Chapter 16

Fig. 303.Fossil Tree Stumps of a Carboniferous Forest, Scotland

These conditions are fairly met in the great swamps of river plains and deltas and of coastal plains, such as the great Dismal Swamp, where thousands of generations of forests with their undergrowths contribute their stems and leaves to form thick beds of peat. A coal seam is a fossil peat bed.

Geographical conditions during the Pennsylvanian.The Carboniferous peat swamps were of vast extent. A map of the Coal Measures (Fig. 260) shows that the coal marshes stretched, with various interruptions of higher ground and straits of openwater, from eastern Pennsylvania into Alabama, Texas, and Kansas. Some individual coal beds may still be traced over a thousand square miles, despite the erosion which they have suffered. It taxes the imagination to conceive that the varied region included within these limits was for hundreds of thousands of years a marshy plain covered with tropical jungles such as that pictured inFigure 304.

On the basis that peat loses four fifths of its bulk in changing to coal, we may reckon the thickness of these ancient peat beds. Coal seams six and ten feet thick, which are not uncommon, represent peat beds thirty and fifty feet in thickness, while mammoth coal seams fifty feet thick have been compressed from peat beds two hundred and fifty feet deep.

At the same time, the thousands of feet of marine and freshwater sediments, with their repeated alternations of limestones, sandstones, and shales, in which the seams of coal occur, prove a slow subsidence, with many changes in its rate, with halts when the land was at a stillstand, and with occasional movements upward.

When subsidence was most rapid and long continued the sea encroached far and wide upon the lowlands and covered the coal swamps with sands and muds and limy oozes. When subsidence slackened or ceased the land gained on the sea. Bays were barred, and lagoons as they gradually filled with mud became marshes. River deltas pushed forward, burying with their silts the sunken peat beds of earlier centuries, and at the surface emerged in broad, swampy flats,—like those of the deltas of the Mississippi and the Ganges,—which soon were covered with luxuriant forests. At times a gentle uplift brought to sea level great coastal plains, which for ages remained mantled with the jungle, their undeveloped drainage clogged with its débris, and were then again submerged.

Fig. 304.Ideal Landscape of the Pennsylvanian Epoch

Physical geography of the several regions.The Acadian regionlay on the eastern side of the northern land, where now are New Brunswick and Nova Scotia, and was an immense river delta. Here river deposits rich in coal accumulated to a depth of sixteen thousand feet. The area of this coal field is estimated at about thirty-six thousand square miles.

The Appalachian regionskirts the Appalachian oldland on the west from the southern boundary of New York to northern Alabama, extending west into eastern Ohio. The Cincinnati anticline was now a peninsula, and the broad gulf which had lain between it and Appalachia was transformed at the beginning of the Pennsylvanian into wide marshy plains, now sinking beneath the sea and now emerging from it. This area subsided during the Carboniferous period to a depth of nearly ten thousand feet.

The Central regionlay west of the peninsula of the Cincinnati anticline, and extended from Indiana west into eastern Nebraska, and from central Iowa and Illinois southward about the ancient island in Missouri and Arkansas into Oklahoma and Texas. On the north the subsidence in this area was comparatively slight, for the Carboniferous strata scarcely exceed two thousand feet in thickness. But in Arkansas and Indian Territory the downward movement amounted to four and five miles, as is proved by shoal water deposits of that immense thickness.

The coal fields of Indiana, and Illinois are now separated by erosion from those lying west of the Mississippi River. At the south the Appalachian land seems still to have stretched away to the west across Louisiana and Mississippi into Texas, and this westward extension formed the southern boundary of the coal marshes of the continent.

The three regions just mentioned include the chief Carboniferous coal fields of North America. Including a field in central Michigan evidently formed in an inclosed basin (Fig. 260), and one in Rhode Island, the total area of American coal fields has been reckoned at not less than two hundred thousand square miles. We can hardly estimate the value of these great storesof fossil fuel to an industrial civilization. The forests of the coal swamps accumulated in their woody tissues the energy which they received from the sun in light and heat, and it is this solar energy long stored in coal seams which now forms the world’s chief source of power in manufacturing.

The western area.On the Great Plains beyond the Missouri River the Carboniferous strata pass under those of more recent systems. Where they reappear, as about dissected mountain axes or on stripped plateaus, they consist wholly of marine deposits and are devoid of coal. The rich coal fields of the West are of later date.

