other aqueous, rocks spread in horizontal layers at the bottom of water, yet sometimes made wholly or partly of the broken, or finely ground, remains of organic forms, or sometimes formed by direct chemical action.

The Azoic rocks show that there was a long time after the waters covered the surface, during which there was too much heat, and probably, also, too strong infusions of chemical substances unfriendly to life to permit its introduction. The rocks of that period were deeply affected by heat, sometimes rendering it difficult to tell that they were ever stratified or formed of successive layers, as is the case with all aqueous rocks. It is believed that a deep layer of this primitive rock was spread over all regions before any land was raised out of the universal sea. Then contraction displayed its forces in making the first land, the waters must have cooled, while the chemical substances in them diminished, being deposited as rock.

Arrived at this point, seaweeds and the first animals ap peared in the waters, vast beds of iron and copper were formed, and Palæozoic time had begun. The waters were soon alive with animals. One of the principal uses of the shell fish, so extremely abundant at this time, was to form limestone after their death from the stony covering in which they inclosed themselves in life. It was a very long period. Slowly the surface of the Valley rose and fell. After each change different classes of rocks were formed, different species of animals flourished in the waters and different varieties of plants appeared on the land.

So

No land animals are known to have existed then, and it was only in the latter part of this period that fishes appeared in the seas. Nature makes her great changes very slowly. quietly were the elevations and depressions of the central Valley made that the rocks there were but little displaced or bent, and the sinking along the site of the Alleghany mountains was so slow that the formation of rock could keep pace

THE CLOSE OF ANCIENT TIME.

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with it; and when, at the close of this long period, these mountains were raised, it was so long in the doing that an eminent authority says, "motion by the few inches (or, at most, a few feet) a century accords best with the facts."

Nearly all the rock-making of the region east of the Mississippi River from the upper part of the Gulf States except the immediate vicinity of the river below the mouth of the Ohio, and perhaps all of Minnesota, Iowa and Northern Missouri, was done in this ancient time. On the northern and especially the eastern sides a large part of the material for rocks was obtained from the lands where other rocks were worn down and carried as mud, sand and pebbles to the sea; but in the quiet interior the rocks were chiefly limestone formed from the shells of its immense swarms of animals.

Toward the close of this period the sea seems to have been largely shut out. The general surface lay very near the level of the water and vegetable life, for the first time, predominated. When a vast amount of forest growth had been gathered, the surface sunk beneath the waters and the vegetable material was buried beneath mud and other rock-making material. A rise then occurred bringing the surface to its former position, the forest growth again springing up to be again buried, and so on many times in succession, each time furnishing material for a layer of coal. After a period of rest which allowed this to consolidate, the first great mountain-making period closed Ancient, or Palæozoic time. Only the surface of the upper and eastern Valley was afterwards modified or received additional material.

During Mesozoic time much, though not all, of the remainder of the Valley was filled out. In Texas there was a shallow sea, and a great thickness of limestone formed, while in the waters of the upper part of the Gulf States-which took in some of Tennessee and much of Mississippi and Alabama-there was probably a greater depth in which the chalk and flint formation was laid from shells of minute animals, and sandstones

were formed from the material washed down from the lands to the north. What was done during this time in the broad channel between the Missouri River and the site of the mountains on the west is not so well known except in Nebraska and near the Black Hills where the rocks of the period are rich in the remains of the life of that time. Toward the close of the Mesozoic, the symptoms of the coming vast elevations of the great Mountain-making Era began to appear by the elevation of the sea-bottom near to, or just above, the surface of the water and much coal was made, in places, amounting to about fifteen thousand square miles in the Western Valley.

There was a very great change in animal and vegetable life, which shows that the climate and the general conditions on which the development of life forms largely depends were very much altered during the coal and mountain-making periods which closed ancient time. It was a transition from the Old to the New and closed with the supreme display of force which produced the largest mountain and high platean systems of all the continents.

This elevation was not wholly completed until Cenozoic or recent time, during the first part of which the low lands bordering the Gulf were completed to about their present extent. The western plains in the Valley, which continued for a while to be a region of marshes and fresh water lakes, were then filled up and elevated.

This substantially completed the structural work of the Valley and of the continent, and introduced the general conditions of climate which still exist. With all the great changes which occurred on three of its borders, the Valley itself was a generally quiet region, even the elevation of the mountains, in which it shared, disturbing its rocks but slightly. Yet, slight as they were, these disturbances were of great importance. They produced a displacement, for instance, across Illinois, Northeastern Iowa and Southwestern Indiana crossing the Ohio River at Louisville, giving access

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