rock salt inclosed in beds of pervious sandstone?

The salt beds of the Salina are of great value. They are reached by well borings, and their brines are evaporated by solar heat and by boiling. The rock salt is also mined from deep shafts.

Similar deposits of salt, formed under like conditions, occur in the rocks of later systems down to the present. The salt beds of Texas are Permian, those of Kansas are Permian, and those of Louisiana are Tertiary.

The Mississippi valley.The heavy near-shore formations of the Silurian in the Appalachian region thin out toward the west. The Medina and the Clinton sandstones are not found west of Ohio, where the first passes into a shale and the second into a limestone. The Niagara limestone, however, spreads from the Hudson River to beyond the Mississippi, a distance of more than a thousand miles. During the Silurian period the Mississippi valley region was covered with a quiet, shallow, limestone-making sea, which received little waste from the low lands which bordered it.

The probable distribution of land and sea in eastern North America and western Europe is shown inFigure 287. The fauna of the interior region and of eastern Canada are closely allied with that of western Europe, and several species are identical. We can hardly account for this except by a shallow-water connection between the two ancient epicontinental seas. It was perhaps along the coastal shelves of a northern land connecting America and Europe by way of Greenland and Iceland that the migration took place, so that the same species came to live in Iowa and in Sweden.

Fig. 287.Hypothetical Map of Parts of North America and Europe in Silurian TimeShaded areas, probable seas; broken lines, approximate shorelines

The western United States.So little is found of the rocks of the system west of the Missouri River that it is quite probable that the western part of the United States had for the most part emerged from the sea at the close of the Ordovician and remained land during the Silurian. At the same time the western land was perhaps connected with the eastern land of Appalachia across Arkansas and Mississippi; for toward the south the Silurian sediments indicate an approach to shore.

Life of the Silurian

In this brief sketch it is quite impossible to relate the many changes of species and genera during the Silurian.

Fig. 288.A Compound Cup Coral

Fig. 289.A Simple Cup Coral

Corals.Some of the more common types are familiarly known as cup corals, honeycomb corals, and chain corals. In thecup coralsthe most important feature is the development of radiating vertical partitions, orsepta, in the cell of the polyp. Some of the cup corals grew in hemispherical colonies (Fig. 288), while many were separate individuals (Fig. 289), building a single conical, or horn-shaped cell, which sometimes reached the extreme size of a foot in length and two or three inches in diameter.

Honeycomb coralsconsist of masses of small, close-set prismatic cells, each crossed by horizontal partitions, ortabulæ, while the septa are rudimentary, being represented by faintly projecting ridges or rows of spines.

Fig. 290.Honeycomb Corals

Fig. 291.A Chain Coral

Fig. 292.A Syringopora Coral

Chain coralsare also marked by tabulæ. Their cells form elliptical tubes, touching each other at the edges, and appearing in cross section like the links of a chain. They became extinct at the end of the Silurian.

The corals of theSyringoporafamily are similar in structure to chain corals, but the tubular columns are connected only in places.

Fig. 293.A Blastoid:A, side view, showing portion of the stem;B, summit of calyx (species Carboniferous)

Fig. 294.A Silurian Scorpion

To the echinoderms there is now added theblastoid(bud-shaped). The blastoid is stemmed and armless, and its globular “head” or “calyx,” with its five petal-like divisions, resembles a flower bud. The blastoids became more abundant in the Devonian, culminated in the Carboniferous, and disappeared at the end of the Paleozoic.

The great eurypterids—some of which were five or six feet in length—and the cephalopods were still masters of the seas.Fishes were as yet few and small; trilobites and graptolites had now passed their prime and had diminished greatly in numbers. Scorpions are found in this period both in Europe and in America. The limestone-making seas of the Silurian swarmed with corals, crinoids, and brachiopods.

With the end of the Silurian period theAge of Invertebratescomes to a close, giving place to the Devonian, theAge of Fishes.

Fig. 295.Block of Limestone showing Interior Casts of Pentamerus oblongus, a Common Silurian Brachiopod

CHAPTER XVIII

THE DEVONIAN

In America the Silurian is not separated from the Devonian by any mountain-making deformation or continental uplift. The one period passed quietly into the other. Their conformable systems are so closely related, and the change in their faunas is so gradual, that geologists are not agreed as to the precise horizon which divides them.