On the whole the Carboniferous seems to have been a time of subsidence in the West. Throughout the period a sea covered the Great Basin and the plateaus of the Colorado River. At the time of the greatest depression the sites of the central chains of the Rockies were probably islands, but early in the period they may have been connected with the broad lands to the south and east. Thousands of feet of Carboniferous sediments were deposited where the Sierra Nevada Mountains now stand.

The Permian.As the Carboniferous period drew toward its close the sea gradually withdrew from the eastern part of the continent. Where the sea lingered in the deepest troughs, and where inclosed basins were cut off from it, the strata of the Permian were deposited. Such are found in New Brunswick, in Pennsylvania and West Virginia, in Texas, and in Kansas. In southwestern Kansas extensive Permian beds of rock salt and gypsum show that here lay great salt lakes in which these minerals were precipitated as their brines grew dense and dried away.

In the southern hemisphere the Permian deposits are so extraordinary that they deserve a brief notice, although we have so far omitted mention of the great events which characterized the evolution of other continents than our own. The Permian fauna- flora of Australia, India, South Africa, and the southern part of South America are so similarthat the inference is a reasonable one that these widely separated regions were then connected together, probably as extensions of a great antarctic continent.Interbedded with the Permian strata of the first three countries named are extensive and thick deposits of a peculiar nature which are clearly ancient ground moraines. Clays and sand, now hardened to firm rock, are inset with unsorted stones of all sizes, which often are faceted and scratched. Moreover, these bowlder clays rest on rock pavements which are polished and scored with glacial markings. Hence toward the close of the Paleozoic the southern lands of the eastern hemisphere were invaded by great glaciers or perhaps by ice sheets like that which now shrouds Greenland.These Permian ground moraines are not the first traces of the work of glaciers met with in the geological record. Similar deposits prove glaciation in Norway succeeding the pre-Cambrian stage of elevation, and Cambrian glacial drift has recently been found in China.

Interbedded with the Permian strata of the first three countries named are extensive and thick deposits of a peculiar nature which are clearly ancient ground moraines. Clays and sand, now hardened to firm rock, are inset with unsorted stones of all sizes, which often are faceted and scratched. Moreover, these bowlder clays rest on rock pavements which are polished and scored with glacial markings. Hence toward the close of the Paleozoic the southern lands of the eastern hemisphere were invaded by great glaciers or perhaps by ice sheets like that which now shrouds Greenland.

These Permian ground moraines are not the first traces of the work of glaciers met with in the geological record. Similar deposits prove glaciation in Norway succeeding the pre-Cambrian stage of elevation, and Cambrian glacial drift has recently been found in China.

The Appalachian deformation.We have seen that during Paleozoic times a long, narrow trough of the sea lay off the western coast of the ancient land of Appalachia, where now are the Appalachian Mountains. During the long ages of this era the trough gradually subsided, although with many stillstands and with occasional slight oscillations upward. Meanwhile the land lying to the east was gradually uplifted at varying rates and with long pauses. The waste of the rising land was constantly transferred to the sinking marginal sea bottom, and on the whole the trough was filled with sediments as rapidly as it subsided. The sea was thus kept shallow, and at times, especially toward the close of the era, much of the area was upbuilt or raised to low, marshy, coastal plains. When the Carboniferous was ended the waste which had been removed from the land and laid along its margin in the successive formations of the Paleozoic had reached a thickness of between thirty and forty thousand feet.

Both by sedimentation and by subsidence the trough had now become a belt of weakness in the crust of the earth. Herethe crust was now made of layers to the depth of six or seven miles. In comparison with the massive crystalline rocks of Appalachia on the east, the layered rock of the trough was weak to resist lateral pressure, as a ream of sheets of paper is weak when compared with a solid board of the same thickness. It was weaker also than the region to the west, since there the sediments were much thinner. Besides, by the long-continued depression the strata of the trough had been bent from the flat-lying attitude in which they were laid to one in which they were less able to resist a horizontal thrust.

