The Music of the Spheres/Chapter 16
CHAPTER XVI
THE EARTH
A Planet at Close Range
Diameter—7927 miles
Volumes could not cover the extremely complex history of the earth, nor picture with justice the exquisite construction of even its smallest features. It is here that the astronomer puts away his glass and turns detective with the geologist on his own home ground.
Astronomers, basing their computations of the earth's age upon the rate at which the sun is shrinking or contracting, have placed it at 25,000,000 years, but geologists claim that this is too short a time to account for the enormous thickness of stratified deposits, the salinity of the ocean, and other similar evidence which seems to prove that it is probably 100,000,000 years old, or even older. Later estimates based upon other modes of production of solar heat may extend this estimate several hundred fold.
The moon is believed by some to have once been a part of the earth and that it was formed through a division of the earth's ball when our world was in an early stage of evolution. Becoming two separated bodies, they have remained bound together by mutual attraction, and thus move around the sun.
According to the Nebular Hypothesis expounded by La Place, the earth and moon once shone like stars but, through the long ages of time they have gradually cooled, and, their lights extinguished, two cold-surfaced globes now wend their way around the sun. The smaller of these two globes is now scarred and old but _-_figure_155.jpg)
THE EARTH, SEEN FROM A DISTANCE.
(Used by permission of the publishers, Rand McNally & Company, from Dodge's "Elementary Geography.") the other is still fresh with sparkling waters, green verdure, and is teeming with life.
According to the later and more generally accepted Planetesimal Theory of Moulton and Chamberlain, these globes were cold in the beginning but became hot through compression as the material of which they were composed was drawn toward their centers by the pull of gravitation.
The four outer planets are mostly gaseous; the four inner ones have hardened crusts. Let us scan for a moment the various surface features of the third one in order from the sun,—the Earth, our own globe, and consider the unique and marvelous manner in which its various features have come to be.
In the course of millions of years, the earth, through the pull of gravitation, became gradually more and more dense. Different elements in its gases (which according to the later theory were squeezed out by pressure), combined to form rock, water and atmosphere. Hydrogen united with oxygen formed water; nitrogen, oxygen and small quantities of carbonic acid gas formed the atmosphere; oxygen mixed with silicon made quartz, the earth's first rock. This rock, called granite, rose to the top of the molten liquid material and formed a thin film of crust, for it was lighter than the metallic substance which sank to form the earth's core.
This thin, rocky film of crust which covered the hot liquid matter was at first so hot that the gases which had formed the water could not stay on its surface but rose in the air as steam. After the temperature became lower and the rocks cooled, rain poured down in sheets of water, remained on the crust and formed a warm sea.
For a long time this film was but a slight obstruction to the raging fiery forces confined beneath it and again and again these forces tore the crust asunder and made the surface of the earth a scene of wild commotion. But each time the cooling gained a slight mastery over the heat, and by cooling the earth's film was continually shrinking thus forming a thicker and more solid protective crust. After an heroic struggle between these mighty forces the crust was at last strong enough even to sink down in places to fit the inner core; the sea then ran into these hollows and into the wrinkles formed from the shrinking, and the first ridges or islands appeared above the surface of the water.
These ridges or islands were then attacked by the elements, eroded by the weather and the rains, and the granite which formed them was crumbled into quartz, feldspar and mica. The torrents of rain formed rivers, and these, seeking a level, carried the débris down to the shores of the ridges,—the finer sediment, the feldspar and mica, being washed far out from the rocky land where it settled to the bottom of the sea and in time formed mud-banks, which under pressure turned to clay, while the clay under greater pressure, turned to slate. The quartz of the granite, ground to fine particles, stayed near the shore and formed sandy beaches, which if covered under great pressure, became sandstone. Thus were some of our various rocks formed from the original rock of granite, and no matter where one sees sandstone or claybanks, even if on top of a mountain, such a place was once under water, for that is the only place where such sedimentary material could form.
Draw down Æonian hills, and sow
The dust of continents to be."
This slow disintegration of the earth's first land continued for millions of years, the waste accumulating along the shores, and as far out as the finer sediment could be carried. Finally the pressure on the edges of this land caused the thin crust to soften and break, the molten magma poured forth in great masses, and the thick sediments were smashed together, folded up and thrust _-_figure_156.jpg)
A PORTION OF THE COLUMBIAN LAVA FLOW IN THE STATE OF WASHINGTON.