Subdivisions and physical geography.The Devonian is represented in New York and southward by the following five series. We add the rocks of which they are chiefly composed.

The Helderberg is a transition epoch referred by some geologists to the Silurian. The thin sandstones of the Oriskany mark an epoch when waves worked over the deposits of former coastal plains. The limestones of the Corniferous testify to a warm and clear wide sea which extended from the Hudson to beyond the Mississippi. Corals throve luxuriantly, and their remains, with those of mollusks and other lime-secreting animals, built up great beds of limestone. The bordering continents, as during the later Silurian, must now have been monotonous lowlands which sent down little of even the finest waste to the sea.

In the Hamilton the clear seas of the previous epoch became clouded with mud. The immense deposits of coarse sandstonesand sandy shales of the Chemung, which are found off what was at the time the west coast of Appalachia, prove an uplift of that ancient continent.

The Chemung seriesextends from the Catskill Mountains to northeastern Ohio and south to northeastern Tennessee, covering an area of not less than a hundred thousand square miles. In eastern New York it attains three thousand feet in thickness; in Pennsylvania it reaches the enormous thickness of two miles; but it rapidly thins to the west. Everywhere the Chemung is made of thin beds of rapidly alternating coarse and fine sands and clays, with an occasional pebble layer, and hence is a shallow-water deposit. The fine material has not been thoroughly winnowed from the coarse by the long action of strong waves and tides. The sands and clays have undergone little more sorting than is done by rivers. We must regard the Chemung sandstones as deposits made at the mouths of swift, turbid rivers in such great amount that they could be little sorted and distributed by waves.Over considerable areas the Chemung sandstones bear little or no trace of the action of the sea. The Catskill Mountains, for example, have as their summit layers some three thousand feet of coarse red sandstones of this series, whose structure is that of river deposits, and whose few fossils are chiefly of fresh-water types. The Chemung is therefore composed of delta deposits, more or less worked over by the sea. The bulk of the Chemung equals that of the Sierra Nevada Mountains. To furnish this immense volume of sediment a great mountain range, or highland, must have been upheaved where the Appalachian lowland long had been. To what height the Devonian mountains of Appalachia attained cannot be told from the volume of the sediments wasted from them, for they may have risen but little faster than they were worn down by denudation. We may infer from the character of the waste which they furnished to the Chemung shores that they did not reach an Alpine height. The grains of the Chemung sandstones are not those which would result from mechanical disintegration, as by frost on high mountain peaks, but are rather those which would be left from the long chemical decay of siliceous crystalline rocks; for the more soluble minerals are largely wanting. The red color of much of the deposits points to the same conclusion. Red residual clays accumulated on the mountain sides and upland summits, and were washed as ocherous silt to mingle with the delta sands. The iron- bearing igneous rocksof the oldland also contributed by their decay iron in solution to the rivers, to be deposited in films of iron oxide about the quartz grains of the Chemung sandstones, giving them their reddish tints.

Over considerable areas the Chemung sandstones bear little or no trace of the action of the sea. The Catskill Mountains, for example, have as their summit layers some three thousand feet of coarse red sandstones of this series, whose structure is that of river deposits, and whose few fossils are chiefly of fresh-water types. The Chemung is therefore composed of delta deposits, more or less worked over by the sea. The bulk of the Chemung equals that of the Sierra Nevada Mountains. To furnish this immense volume of sediment a great mountain range, or highland, must have been upheaved where the Appalachian lowland long had been. To what height the Devonian mountains of Appalachia attained cannot be told from the volume of the sediments wasted from them, for they may have risen but little faster than they were worn down by denudation. We may infer from the character of the waste which they furnished to the Chemung shores that they did not reach an Alpine height. The grains of the Chemung sandstones are not those which would result from mechanical disintegration, as by frost on high mountain peaks, but are rather those which would be left from the long chemical decay of siliceous crystalline rocks; for the more soluble minerals are largely wanting. The red color of much of the deposits points to the same conclusion. Red residual clays accumulated on the mountain sides and upland summits, and were washed as ocherous silt to mingle with the delta sands. The iron- bearing igneous rocksof the oldland also contributed by their decay iron in solution to the rivers, to be deposited in films of iron oxide about the quartz grains of the Chemung sandstones, giving them their reddish tints.