There now occurred one of the critical stages in the history of the planet, when the crust crumples under its own weight and shrinks down upon a nucleus which is diminishing in volume and no longer able to support it. Under slow but resistless pressure the strata of the Appalachian trough were thrust against the rigid land, and slowly, steadily bent into long folds whose axes ran northeast-southwest parallel to the ancient coast line. It was on the eastern side next the buttress of the land that the deformation was the greatest, and the folds most steep and close. In central Pennsylvania and West Virginia the folds were for the most part open. South of these states the folds were more closely appressed, the strata were much broken, and the great thrust faults were formed which have been described already (p. 218). In eastern Pennsylvania seams of bituminous coal were altered to anthracite, while outside the region of strong deformation, as in western Pennsylvania, they remained unchanged. An important factor in the deformation was the massive limestones of the Cambrian-Ordovician. Because of these thick, resistant beds the rocks were bent into wide folds and sheared in places with great thrust faults. Had the strata been weak shales, an equal pressure would have crushed and mashed them.

Although the great earth folds were slowly raised, and no doubt eroded in their rising, they formed in all probability arange of lofty mountains, with a width of from fifty to a hundred and twenty-five miles, which stretched from New York to central Alabama.

From their bases lowlands extended westward to beyond the Missouri River. At the same time ranges were upridged out of thick Paleozoic sediments both in the Bay of Fundy region and in the Indian Territory. The eastern portion of the North American continent was now well-nigh complete.

The date of the Appalachian deformation is told in the usual way. The Carboniferous strata, nearly two miles thick, are all infolded in the Appalachian ridges, while the next deposits found in this region—those of the later portion of the first period (the Trias) of the succeeding era—rest unconformably on the worn edges of the Appalachian folded strata. The deformation therefore took place about the close of the Paleozoic. It seems to have begun in the Permian, in, eastern Pennsylvania,—for here the Permian strata are wanting,—and to have continued into the Trias, whose earlier formations are absent over all the area.

With this wide uplift the subsidence of the sea floor which had so long been general in eastern North America came to an end. Deposition now gave place to erosion. The sedimentary record of the Paleozoic was closed, and after an unknown lapse of time, here unrecorded, the annals of the succeeding era were written under changed conditions.

In western North America the closing stages of the Paleozoic were marked by important oscillations. The Great Basin, which had long been a mediterranean sea, was converted into land over western Utah and eastern Nevada, while the waves of the Pacific rolled across California and western Nevada.

The absence of tuffs and lavas among the Carboniferous strata of North America shows that here volcanic action was singularly wanting during the entire period. Even the Appalachian deformation was not accompanied by any volcanic outbursts.

Life of the Carboniferous

Fig. 305.Carboniferous Ferns

Fig. 306.Calamites

Plants.The gloomy forests and dense undergrowths of the Carboniferous jungles would appear unfamiliar to us could we see them as they grew, and even a botanist would find many of their forms perplexing and hard to classify. None of our modern trees would meet the eye. Plants with conspicuous flowers of fragrance and beauty were yet to come. Even mosses and grasses were still absent.

Ferns we should recognize at once by their delicate fronds with the spore cases underneath, and a botanist would notice that certain species belong to families which still exist. As at the present, some were lowly herbaceous plants, and some were tree ferns, lifting their crown of feathery fronds high in the air on trunks of woody tissue.

Dense thickets, like cane or bamboo brakes, were composed of thick clumps ofCalamites, whose slender, jointed stems shot up to a height of forty feet, and at the joints bore slenderbranches set with whorls of leaves. These were close allies of the Equiseta or “horsetails,” of the present; but they bore characteristics of higher classes in the woody structures of their stems.

There were also vast monotonous forests, composed chiefly of trees belonging to the lycopods, and whose nearest relatives to-day are the little club mosses of our eastern woods. Two families of lycopods deserve special mention,—the Lepidodendrons and the Sigillaria.

Fig. 308.Sigillaria

Fig. 307.Lepidodendron

TheLepidodendron, or “scale tree,” was a gigantic club moss fifty and seventy-five feet high, spreading toward the top into stout branches, at whose ends were borne cone-shaped spore cases. The younger parts of the tree were clothed with stiff needle-shaped leaves, but elsewhere the trunk and branches were marked with scalelike scars, left by the fallen leaves, and arranged in spiral rows.

TheSigillaria, or “seal tree,” was similar to the Lepidodendron, but its fluted trunk divided into even fewer branches, and was dotted with vertical rows of leaf scars, like the impressions of a seal.

