RAISING OF CONTINENTS AND MOUNTAINS.

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western and southern parts of the Valley were raised at the same time. All the highest plateaus and loftiest mountain ranges of other continents also date from this period. All this was accompanied with fearful earthquakes, with immense activity in volcanoes and the gushing forth of vast quantities of lava from long clefts in the rocks which must have been many miles deep.

This seemed to have been the great and, in some degree, definite adjustment of the surface of the earth to what lay beneath it. Apparently the surface, or crust of the earth, had become extremely thick and solid, and the former elevations of land and mountains had only partially relieved the strain, which continued to accumulate while the thickness and solidity of the crust also increased, until the pent-up giant force could only be relieved by these vast elevations.

There were frequent changes of level over wide regions in later times and there is much local movement to this day; but it appears to be chiefly a temporary shifting of level without any great world-wide or very permanent changes. What is the present condition of the interior of the earth, is a question on which geologists are not fully agreed. To settle it requires a comprehensiveness of knowledge not yet acquired. Many of the most eminent authorities consider it probable that pressure has so far overcome the expansive force of heat that the center of the glowing mass is solid and that a fluid mass lies between it and the surface crust. The mysterious behavior of magnetic forces has suggested that as an explanation. Others suppose that there has never been such a sea of molten fire beneath the cold crust as has been described; that pressure and the cooling process hardened the surface and the interior at the same time.

This view allows the same degree of heat in the interior but contends that it did not prevent the solidifying process. The heat has always been escaping-ascending from below through the colder rocks-and the surface changes-sinking of ocean

beds, raising of continents and mountains, and other displays of immense force-are due to the unequal contraction of the cooling rocks lying below those already cooled, and to the unequal qualities of the surface rocks as conductors of heat. This leaves the same horizontal strain in the surface rocks, and the way in which the force is applied to produce the great elevations and constant movements noticed is explained with much plausibility.

It is, however, a recent theory, requires mature consideration, and is not yet received by the exceedingly respectable authorities here followed. Still, it may prove to be true. The earth, as a whole, has been proved to be more than twice as heavy as the weight of its surface rocks would make it, so that extreme density for the interior or a vastly heavier substance must be supposed. It is still an open question how this is to be explained, and it was one of the chief reasons for the acceptance by some of the solid theory. To accept it would vary the explanation of continent outlining and mountain making, but would not demand any other change.

CHAPTER II.

HOW ROCKS ARE MADE AND HOW THEIR STORY' IS READ.

We have seen that, amidst all the seeming confusion of the earth in its earlier periods, an orderly and measured progress appears to have ruled from the first. Motion produced notable changes and change was controlled and guided towards certain definite ends. The materials that came finally together to produce the earth, as we now see it, were all scattered over an unspeakably vast space as vapor or "star dust." Examples of that state of things are believed to exist still in the Universe by astronomers. They are called Nebulæ.

By some means movement was commenced among these thinly diffused particles of matter-they attracted and repelled each other; from this proceeded heat. Particles of the same kind attracted each other most strongly and produced separation and concentration, and progress was commenced. This continued until the highest degree of heat was produced and then concentration was carried forward by the process of cooling until the separation of the mass of heavier material from the lighter, by the formation of a crust, made another long step forward. These lighter materials took the form of air and water; the water fell to the surface or floated as vapor in the air, and these two, assisted by powerful chemical agents and the vast forces we considered in the previous chapter, commenced the work of reconstructing the surface material of the hard-crust-that is, began a new process of rock making.

At first chemical and mechanical forces worked alone. After a time another agent appeared--the Life Force. This

busy and intelligent workman was a remarkably skillful chemist and builder, varied the style and the aims of its work according to circumstances, and so distinctly different are the forms it produced in each period that the geologist uses them as a guide in his researches.

The position of the rocks and some of their more general, as well as peculiar, features show in what age they were made. But these characteristics are not always present or may not always be distinct enough to make them reliable as a guide. So many changes have occurred that nowhere in the world do the entire series of rocks lie in regular succession one above the other. When any part of the crust of the earth was raised out of the water no rock was formed, and sometimes, while so raised, many layers already formed were in part or in whole washed away. Then the same surface was often sunk under water again and another series was formed of a later period, leaving a vast break in the series at that point. Sometimes they were so disturbed by elevating forces that it would be difficult to tell where they belonged but for the animal or vegetable remains in them.

A careful study of these remains reveals the remarkable fact that some classes of animals are wholly confined to certain series of rocks, and that the varying tribes, families and species of these classes are limited to particular layers in the series formed during a certain period. These remains, therefore, are a most important aid in classifying rocks. The life force, as has been said, varies the forms according to the condition of the climate, of the air and the water, and a thousand local or general circumstances. These indications, joined with the chemical structure of the rocks, the special materials of which they are composed, the marks of mechanical force which they bear and their position, furnish the alphabet of the language in which they tell their story.

It is a language that requires to be learned by study and pains; but when once mastered it is very clear and definite in

THE FOUR CLASSES OF ROCKS.

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conveying information. It is the most reliable of histories, for it is the record made by the events themselves as they passed. It is the phonograph of the long ages before there was a human observer, repeating the story of its own times to us much more exactly than such an observer could have learned it.

This story is told in four different volumes; that is to say, there are four periods and four classes of rocks called Azoic, Palæozoic, Mesozoic, and Cenozoic. These are Greek terms: Azoic meaning without life; Palæozoic, ancient life; Mesozoic, mediaval or middle life, and Cenozoic, recent life. The Azoic rocks contain no traces of life; the Palæozoic rocks, lying above the first, inclose the oldest remains of life that have been preserved; the Mesozoic rocks rest above the Palæozoic, and contain remains of plants and animals more like those which now exist; and the Cenozoic rocks, lying highest of all, except when they have been thrown out of place by elevating or disturbing forces, contain recent forms of life or which bear a close resemblance to those now existing.

The igneous rocks (ignis is Latin for fire)—those which cooled after having been melted-lie at the bottom underneath all the rest, except in cases where they have been thrown out of volcanoes, or have otherwise burst up and overflowed the surface through breaks in the crust, both which cases have been very numerous. Sometimes the rocks originally lying above them have been thrown off in mountain-making or have been quite worn away by the atmosphere, rains, and ice. These forces have always been actively at work crumbling away the elevated surfaces of the land and carrying away fragments and fine material in blocks, pebbles, sand, and mud, to form new layers in the waters. These layers form " "aqueous rocks" (aqua is Latin for water).

All the four classes above-mentioned differ from the igneous rocks. They are the finely worn material of the igneous, or of