Life of the Devonian

Plants.The lands were probably clad with verdure during Silurian times, if not still earlier; for some rare remains of ferns and other lowly types of vegetation have been found in the strata of that system. But it is in the Devonian that we discover for the first time the remains of extensive and luxuriant forests. This rich flora reached its climax in the Carboniferous, and it will be more convenient to describe its varied types in the next chapter.

Rhizocarps.In the shales of the Devonian are found microscopic spores of rhizocarps in such countless numbers that their weight must be reckoned in hundreds of millions of tons. It would seem that these aquatic plants culminated in this period, and in widely distant portions of the earth swampy flats and shallow lagoons were filled with vegetation of this humble type, either growing from the bottom or floating free upon the surface. It is to the resinous spores of the rhizocarps that the petroleum and natural gas from Devonian rocks are largely due. The decomposition of the spores has made the shales highly bituminous, and the oil and gas have accumulated in the reservoirs of overlying porous sandstones.

Invertebrates.We must pass over the ever-changing groups of the invertebrates with the briefest notice. Chain corals became extinct at the close of the Silurian, but other corals were extremely common in the Devonian seas. At many places corals formed thin reefs, as at Louisville, Kentucky, where the hardness of the reef rock is one of the causes of the Falls of the Ohio.

Sponges, echinoderms, brachiopods, and mollusks were abundant. The cephalopods take a new departure. So far in alltheir various forms, whether straight, as the Orthoceras, or curved, or close-coiled as in the nautilus, the septum, or partition dividing the chambers, met the inner shell along a simple line, like that of the rim of a saucer. There now begins a growth of the septum by which its edges become sharply corrugated, and the suture, or line of juncture of the septum and the shell, is thus angled. The group in which this growth of the septum takes place is called theGoniatite(Greekgōnia, angle).

Fig. 296.A Goniatite

Vertebrates.It is with the greatest interest that we turn now to study the backboned animals of the Devonian; for they are believed to be the ancestors of the hosts of vertebrates which have since dominated the earth. Their rudimentary structures foreshadowed what their descendants were to be, and give some clue to the earliest vertebrates from which they sprang. Like those whose remains are found in the lower Paleozoic systems, all of these Devonian vertebrates were aquatic and go under the general designation of fishes.

Fig. 297.Palæospondylus

The lowest in grade and nearest, perhaps, to the ancestral type of vertebrates, was the problematic creature, an inch or so long, ofFigure 297. Note the circular mouth not supplied with jaws, the lack of paired fins, and the symmetric tail fin, with the column of cartilaginous, ringlike vertebræ running through it to the end. The animal is probably to be placed with the jawless lampreys and hags,—a group too low to be included among true fishes.

Ostracoderms.This archaic group, long since extinct, is also too lowly to rank among the true fishes, for its membershave neither jaws nor paired fins. These small, fishlike forms were cased in front with bony plates developed in the skin and covered in the rear with scales. The vertebræ were not ossified, for no trace of them has been found.

Fig. 298.An Ostracoderm

Devonian fishes.Thetrue fishesof the Devonian can best be understood by reference to their descendants now living. Modern fishes are divided into several groups:sharksand their allies;dipnoans;ganoids, such as the sturgeon and gar; andteleosts,— most common fishes, such as the perch and cod.

Fig. 299.A Paleozoic Shark

Sharks.Of all groups of living fishes the sharks are the oldest and still retain most fully the embryonic characters of their Paleozoic ancestors. Such characters are the cartilaginous skeleton, and the separate gill slits with which the throat wall is pierced and which are arranged in line like the gill openings of the lamprey. The sharks of the Silurian and Devonian are known to us chiefly by their teeth and fin spines, for they were unprotected by scales or plates, and were devoid of a bonyskeleton.Figure 299is a restoration of an archaic shark from a somewhat higher horizon. Note the seven gill slits and the lappetlike paired fins. These fins seem to be remnants of the continuous fold of skin which, as embryology teaches, passed from fore to aft down each side of the primitive vertebrate.