Both Lepidodendron and Sigillaria were anchored by means of great cablelike underground stems, which ran to long distances through the marshy ground. The trunks of both trees had a thick woody rind, inclosing loose cellular tissue and a pith. Their hollow stumps, filled with sand and mud, are common in the Coal Measures, and in them one sometimes finds leaves and stems, land shells, and the bones of little reptiles of the time which made their home there.

It is important to note that some of these gigantic lycopods, which are classed with thecryptogams, or flowerless plants, had pith and medullary rays dividing their cylinders into woody wedges. These characters connect them with thephanerogams, or flowering plants. Like so many of the organisms of the remote past, they were connecting types from which groups now widely separated have diverged.

Gymnosperms, akin to the cycads, were also present in the Carboniferous forests. Such were the different species ofCordaites, trees pyramidal in shape, with strap-shaped leaves and nutlike fruit. Other gymnosperms were related to the yews, and it was by these that many of the fossil nuts found in the Coal Measures were borne. It is thought by some that the gymnosperms had their station on the drier plains and higher lands.

The Carboniferous jungles extended over parts of Europe and of Asia, as well as eastern North America, and reached from the equator to within nine degrees of the north pole. Even in these widely separated regions the genera and species of coal plants are close akin and often identical.

Invertebrates.Among the echinoderms, crinoids are now exceedingly abundant, sea urchins are more plentiful, and sea cucumbers are found now for the first time. Trilobites are rapidly declining, and pass away forever with the close of the period. Eurypterids are common; stinging scorpions are abundant; and here occur the first-known spiders.

We have seen that the arthropods were the first of all animals to conquer the realm of the air, the earliest insects appearing in the Ordovician. Insects had now become exceedingly abundant, and the Carboniferous forests swarmed with the ancestral types of dragon flies,—some with a spread of wing of more than two feet,— May flies, crickets, and locusts. Cockroaches infested the swamps, and one hundred and thirty-three species of this ancient order have been discovered in the Carboniferous of North America. The higher flower-loving insects are still absent; the reign of the flowering plants has not yet begun. The Paleozoic insects were generalized types connecting the present orders. Their fore wings were still membranous and delicately veined, and used in flying; they had not yet become thick, and useful only as wing covers, as in many of their descendants.

Fig. 309.Carboniferous BrachiopodsA, Productus;B, Spirifer, the right-hand figure showing the interior with the calcareous spires for the support of the arms

Fishesstill held to the Devonian types, with the exception that the strange ostracoderms now had perished.

Amphibians.Footprints of amphibians are found in the Devonian. The earliest Carboniferous amphibians were small newtlike creatures possessing not only the typical amphibian double breathing system of gills and lungs but also a double locomotive apparatus of short, weak legs for crawling on land and atail for propulsion in the water. They branched into a variety of types,—some large and crocodilian, some with well-developed legs for running, some with large heads like giant tadpoles, and some eel-like and limbless.

Fig. 310.A Carboniferous Dragon FlyOne tenth natural size

Fig. 311.A Carboniferous Amphibian

The earliest amphibians differ from those of to-day in a number of respects. They were connecting types linking together fishes, from which they were descended, with reptiles, of which they were the ancestors. They retained the evidence of their close relationship with the Devonian fishes in their cold blood, their gills and aquatic habit during their larval stage, their teeth with dentine infolded like those of the Devonian ganoids but still more intricately, and their biconcave vertebræ which never completely ossified. These, the highest vertebrates of the time, had not yet advanced beyond the embryonic stage of the more or less cartilaginous skeleton and the persistent notochord.

Fig. 312.Transverse Section of Segment of Tooth of Carboniferous Amphibian

On the other hand, the skull of the Carboniferous amphibians was made of close-set bony plates, like the skull of the reptile, rather than like that of the frog, with its open spaces (Figs.313and314). Unlike modern amphibians, with their slimy skin, the Carboniferous amphibians wore an armor of bony scales over the ventral surface and sometimes over the back as well.

It is interesting to notice from the footprints and skeletons of these earliest-known vertebrates of the land what was the primitive number of digits. The Carboniferous amphibians had five- toed feet, the primitive type of foot, from which their descendants of higher orders, with a smaller number of digits, have diverged.

The Carboniferous was the age of lycopods and amphibians, as the Devonian had been the age of rhizocarps and fishes.

