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The Silurian Age, compared with the more stable formations of subsequent times, was one of commotion, in which fire and water played a conspicuous part. Earthquakes and volcanoes furrowed the yielding crust with ridges, and threw up islands whose craggy summits, here and there, stood like sentinels above the murky deep which dashed against their shores. The present diversities of climate did not exist, as the temperature was mostly due to the escape of internal heat, which was the same over every part of the surface. As the radiation of heat in future ages declined, the sun became the controlling power, and zones of climate appeared as the result of solar domination.' Uniform thermal conditions imparted a corresponding character to vegetable and animal life, and one universal fauna and flora extended from the equator to the poles. These hardy marine types consisted of Radiates, Mollusks and Articulates, three of the four sub-kingdoms of animal life. Seaweed, which served as food for the animals, was the only plant of which any traces remain. During the Silurian age North America, like its inhabitants, was mostly submarine, as proved by wave-lines on the emerging lands. There lay along the eastern border of the continent an extended ridge, which served as a breakwater to the waves of the Atlantic. The region of the Alleghanies was subject to great elevations and depressions, and the latter largely preponderating, caused the deposit of some twelve thousand feet of strata. Although mostly under water, there was added to the original nucleus of the continent formations now found in New York, Michigan, Illinois, Wisconsin and Minnesota. Niagara limestone, a Silurian formation, is found over a large exteut of country in northern Illinois, beyond the limits of the coal-fields. It is a compact grayish stone, susceptible of a high polish, and at Athens and Joliet is extensively quarried for building purposes, and shipped to different parts of the State. The new Capitol is being erected of this material. The Galena limestone, another Silurian deposit, is interesting, from the fact that it contains the lead and zinc ores of the State. St. Peters sandstone belongs also to the same system. Besides outcropping in a number of other localities, it appears in the bluffs of the Ilinois, where it forms the island like plateau known as Starved Rock. In some localities, being composed almost entirely of silica and nearly free from coloring matter, it is the best material in the West for the manufacture of glass.
The Devonian Age is distinguished for the introduction of Vertebrates, or the fourth sub-kingdom of animal life and the beginning of terrestrial vegetation. The latter appeared in two classes, the highest of the flowerless and the lowest of the flowering plants. The Lepidoclendron, a noted instance of the former, was a majestic upland forest tree, which, during the coal period, grew to a hight of 80 feet, and had a base of more than 3 feet in diameter. Beautiful spiral flutings, coiling in opposite directions and crossing each other at fixed angles, carved the trunks and branches into rhomboidal eminences, each of which was scarred with the mark of a falling leaf. At an altitude of 60 feet it sent off arms, each separating into branchlets covered with a needle-like foliage, destitute of flowers. It grew, not by internal or external accretions, as plants of the present day, but like the building of a monument, by additions to the top of its trunk. Mosses, rushes and other diminutive flowerless plants are now the only surviving representative of this cryptogamic vegetation, which so largely predominated in the early botany of the globe. Floral beauty and fragrance were not characteristic of the old Devonian woods. No bird existed to enliven their silent groves with song, no serpent to hiss in their fenny brakes, nor beast to pursue, with hideous yells, its panting prey.
The vertebrates consisted of fishes, of which the Ganoids and Placoids were the principal groups. The former were the forerunners of the reptile, which in many respects they closely resembled. They embraced a large number of species, many of which grew to a gigantic size; but with the exception of the gar and sturgeon, they have no living representatives. The Placoids, structurally formed for advancement, still remain among the highest types of the present seas. The shark, a noted instance, judging from its fossil remains, must have attained 100 feet in length. Both groups lived in the sea, and if any fresh water animals existed their remains have either perished or not been found. So numerous were the inhabitants of the ocean, that the Devonian has been styled the age of fishes. In their anatomical structure was foreshadowed the organization of man; reptiles, birds and mammals being the intermediate gradations. The continental sea of the preceding age still covered the larger part of North America, extending far northwest and opening south into the Gulf of Mexico. In its shallow basins were deposited sandstones, shales and limestones, which westerly attained a thickness of 500 feet, and in the region of the Alleghanies 1,500 feet. The great thickness of the latter deposits indicated oscillations, in which the downward inovement exceeded the upward. Shallow waters, therefore, interspersed with reefs and islands, still occupied the sites of the Alleghanies and Rocky Mountains, which now look down from above the clouds on the finished continent. The St. Lawrence and the Hudson may have existed in miniature, but the area of land was too small for rivers and other bodies of fresh water of considerable extent. In the disturbances closing the Devonian age additions were made to the surface in Iowa, Wisconsin and Illinois. The two resulting formations in this State are the Devonian limestone and the Oriskany sandstone. There are outcrops of the former in the bluffs of the Mississippi, Rock and Illinois rivers. It contains a great variety of fossils, and is used for building material and the manufacture of quicklime. The latter appears in Union, Alexander and Jackson counties, and is used to some extent in the manufacture of glass.