Devonian sharks were comparatively small. They had not evolved into the ferocious monsters which were later to be masters of the seas.

Fig. 300.A Devonian Dipnoan

Dipnoans, or lung fishes.These are represented to-day by a few peculiar fishes and are distinguished by some high structures which ally them with amphibians. An air sac with cellular spaces is connected with the gullet and serves as a rudimentary lung. It corresponds with the swim bladder of most modern fishes, and appears to have had a common origin with it. We may conceive that the primordial fishes not only had gills used in breathing air dissolved in water, but also developed a saclike pouch off the gullet. This sac evolved along two distinct lines. On the line of the ancestry of most modern fishes its duct was closed and it became the swim bladder used in flotation and balancing. On another line of descent it was left open, air was swallowed into it, and it developed into the rudimentary lung of the dipnoans and into the more perfect lungs of the amphibians and other air- breathing vertebrates.

One of the ancient dipnoans is illustrated inFigure 300. Some of the members of this order were, like the ostracoderms, cased in armor, but their higher rank is shown by their powerful jaws and by other structures. Some of these armoredfishes reached twenty-five feet in length and six feet across the head. They were the tyrants of the Devonian seas.

Fig. 301.A Devonian Fringe-Finned Ganoid

Ganoids.These take their name from their enameled plates or scales of bone. The few genera now surviving are the descendants of the tribes which swarmed in the Devonian seas. A restoration of one of a leading order, thefringe-finnedganoids, is given inFigure 301. The side fins, which correspond to the limbs of the higher vertebrates, are quite unlike those of most modern fishes. Their rays, instead of radiating from a common base, fringe a central lobe which contains a cartilaginous axis. The teeth of the Devonian ganoids show a complicated folded structure.

General characteristics of Devonian fishes.The notochord is persistent.The notochord is a continuous rod of cartilage, or gristle, which in the embryological growth of vertebrate animals supports the spinal nerve cord before the formation of the vertebræ. In most modern fishes and in all higher vertebrates the notochord is gradually removed as the bodies of the vertebræ are formed about it; but in the Devonian fishes it persists through maturity and the vertebræ remain incomplete.

The skeleton is cartilaginous.This also is an embryological characteristic. In the Devonian fishes the vertebræ, as well as the other parts of the skeleton, have not ossified, or changed to bone, but remain in their primitive cartilaginous condition.

Fig. 302.Vertebræ of Sturgeon in side viewA; and vertical transverse sectionB, showing Notochordch, and Neural Canalm

The tail fin is vertebrated.The backbone runs through the fin and is fringed above and below with its vertical rays. In some fishes with vertebrated tail fins the fin is symmetric (Fig. 300), and this seems to be the primitive type. In others the tail fin is unsymmetric: the backbone runs into the upper lobe, leaving the two lobes of unequal size. In most modern fishes (theteleosts) the tail fin is not vertebrated: the spinal column ends in a broad plate, to which the diverging fin rays are attached.

But along with these embryonic characters, which were common to all Devonian fishes, there were other structures in certain groups which foreshadowed the higher structures of the land vertebrates which were yet to come: air sacs which were to develop into lungs, and cartilaginous axes in the side fins which were a prophecy of limbs. The vertebrates had already advanced far enough to prove the superiority of their type of structure to all others. Their internal skeleton afforded the best attachment for muscles and enabled them to become the largest and most powerful creatures of the time. The central nervous system, with the predominance given to the ganglia at the fore end of the nerve cord,—the brain,— already endowed them with greater energy than the invertebrates; and, still more important, these structures contained the possibility of development into the more highly organized land vertebrates which were to rule the earth.

Teleosts.The great group of fishes called the teleosts, or those with complete bony skeletons, to which most modern fishes belong, may be mentioned here, although in the Devonian they had not yet appeared. The teleosts are a highly specialized type, adapted most perfectly to their aquatic environment. Heavy armor has been discarded, and reliance is placed instead on swiftness. The skeleton is completely ossified and the notochord removed. The vertebræ have been economically withdrawn from the tail, and the cartilaginous axis of the side fins has been found unnecessary. The air sac has become a swim bladder. In this complete specialization they have long since lost the possibility of evolving into higher types.It is interesting to note that the modern teleosts in their embryological growth pass through the stages which characterized the maturity of their Devonian ancestors; their skeleton is cartilaginous and their tail fin vertebrated.