Fig. 313.Skull of a Permian Amphibian from Texas

Fig. 314.Skull of a Frog

Life of the Permian.The close of the Paleozoic was, as we have seen, a time of marked physical changes. The upridging of the Appalachians had begun and a wide continental uplift—proved by the absence of Permian deposits over large areas where sedimentation had gone on before—opened new landsfor settlement to hordes of air-breathing animals. Changes of climate compelled extensive migrations, and the fauna of different regions were thus brought into conflict. The Permian was a time of pronounced changes in plant and animal life, and a transitional period between two great eras. The somber forests of the earlier Carboniferous, with their gigantic club mosses, were now replaced by forests of cycads, tree ferns, and conifers. Even in the lower Permian the Lepidodendron and Sigillaria were very rare, and before the end of the epoch they and the Calamites also had become extinct. Gradually the antique types of the Paleozoic fauna died out, and in the Permian rocks are found the last survivors of the cystoid, the trilobite, and the eurypterid, and of many long-lived families of brachiopods, mollusks, and other invertebrates. The venerable Orthoceras and the Goniatite linger on through the epoch and into the first period of the succeeding era. Forerunners of the great ammonite family of cephalopod mollusks now appear. The antique forms of the earlier Carboniferous amphibians continue, but with many new genera and a marked increase in size.

A long forward step had now been taken in the evolution of the vertebrates. A new and higher type, the reptiles, had appeared, and in such numbers and variety are they found in the Permian strata that their advent may well have occurred in a still earlier epoch. It will be most convenient to describe the Permian reptiles along with their descendants of the Mesozoic.

CHAPTER XX

THE MESOZOIC

With the close of the Permian the world of animal and vegetable life had so changed that the line is drawn here which marks the end of the old order and the beginning of the new and separates the Paleozoic from the succeeding era,—the Mesozoic, the Middle Age of geological history. Although the Mesozoic era is shorter than the Paleozoic, as measured by the thickness of their strata, yet its duration must be reckoned in millions of years. Its predominant life features are the culmination and the beginning of the decline of reptiles, amphibians, cephalopod mollusks, and cycads, and the advent of marsupial mammals, birds, teleost fishes, and angiospermous plants. The leading events of the long ages of the era we can sketch only in the most summary way.

The Mesozoic comprises three systems,—theTriassic, named from its threefold division in Germany; theJurassic, which is well displayed in the Jura Mountains; and theCretaceous, which contains the extensive chalk (Latin,creta) deposits of Europe.

In eastern North America the Mesozoic rocks are much less important than the Paleozoic, for much of this portion of the continent was land during the Mesozoic era, and the area of the Mesozoic rocks is small. In western North America, on the other hand, the strata of the Mesozoic—and of the Cenozoic also—are widely spread. The Paleozoic rocks are buried quite generally from view except where the mountain makings and continental uplifts of the Mesozoic and Cenozoic have allowed profound erosion to bring them to light, as in deep canyons and about mountain axes. The record of many of the most important events in the development of the continent during the Mesozoic and Cenozoic eras is found in the rocks of our western states.

The Triassic and Jurassic

Eastern North America.The sedimentary record interrupted by the Appalachian deformation was not renewed in eastern North America until late in the Triassic. Hence during this long interval the land stood high, the coast was farther out than now, and over our Atlantic states geological time was recorded chiefly in erosion forms of hill and plain which have long since vanished. The area of the later Triassic rocks of this region, which take up again the geological record, is seen in the map ofFigure 260. They lie on the upturned and eroded edges of the older rocks and occupy long troughs running for the most part parallel to the Atlantic coast. Evidently subsidence was in progress where these rocks were deposited. The eastern border of Appalachia was now depressed. The oldland was warping, and long belts of country lying parallel to the shore subsided, forming troughs in which thousands of feet of sediment now gathered.

These Triassic rocks, which are chiefly sandstones, hold no marine fossils, and hence were not laid in open arms of the sea. But their layers are often ripple-marked, and contain many tracks of reptiles, imprints of raindrops, and some fossil wood, while an occasional bed of shale is filled with the remains of fishes. We may conceive, then, of the Connecticut valley and the larger trough to the southwest as basins gradually sinking at a rate perhaps no faster than that of the New Jersey coast to-day, and as gradually aggraded by streams from the neighboring uplands. Their broad, sandy flats were overflowed by wandering streams, and when subsidence gained on deposition shallow lakes overspread the alluvial plains. Perhaps now and then the basins became long, brackish estuaries, whose low shores were swept by the incoming tide and were in turn left bare at its retreat to receive the rain prints of passing showers and the tracks of the troops of reptiles which inhabited these valleys.

