THE EFFECTS OF CONTRACTION.

37

ing the wide Pacific have broad plateaus and high peaks, while those near the narrower Atlantic are more modest in all their proportions. The same peculiarity is observed on all the continents, which points to a general and uniform law of elevation.

The almost inconceivable power producing this elevation is thus the result of the contraction which steadily follows the cooling of the earth, and possibly, to some extent, the chemical changes and the force of gravity which consolidate the materials of the rocks so that they occupy less and less space. Since rocks, in cooling, lose from eight to twelve per cent in bulk, the surface crust was obliged by its weight to follow the contraction beneath.

This process is extremely slow, and the strain produced on all the surface rocks by contraction seems to have had its long periods of accumulation during which it manifested itself by a slow rise and fall of the surface over the continental regions. In some places the changes of level were great and long continued, in others slight but changing more often. Along the site of the Alleghany Mountains there was a long period of slow sinking. Nearly eight miles in thickness of rock was there formed. The character of the various layers showed that they were all formed not far below the surface of the water--the sinking and the formation of rock continuing to be about equal during the whole period. There were frequent changes in the direction of the movement over the general surface of the future Valley of the Mississippi, but its range seems to have been small, only about 4,000 feet of rock being formed in the Central Valley.

There were several periods during which this force violently eased itself by permanently raising some part of the crust high above the rest. During the first of these periods of permanent rising the Great Valley seems to have been outlined.

Land was first made along the northern border, and it is

4

believed to have formed the oldest of all the continents. It stretched from Labrador southwest, along the northern rim of the Great Lakes to Minnesota, with another branch from Lake Superior northwest far toward the Pole. The eastern side of the Valley next the Atlantic was then raised, and if land near the Pacific was not made then there was, at least, a sub-marine ridge, and ever thereafter the site of the Valley remained enclosed, sometimes as an interior shallow sea, and at others as low-lying land. The surface of the Valley was always the most stable part of the Continent.

After these liftings and some efforts at making mountains in a comparatively small way in Canada and New England, there was a very long period of uneasy movement, during which the land slowly gained on the water along the northern and eastern border of the Valley; but no great or extensive elevations were made. All the rocks and minerals of the Northern and Central Valley east of the Mississippi were made during this time, which was followed by a great display of force. The Alleghany Mountains were raised, and with them probably more than half, possibly two thirds, of the Valley became permanently dry land. This was a far greater display of force than any former elevation, and it is believed by some that the Alleghanies made the first great mountain chain raised on any continent.

Much of the surface of the Gulf States was still under water and an arm of the sea, or a channel some hundreds of miles wide, lay between the Missouri River and the site of the future Rocky Mountains. Another period of comparative quiet followed; but still greater forces were gathering, and finally, in the early part of modern geological time, made the grandest show of power the history of the earth can present. The long chain of the Rocky and Andes Mountains was raised, during which period of elevation a region of the continent a thousand miles wide was lifted into high plateaus, which served as a basis for many lofty mountain ranges. The

RAISING OF CONTINENTS AND MOUNTAINS.

39

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