A few million years passed by and these same deposits may again be raised and become dry land. The great stretches of land between the mountains of the eastern and western portion of America were formed from the sediment washed down from these mountains and deposited in the sea which lay between.
The crust of the earth is still rising and sinking, the land gaining on the water, but as this is usually such a slow movement, it can only be traced through a period of many years.
"The notion that the ground is naturally steadfast is an error. The idea of terra firma belongs with the ancient belief that the earth was the center of the universe. It is, indeed, by their mobility that the continents survive the increasing assaults of the ocean waves, and the continuous down-wearing which the rivers and glaciers bring about."
—Professor Shaler.
Nearly the whole of Sweden and Norway, for instance, has been rising for thousands of years. This is proved by the fact that old sea-beaches full of shells of species now living in neighboring seas, are found from fifty to seventy miles inland, and several hundred feet above the present sea-level on both sides of the Scandinavian peninsula.[2] Such sea-beaches are found high above the present water level in Chile and Patagonia, and the chalk cliffs of England were formed under the waves of the ocean. Le Conte says that one of the most evident proofs of crustal movements in ancient geologic times is the great thickness of shallow water sediments (sandstones, shales and limestones) over the greater portions of all of the continents, which represent areas of slow subsidence; and the great breaks or unconformities between the series of strata, which represent areas of uplift and atmospheric erosion.
Although sand and gravel are distributed over the bottom of the ocean near the land, dredging expeditions reaching down their metal cylinders for samples of the sea-floor at greater depths in various sections of the globe, have discovered that it is of a different composition, one-third of its area being covered with red clay. Red clay is composed of iron and silica and represents all that is left of the minute shells which cover the tiny sea-animals which flourish in such countless multitudes throughout the ocean. These shells, after the animal dies, drift downward in ceaseless showers and the lime, of which they are largely composed, is dissolved during the long journey to the bottom. "Ooze" is found in water less deep and is formed by the accumulation of minute shells of different species. Minute shells of the same species have formed great thicknesses of limestone or of chalk even as grains of sand build in time their banks of sandstone. Such information as this is, of course, disclosed by the penetrating eye of the microscope. Chalk cliffs, now dry land, form thousands of miles of habitable land in Europe, while coral sand pressed into rock forms the unique foundation of the Bahama Islands, the Bermudas, many islands of the Pacific, and the Great Barrier Reef extending for a thousand miles along the shores of northern Australia!
Looking again about our own country on the land that lies cradled between the eastern and the western mountains of the United States, one cannot help but regard with awe the magnificent stretches of time it must have taken for this great area to have been formed beneath the sea. This alone proves the great age of the earth. Because it was raised slowly, the strata lie there little disturbed, one over the other, "like the leaves of a great stone book," containing the history of the earth. A good view of the river strata may be obtained in the canyon of the Colorado river, for this mighty river has carved a bed through the stratified deposits to a depth of from 3000 to 6000 feet.
"To quarry the heart of the earth
Till, in the rocks red rise,
Its age and birth, through an awful girth
Of strata, should show the wonder-worth
Of patience to all eyes."
The elevated sea-bottom is a fertile field for the fossil hunter, affording evidence that enables the geologist to reconstruct the evolutionary development of life on our planet in its divinely ordained stages. There is often but little resemblance in the fossils found in the lower strata to the plants and animals now in existence, but traced from stratum to stratum, the connection is obvious. The "fire-formed" rocks of the Archæan Age, which support all the other strata, contain no evidence of life; neither do the first sedimentary rocks which were deposited above them. The first fossils are found in the strata overlying the first sedimentary rock.
Geologists have divided the history of the earth into seven ages:
1. The Archæan Age, represented by the Archæan system of
rocks.