The Triassic rocks are mainly red sandstones,—often feldspathic, or arkose, with some conglomerates and shales. Considering the large amount of feldspathic material in these rocks, do you infer that they were derived from the adjacent crystalline and metamorphic rocks of the oldland of Appalachia, or from the sedimentary Paleozoic rocks which had been folded into mountains during the Appalachian deformation? If from the former, was the drainage of the northern Appalachian mountain region then, as now, eastward and southeastward toward the Atlantic? The Triassic sandstones are voluminous, measuring at least a mile in thickness, and are largely of coarse waste. What do you infer as to the height of the lands from which the waste was shed, or the direction of the oscillation which they were then undergoing? In the southern basins, as about Richmond, Virginia, are valuable beds of coal; what was the physical geography of these areas when the coal was being formed?

Fig. 315.Section of Triassic Sandstones of the Connecticut Valleyss, sandstones;ll, lava sheets;cc, crystalline igneous and metamorphic rocks

Interbedded with the Triassic sandstones are contemporaneous lava beds which were fed from dikes. Volcanic action, which had been remarkably absent in eastern North America during Paleozoic times, was well-marked in connection with the warping now in progress. Thick intrusive sheets have also been driven in among the strata, as, for example, the sheet of the Palisades of the Hudson, described onpage 269.

The present condition of the Triassic sandstones of the Connecticut valley is seen inFigure 315. Were the beds laid in their present attitude? What was the nature of the deformation which they have suffered? When did the intrusion of lava sheets take place relative to the deformation? What effect have these sheets on the present topography, and why? Assuming that the Triassic deformation went on more rapidly than denudation, what was its effect on the topography of the time? Are there any of its results remaining in the topography of to-day? Do theTriassic areas now stand higher or lower than the surrounding country, and why? How do the Triassic sandstones and shales compare in hardness with the igneous and metamorphic rocks about them? The Jurassic strata are wanting over the Triassic areas and over all of eastern North America. Was this region land or sea, an area of erosion or sedimentation, during the Jurassic period? In New Jersey, Pennsylvania, and farther southwest the lowest strata of the next period, the Cretaceous, rest on the eroded edges of the earlier rocks. The surface on which they lie is worn so even that we must believe that at the opening of the Cretaceous the oldland of Appalachia, including the Triassic areas, had been baseleveled at least near the coast. When, therefore, did the deformation of the Triassic rocks occur?

Western North America.Triassic strata infolded in the Sierra Nevada Mountains carry marine fossils and reach a thickness of nearly five thousand feet. California was then under water, and the site of the Sierra was a subsiding trough slowly filling with waste from the Great Basin land to the east.

Over a long belt which reaches from Wyoming across Colorado into New Mexico no Triassic sediments are found, nor is there any evidence that they were ever present; hence this area was high land suffering erosion during the Triassic. On each side of it, in eastern Colorado and about the Black Hills, in western Texas, in Utah, over the site of the Wasatch Mountains, and southward into Arizona over the plateaus trenched by the Colorado River, are large areas of Triassic rocks, sandstones chiefly, with some rock salt and gypsum. Fossils are very rare and none of them marine. Here, then, lay broad shallow lakes often salt, and warped basins, in which the waste of the adjacent uplands gathered. To this system belong the sandstones of the Garden of the Gods in Colorado, which later earth movements have upturned with the uplifted mountain flanks.

The Jurassic was marked with varied oscillations and wide changes in the outline of sea and land.

Jurassic shales of immense thickness—now metamorphosed into slates—are found infolded into the Sierra Nevada Mountains. Hence during Jurassic times the Sierra trough continuedto subside, and enormous deposits of mud were washed into it from the land lying to the east. Contemporaneous lava flows interbedded with the strata show that volcanic action accompanied the downwarp, and that molten rock was driven upward through fissures in the crust and outspread over the sea floor in sheets of lava.