It is interesting to note that the modern teleosts in their embryological growth pass through the stages which characterized the maturity of their Devonian ancestors; their skeleton is cartilaginous and their tail fin vertebrated.

CHAPTER XIX

THE CARBONIFEROUS

The Carboniferous system is so named from the large amount of coal which it contains. Other systems, from the Devonian on, are coal bearing also, but none so richly and to so wide an extent. Never before or since have the peculiar conditions been so favorable for the formation of extensive coal deposits.

With few exceptions the Carboniferous strata rest on those of the Devonian without any marked unconformity; the one period passed quietly into the other, with no great physical disturbances.

The Carboniferous includes three distinct series. The lower is called theMississippian, from the outcrop of its formations along the Mississippi River in central and southern Illinois and the adjacent portions of Iowa and Missouri. The middle series is called thePennsylvanian(or Coal Measures), from its wide occurrence over Pennsylvania. The upper series is named thePermian, from the province of Perm in Russia.

The Mississippian series.In the interior the Mississippian is composed chiefly of limestones, with some shales, which tell of a clear, warm, epicontinental sea swarming with crinoids, corals, and shells, and occasionally clouded with silt from the land.

In the eastern region, New York had been added by uplift to the Appalachian land which now was united to the northern area. From eastern Pennsylvania southward there were laid in a subsiding trough, first, thick sandstones (the Pocono sandstone), and later still heavier shales,—the two together reaching the thickness of four thousand feet and more. We infer a reneweduplift of Appalachia similar to that of the later epochs of the Devonian, but as much less in amount as the volume of sediments is smaller.

The Pennsylvanian Series

The Mississippian was brought to an end by a quiet oscillation which lifted large areas slightly above the sea, and the Pennsylvanian began with a movement in the opposite direction. The sea encroached on the new land, and spread far and wide a great basal conglomerate and coarse sandstones. On this ancient beach deposit a group of strata rests which we must now interpret. They consist of alternating shales and sandstones, with here and there a bed of limestone and an occasional seam of coal. A stratum of fire clay commonly underlies a coal seam, and there occur also beds of iron ore. We give a typical section of a very small portion of the series at a locality in Pennsylvania. Although some of the minor changes are omitted, the section shows the rapid alternation of the strata:

This section shows more coal than is usual; on the whole, coal seams do not take up more than one foot in fifty of the Coal Measures. They vary also in thickness more than is seen in the section, some exceptional seams reaching the thickness of fifty feet.

How coal was made.1. Coal is of vegetable origin. Examined under the microscope even anthracite, or hard coal, is seen to contain carbonized vegetal tissues. There are also all gradations connecting the hardest anthracite—through semibituminous coal, bituminous or soft coal, lignite (an imperfect coal in which sometimes woody fibers may be seen little changed)—with peat and decaying vegetable tissues. Coal is compressed and mineralized vegetal matter. Its varieties depend on the perfection to which the peculiar change called bituminization has been carried, and also, as shown in the table below, on the degree to which the volatile substances and water have escaped, and on the per cent of carbon remaining.

2. The vegetable remains associated with coal are those of land plants.

3. Coal accumulated in the presence of water; for it is only when thus protected from the air that vegetal matter is preserved.

4. The vegetation of coal accumulated for the most part where it grew; it was not generally drifted and deposited by waves and currents. Commonly the fire clay beneath the seam is penetrated with roots, and the shale above is packed with leaves of ferns and other plants as beautifully pressed as in a herbarium. There often is associated with the seam a fossil forest, with the stumps, which are still standing where they grew, their spreading roots, and the soil beneath, all changed to stone (Fig. 303). In the Nova Scotia field, out of seventy-six distinct coal seams, twenty are underlain by old forest grounds.

The presence of fire clay beneath a seam points in the same direction. Such underclays withstand intense heat and are used in making fire brick, because their alkalies have been removed by the long-continued growth of vegetation.

Fuel coal is also too pure to have been accumulated by driftage. In that case we should expect to find it mixed with mud, while in fact it often contains no more ash than the vegetal matter would furnish from which it has been compressed.

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