DISTURBANCES IN THE VALLEY.

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to the strata laid in what had been, for the most part, the quietest region of the northern valley. Nature thus opened the book for science to read and, at the same time, accomplished various other important ends. These uplifts, when they broke and turned up the edges of the rocks, produced a great amount of heat, and the quality of the coal beds previously formed there was much improved thereby. The various layers of rock were also hardened and rendered more valuable as building material and the drainage was more or less improved.

Parts of Ohio, Kentucky, Tennessee, Missouri and Arkansas shared in these disturbances, during which nature took occasion to distribute some of the most valuable minerals where they would exert a powerful influence on the welfare of its future inhabitants. Wishing to render the central point attractive and valuable for historical and industrial purposes, she took much pains to enrich Missouri with minerals and to supply Illinois with a good quality of coal with which to work them at the least expense.

Thus all the rocks were formed with a variety of intelligent and benevolent purposes in view.

CHAPTER III.

HOW NATURE FINISHED THE VALLEY AND PREPARED IT FOR MAN.

The last part of the middle period, or Mesozoic time, and the first part of the recent period were occupied in the production of the vast mountain systems which left the continents at their present elevation, and with the same general relations to the seas and to each other as now. The division between the middle and recent times is made at the point where the forms of life that still exist began to appear in the rocks. Cenozoic time is divided by geologists into two parts, the first called the Tertiary, the second and last, which includes the present, the Quaternary.

The Tertiary is divided into three parts, according to the abundance of the species of life forms that still exist. The last of these is called the Pliocene, which means "more recent." A large proportion of the species of plants and animals found preserved in its rocks still remain. The next before it, and further back from us in time, is called Miocene, meaning "less recent." The first era of the Tertiary is called the Eocene, which means "the dawn of the recent." There are rocks of all these periods in the western and southern Valley, for the full outlines of those sections were not gained until the mountains and plateaus had reached their present elevation.

The gains of land in the Valley were not remarkably large in any of these three eras, for the general surface was already above the reach of the sea, but the rocks of those times that were made are of very great interest. They were chiefly fresh water formations from the Black Hills southward, and eastward from the base of the Rocky Mountains, and contain a very interesting class of fossil forms of the land animals of

THE PLAINS DURING THE TERTIARY.

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the three eras immediately preceding the age of ice and the appearance of man. A part of Dakota, Nebraska, Kansas, and portions of the territories bordering them on the west and south formed a lake region all through the Tertiary, or at least through the most of it. It was then a region of unstable level, very much like the eastern Valley during the age of coal-making, and the results were similar, for some 15,000 square miles of that region have beds of workable coal, which date from these deposits.

There is much that is extremely interesting in these coals and rocks besides the animal remains they inclose. Much of the coal is only partially reduced. It is called lignite, or woody coal, for the structure of the wood is often very evident and it has not all the density of true coal, nor its value for all purposes. The rocks are also less compact, in general, though in some situations, and when chemical conditions were favorable, very solid and firm building stone is found. Yet, the surface deposits were generally soft and loose, they did not have time to consolidate, and, being mostly formed in fresh water, which had less of chemical substances to unite and compact the materials, it was left comparatively friable.

For this reason the surface rocks were very heavily worn down and washed away in later periods, leaving deep river beds and here and there isolated embankments, pyramids, and figures of strange and fanciful shapes-the remains of the original layers. These sometimes very much resemble monuments of human labor, yet are always distinguishable by being stratified. Kansas, Nebraska, and Colorado furnish much curious and interesting scenery varied and beautified by this means. The strata of Tertiary times in the lower Valley were not washed and worn as much as on the plains and, for the most part, were more suddenly raised out of the sea.

The climate of this period was warm-temperate-very much like that of the southern Valley at the present day. Vegetation was therefore luxuriant and animal life abundant. With

the close of the Tertiary the Valley was fairly complete in its outlines, in its general provision of metals and coal, and in the rocks suitable for the purposes of the future. When these should be sufficiently pulverized they would furnish elements of inexhaustible fertility to the soil.

The final processes that were to give to this broad region its crowning value for man were reserved to the last part of Cenozoic time, called the Quaternary period. In a region where the rocks next the surface are Azoic, or where they are of igneous origin, the soil is thin and of moderate fertility, often barren. Even if those parts of it which are crumbled by the atmosphere and frost are not washed away they do not contain the variety of elements necessary to an abundant vegetation; the soil is not deep enough to retain the necessary moisture, or it is too compact and clings too closely to the underlying rock to be sufficiently drained. It was necessary to provide against these disadvantages in the Valley. This was accomplished during the three epochs of the Quaternary.

These three eras are called the Glacial period, the Champlain period and the Terrace period. The last includes the time that is now passing. The causes of the Glacial period, or the Age of Ice, are not clearly understood at least geologists are not agreed upon them—and various theories have been ɛuggested, none of which seem to be entirely satisfactory. Some attribute it to astronomical influences. The orbit of the earth slowly varies during a long period and then returns to its original state. When it was most elliptical, and carried the earth furthest away from the sun in one part of its track, the Glacial era is supposed to have occurred. Some scientific men of great eminence favor this view. Changes in the amount of heat furnished by the sun, changes in the sea bottom of regions near the equator, or the sinking of the isthmus connecting the two parts of the American Continent, have been appealed to, as also changer in the atmosphere.

Studies on these theories are not sufficiently mature to

THE AGE OF ICE AND ITS SERVICES.

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determine what may be their real value as yet. It seems fairly certain that after the vast mountain elevations ceasing before the end of the Tertiary period the northern parts of the continents were considerably raised as a whole. It is thought that Behrings Straits were closed, and that the sea bottom of the Northern Atlantic was raised so that Europe and North America were connected for a time. This would shut out the warin ocean currents from the Arctic regions, and, joined with the general elevation, might account for the vast sheet of mingled ice, snow and water that slowly moved down to the central Valley. Such a condition of things now exists in Greenland, and the evidences of its former state, from the eastern part of New England to the Rocky Mountains, are very numerous and positive.

The flow of ice descended to the Ohio River or its vicinity. The softer rocks on the northern rim of the Valley were ground very fine, while great boulders, or blocks, of the harder rocks were broken off, imbedded in the ice, and brought far down the Valley with immense quantities of smaller fragments, pebbles and coarse gravel. The depressions of the Great Lakes are believed to have been made at first by volcanic action in early times, to have been nearly filled by a deposit of softer rock in the slow progress of Palæozoic and Mesozoic times which was scooped out and crushed by this resistless shovel and mill, and carried down into the Valley.

After this crushing process had accumulated the mass of "drift," as this loose material is called, at the lower extremity of the great glacier, the northern regions slowly sank again, continuing that process far below the present level, when it ceased and a rise again commenced. When the elevation that is supposed to have brought on this Age of Ice ceased and the sinking commenced, the climate began to grow warmer, the ice melted, the glaciers retreated to the neighborhood of the pole, and the Champlain Era began.

During this time the melting of the ice and the lower level