The Sierra deformation.Ever since the middle of the Silurian, the Sierra trough had been sinking, though no doubt with halts and interruptions, until it contained nearly twenty-five thousand feet of sediment. At the close of the Jurassic it yielded to lateral pressure and the vast pile of strata was crumpled and upheaved into towering mountains. The Mesozoic muds were hardened and squeezed into slates. The rocks were wrenched and broken, and underground waters began the work of filling their fissures with gold-bearing quartz, which was yet to wait millions of years before the arrival of man to mine it. Immense bodies of molten rock were intruded into the crust as it suffered deformation, and these appear in the large areas of granite which the later denudation of the range has brought to light.

The same movements probably uplifted the rocks of the Coast Range in a chain of islands. The whole western part of the continent was raised and its seas and lakes were for the most part drained away.

The British Isles.The Triassic strata of the British Isles are continental, and include breccia beds of cemented talus, deposits of salt and gypsum, and sandstones whose rounded and polished grains are those of the wind-blown sands of deserts. In Triassic times the British Isles were part of a desert extending over much of northwestern Europe.

The Cretaceous

The third great system of the Mesozoic includes many formations, marine and continental, which record a long and complicated history marked by great oscillations of the crust and wide changes in the outlines of sea and land.

Early Cretaceous.In eastern North America the lowest Cretaceous series comprises fresh-water formations which are traced from Nantucket across Martha’s Vineyard and Long Island, and through New Jersey southward into Georgia. They rest unconformably on the Triassic sandstones and the older rocks of the region. The Atlantic shore line was still farther out than now in the northern states. Again, as during the Triassic, a warping of the crust formed a long trough parallel to the coast and to the Appalachian ridges, but cut off from the sea; and here the continental deposits of the early Cretaceous were laid.Along the Gulf of Mexico the same series was deposited under like conditions over the area known as the Mississippi embayment, reaching from Georgia northwestward into Tennessee and thence across into Arkansas and southward into Texas.In the Southwest the subsidence continued until the transgressing sea covered most of Mexico and Texas and extended a gulf northward into Kansas. In its warm and quiet waters limestones accumulated to a depth of from one thousand to five thousand feet in Texas, and of more than ten thousand feet in Mexico. Meanwhile the lowlands, where the Great Plains are now, received continental deposits; coal swamps stretched from western Montana into British Columbia.The Middle Cretaceous.This was a land epoch. The early Cretaceous sea retired from Texas and Mexico, for its sediments are overlain unconformably by formations of the Upper Cretaceous. So long was the time gap between the two series that no species found in the one occurs in the other.

Along the Gulf of Mexico the same series was deposited under like conditions over the area known as the Mississippi embayment, reaching from Georgia northwestward into Tennessee and thence across into Arkansas and southward into Texas.

In the Southwest the subsidence continued until the transgressing sea covered most of Mexico and Texas and extended a gulf northward into Kansas. In its warm and quiet waters limestones accumulated to a depth of from one thousand to five thousand feet in Texas, and of more than ten thousand feet in Mexico. Meanwhile the lowlands, where the Great Plains are now, received continental deposits; coal swamps stretched from western Montana into British Columbia.

The Middle Cretaceous.This was a land epoch. The early Cretaceous sea retired from Texas and Mexico, for its sediments are overlain unconformably by formations of the Upper Cretaceous. So long was the time gap between the two series that no species found in the one occurs in the other.

The Upper Cretaceous.There now began one of the most remarkable events in all geological history,—the great Cretaceous subsidence. Its earlier warpings were recorded in continental deposits,—wide sheets of sandstone, shale, and some coal,—which were spread from Texas to British Columbia. These continental deposits are overlain by a succession of marine formations whose vast area is shown on the map,Figure 260. We may infer that as the depression of the continent continued the sea came in far and wide over the coast lands and the plains worn low during the previous epochs. Upper Cretaceous formations show that south of New England the waters of the Atlanticsomewhat overlapped the crystalline rocks of the Piedmont Belt and spread their waste over the submerged coastal plain. The Gulf of Mexico again covered the Mississippi embayment, reaching as far north as southern Illinois, and extended over Texas. A mediterranean sea now stretched from the Gulf to the arctic regions and from central Iowa to the eastern shore of the Great Basin land at about the longitude of Salt Lake City, the Colorado Mountains rising from it in a chain of islands. Along with minor oscillations there were laid in the interior sea various formations of sandstones, shales, and limestones, and from Kansas to South Dakota beds of white chalk show that the clear, warm waters swarmed at times with foraminiferal life whose disintegrating microscopic shells accumulated in this rare deposit.