The Carboniferous Age opened with the deposition of widely extended marine formations. Added to the strata previously deposited, the entire thickness in the region of the Alleghanies, now partially elevated, amounted to 7 miles. Wide areas of permanent elevation occurred between the 34th and 45th degrees of latitude, embracing most of the territory between the eastern continental border and the States of Kansas and Nebraska. Farther westward, and resulting from the gradual emergence of the Pacific coast, was an interior sea whose shallow waters still flowed over the site of the Rocky Mountains. The winter temperature near the poles was 66 degrees. A stagnant and stifling atmosphere rested upon the area now constituting the United States and British
America. The McKenzie river, now filled with icebergs, then flowed through verdant banks to a coral sea, having the same temperature as the Gulf of Mexico at the present day. The most prominent feature of the age was the formation of coal. Being carbonized vegetable tissue, the material furnished for this purpose was the vast forest accumulations peculiar to the period. Vegetation, commencing in the previous age, had now attained an expansion which greatly exceeded the growth of prior or subsequent times. Invigorated by a warın, moist and winterless climate, and an atinosphere surcharged with carbonic acid gas, vast jungles spread! over the marshy plains, and impenetrable forests covereil the upland slopes and hights. The graceful lepidodendron, now fuly developed, was one of the principal coal producing plants; subserving the same purpose and associated with it was the gigantic conifer, a member of the pine family. The ancient fern, another coal plant, grew to a bight of 80 feet. Its trunk, regularly fretted with scars and destitute of branches, terminated in a crown of foliage rivaling that of the palm in profuseness and beauty. The sigillarid, however, as it contributed most largely to the production of coal, was the characteristic plant of the period. The trunk, which rose from 40 to 60 feet high from its alternate flutings and ribs, appeared like a clustered column. At an altitude of 25 or 30 feet it separated into branches, covered with a grass-like foliage intermingled with long catkins of obscure flowers or strings of seed, arranged in whorls about a common stem. The structure of the trunk was peculiar. One, 5 feet in diameter, was surrounded with a bark 13 inches in thickness; within this was a cylinder of wood 12 inches in thickness, and at the center a pith 10 inches in diameter. Such a tree would be useless as timber, but the bark, of which they largely consisted, was impervious to mineral solutions, and valuable for the production of coal. The calamites, growing with the sigillarids, covered with dense brakes the marshy flats. Their hollow stems, marked vertically with flutings and horizontally with joints, grew in clumps to a light of 20 feet. „Some species were branchless, while from the joints of other sprang branches, subdividing into whorls of branchlets.
The vast accumulation of vegetable matter from these and other carboniferous plants, either imbedded in the miry soil in which it grew, or swept from adjacent elevations into shallow lakes, became covered witli sediment, and thus were transformed into coal. It has been estimated that 8 perpendicular feet of wood were required to make 1 foot of bituminous coal, and 12 to make 1 of anthracite. Some beds of the latter are 30 feet in thickness, and hence 360 feet of timber must have been consuned in their production. The process of its formation was exactly the same as practiced in the manufacture of charcoal, by burning wood under a covering of earth. Vegetable tissue consists mostly of carbon and oxygen, and decomposition must take place, either under water or some other impervious covering, to prevent the elements from forming carbonic acid gas, and thus escaping to the atmosphere. Contorining to these requirements, the immense vegetable growths forming the coal-fields subsided with the surface on which they grew, and were buried beneath the succeeding deposits. Nova Scotia has 76 different beds, and Illinois 12; and consequently, in these localities there were as many different fields of
verdure overwhelmed in the dirt-beds of the sea. Thus, long before the starry cycles had measured half the history of the unfolding continent, and when first the expanding stream of life but dimly reflected the coming age of mind, this vast supply of fuel was stored away in the rocky frame-work of the globe. Here it slumbered till man made his appearance and dragged it from its rocky lairs. At his bidding it renders the factory animate with humming spindles, driving shuttles, whirling lathes, and clanking forges. Under his guidance the iron-horse, feeding upon its pitchy fragments, bounds with tireless tread over its far reaching track, dragging after him the products of distant marts and climes. By the skill of the one and the power of the other, the ocean steamer plows the deep in opposition to winds and waves, making its watery home a highway for the commerce of the world.