Fig. 316.Hypothetical Map of Upper Cretaceous Epicontinental SeasShaded areas, probable seas; broken lines, approximate shore lines

Fig. 317.Foraminifera from Cretaceous Chalk, Iowa

At this epoch a wide sea, interrupted by various islands, stretched across Eurasia from Wales and western Spain to China, and spread southward over much of the Sahara. To the west its waters were clear and on its floor the crumbled remains of foraminifers gathered in heavy accumulations of calcareous ooze,— the white chalk of France and England. Sea urchins were also abundant, and sponges contributed their spicules to form nodules of flint.The Laramie.The closing stage of the Cretaceous was marked in North America by a slow uplift of the land. As the interior sea gradually withdrew, the warping basins of its floor were filled with waste from the rising lands about them, and over this wide area there were spread continental deposits in fresh-water lakes like the Great Lakes of the present, in brackish estuaries, and in river plains, while occasional oscillations now and again let in the sea. There were vast marshes in which there accumulated the larger part of the valuable coal seams of the West. The Laramie is the coal-bearing series of the West, as the Pennsylvanian is of the eastern part of our country.

The Laramie.The closing stage of the Cretaceous was marked in North America by a slow uplift of the land. As the interior sea gradually withdrew, the warping basins of its floor were filled with waste from the rising lands about them, and over this wide area there were spread continental deposits in fresh-water lakes like the Great Lakes of the present, in brackish estuaries, and in river plains, while occasional oscillations now and again let in the sea. There were vast marshes in which there accumulated the larger part of the valuable coal seams of the West. The Laramie is the coal-bearing series of the West, as the Pennsylvanian is of the eastern part of our country.

The Rocky Mountain deformation.At the close of the Cretaceous we enter upon an epoch of mountain-making far more extensive than any which the continent had witnessed. The long belt lying west of the ancient axes of the Colorado Islands and east of the Great Basin land had been an area of deposition for many ages, and in its subsiding troughs Paleozoic and Mesozoic sediments had gathered to the depth of many thousand feet. And now from Mexico well-nigh to the Arctic Ocean this belt yielded to lateral pressure. The Cretaceous limestones of Mexico were folded into lofty mountains. A massive range was upfolded where the Wasatch Mountains now are, and various ranges of the Rockies in Colorado and other states were upridged. However slowly these deformations were effected they were no doubt accompanied by world-shaking earthquakes, and it is known that volcanic eruptions took place on a magnificent scale. Outflows of lava occurred along the Wasatch, the laccoliths of the Henry Mountains (P. 271) were formed, while the great masses of igneous rock which constitute the cores of theSpanish Peaks (p. 271) and other western mountains were thrust up amid the strata. The high plateaus from which many of these ranges rise had not yet been uplifted, and the bases of the mountains probably stood near the level of the sea.

North America was now well-nigh completed. The mediterranean seas which so often had occupied the heart of the land were done away with, and the continent stretched unbroken from the foot of the Sierras on the west to the Fall Line of the Atlantic coastal plain on the east.

The Mesozoic peneplain.The immense thickness of the Mesozoic formations conveys to our minds some idea of the vast length of time involved in the slow progress of its successive ages. The same lesson is taught as plainly by the amount of denudation which the lands suffered during the era.

The beginning of the Mesozoic saw a system of lofty mountain ranges stretching from New York into central Alabama. The end of this long era found here a wide peneplain crossed by sluggish wandering rivers and overlooked by detached hills as yet unreduced to the general level. The Mesozoic era was long enough for the Appalachian Mountains, upridged at its beginning, to have been weathered and worn away and carried grain by grain to the sea. The same plain extended over southern New England. The Taconic range, uplifted partially at least at the close of the Ordovician, and the block mountains of the Triassic, together with the pre- Cambrian mountains of ancient Appalachia, had now all been worn to a common level with the Allegheny ranges. The Mesozoic peneplain has been upwarped by later crustal movements and has suffered profound erosion, but the remnants of it which remain on the upland of southern New England and the even summits of the Allegheny ridges suffice to prove that it once existed. The age of the Mesozoic peneplain is determined from the fact that the lower Tertiary sediments were deposited on its even surface when at the close of the era the peneplain was depressed along its edges beneath the sea.

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