Prior to the formation of coal, so great was the volume of carbonic acid gas in the atmosphere that only slow breathing and cold blooded animals could exist. Consequent upon its conversion into coal, all the preceding species of plants and animals perished, and new forms came upon the stage of being with organizations adapted to the improved conditions. In the new economy, as at the present time, stability is maintained in the atmosphere by the reciprocal relations subsisting between it and the incoming types. The animal inspires oxygen and expires carbouic acid gas; the vegetable inspires carbonic acid gas and expires oxygen, thus preserving the equilibrium of this breathing medium. The coal-fields of Europe are estimated at 18,000 square miles, those of the United States at 150,000. The Alleghany coal-field contains 60,000 square miles, with an aggregate thickness of 120 feet. The Illinois and Missouri 60,000 square miles, and an aggregate thickness in some localities of 70 feet. Other fields occur in different localities, of various thicknesses. In Illinois, three-fourths of the surface are underlaid by beds of coal, and the State consequently has a greater area than any other member of the Union. There are 12 different beds, the two most important of which are each from 6 to 8 feet in thickness. The entire carboniferous system, including the coalbeds and the intervening strata, in southern Illinois is 27,000 feet in thickness, and in the northern part only 500.
Next to the immense deposits of coal, the Burlington, Keokuk and St. Louis limestones are the most important formations. They receive their appellations from the cities whose names they bear-where their lithological characters were first studieil-and in the vicinities of which they crop out in Illinois. The Burlington furnishes inexhaustible supplies of building stone and quicklime, but is mostly interesting on account of the immense number of interesting fossils which it contains. Along its northern outcrop Crinoids are found in a profusion unequalled by that of any locality of similar extent in the world. Though untold ages lave elapsed since their incarceration in the rocks, so perfect has been their preservation, their structure can be determined with almost as much precision as if they had perished but yesterday. The Keokuk is extensively used for architectural purposes, and furnished the material for the celebrated Mormon Temple at Nauvoo, the new Post-office at Springfield, and the Custom Houses at Galena and Dubuque. It contains some of the most interesting crystals found in the State. These consist of hollow spheres of
quartz and chalcedony of various sizes, and lined on the inside with crystalets of different minerals. Tons of specimens have been taken from Hancock county and distributed over the United States and Europe, to ornament the cabinets of mineralogists. The St. Louis is almost pure carbonet of lime, and the best material in the State for the manufacture of quick-lime. It is largely quarried at Alton.
The Age of Reptiles is distinguished for changes in the continental borders, which generally ran within their present limits. The sub-marine outlines of the Bay of New York, and the course of the Hudson, indicate that the adjacent shores during the early part of this age were beyond their present limits. Southward the sea line ran within the present shore, the distance increasing from 60 miles in Maryland to 100 in Georgia, and 200 in Alabama. The Texan gulf-shore, and that of the peninsula and State of California, were parallel, and mostly within their present positions. These borders were fringed with deposits, while inland the trough of the old continental sea was becoming more shallow. The altitude of the Alleghanies had nearly reached their present hight. The Rocky Mountains, in the transition from the close of the present to the beginning of the subsequent age, began slowly to emerge from the waters under which they had hitherto slumbered. The Gulf of Mexico formed a deep bay extending to the mouth of the Ohio, and, protruding itself northwesterly, covered the region of the Rocky Mountains. It may have connected with the Arctic Ocean, but observations have been too limited to trace it with certainty beyond the head waters of the Missouri and Yellow Stone. These are, therefore, among the more recently formed rivers, and cannot be compared with the primeval St. Lawrence and Hudson. The Mississippi was a stream of not more than one-half its present length and volume, falling into the gulf not far from the site of Cairo. The Ohio drained substantially the same region it does at the present time. In the earlier part of the age the geographical distribution of fossils indicates a common temperature, from Belring Strait in the Northern to that of Magellan in the Southern Hemisphere. In the latter part, however, a difference is perceptible, indicating also a difference of temperature and the commencement of climatic zones. This change, caused by the partial upheaval of mountain chains north of the Equator, and the decline of internal heat, marked a new era in the physical history of the globe. As the result, currents commenced flowing in the ocean; the constant monotony of previous ages was broken by the pleasant diversities of changing seasons ; life was imparted to the atmosphere, and the breeze came forth laden with the breath of spring; the tempest madly burst into being and began its work of destruction, and the trade-winds commenced blowing, but it was reserved for a future age to make them the common carriers of the ocean's commerce.
The principal formations of the age, none of which exist in Illinois, were sandstones, chalks and limestones, interstratified with deposits of salt and gypsum. Their absence can be explained either upon the supposition that the surface of the State was either above the waters in which they were deposited, or, having originally been deposited, they were subsequently swept away by denuding agencies. The former was perhaps the case, as no aqueous action