Photograph by Asahel Curtis, Seattle. upward to form the earth's first hills. The crust was then so thin and easily broken that only low hills appeared but after it became thicker it was not so easily rent and mountain ranges were born with an accompanying roar of tremendous volcanoes. Geologists, studying the ages of the different ranges in the United States, have concluded that the first to appear were the Appalachian mountains; later the Rocky mountains in the west, and still later the Coast ranges. Mountain ranges are invariably found along lines of very thick sediments. The same strata which along the Appalachian range are 40,000 feet thick, thin out, when traced westward, to only 4000 feet at the Mississippi River.[1] The high tablelands in portions of Idaho, Oregon and Washington were formed by great streams of lava which poured through many of the fissures of the coast ranges and buried the whole country above the hill tops. This lava cooled into thick massive layers of solid rock—an interesting sight to this very day.
Although mountain ranges appear amid violence and their growth, in a geologic sense, is rapid, plains and plateaus are illustrations of sea-bottoms which have been covered with sediment carried down from the mountains,—mainly pebbles, sand and clay,—pressed and cemented together under the water and then gently raised to the surface. Sometimes the sea-bottom is exposed as a vast plain; again the strata are upheaved, bent or broken, forming faults and ridges. Many of the elevated plains and plateaus are later sculptured into hills by rains, winds, frosts and rivers. The hills are then gently torn down, the valleys filled up and the sediment again deposited by the rivers upon the bed of the ocean.
A few million years passed by and these same deposits may again be raised and become dry land. The great stretches of land tween 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."
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.
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.
2. The Age of Invertebrates, represented by the Cambrian, the Lower Silurian and the Upper Silurian rocks.
3. The Age of Fishes, represented by the Devonian rocks.
4. The Age of Amphibians, represented by the Carboniferous rocks.
5. The Age of Reptiles, represented by the Mesozoic rocks.
6. The Age of Mammals, represented by the Cenozoic rocks.
7. The Age of Man, represented by recent rocks.
All of these Ages, which represent the life history of the world, left their impression on the rocks of their times, and each Age is connected with the preceding and the succeeding Age like links in a long, long chain.
In regard to the distinctness and importance of the last great era, the Age of Man, Le Conte, in "Elements of Geology," says that there are two views which will ever divide geologists, depending on the two views regarding the relation of man to Nature:
"From a purely structural and animal point of view, man is very closely united with the animal kingdom. He has no department of his own, but belongs to the vertebrate department, along with the quadrupeds, birds, reptiles and fishes. He has no class of his own, but belongs to the class Mammalia, along with the quadrupeds. Neither has he an order of his own, but belongs to the order of the Primates, along with the monkeys, lemurs, etc. Even a family of his own, the Hominidae, is grudgingly admitted by some. But from a psychical point of view it is simply impossible to overestimate the space which separates man from all lower things. Man must be set off not only against the whole animal kingdom, but against the whole book of Nature besides, as an equivalent: Nature the Book—the revelation—and man the interpreter. So in the history of the earth: from one point of view the era of man is not equivalent to an era, nor to an age, nor to a period, nor even to an epoch. But from another point of view it is the equivalent of the whole geological history of the earth besides. For the history of the earth finds its consummation, and its interpreter, and its significance, in man."
Ah—how exhilarating the thought! Once again with the astronomer, we gaze out on the huge planets of the solar system,—the groups and great galaxies of stars. Does the earth really seem so insignificant in its smallness?
The beautiful little earth-ball, with its precious freight of human beings, softly flies about the sun. Amply protected in a transparent case of atmosphere, it completes its tremendous journey of 576,000,000 miles once every year. During this year its temperature is varied by four seasons which are caused by the earth being inclined 23½ degrees on its axis.
The poles of earth twice ten degrees or more
From the sun's axle; they with labor push'd
Oblique the centric globe."
—Milton's Paradise Lost.
During this journey around the sun, the earth rotates 366¼ times on its axis, making one revolution every 23 hours, 56 minutes and 4.09 seconds. This causes each side to be alternately warmed and lighted every few hours. Cupped close to the darkened hemisphere lies a huge cone of shadow which extends out in space past the moon. When the moon rolls through this shadow it is eclipsed for it has come into our night. _-_figure_157.jpg){kind=small}
This cone of the earth's shadow is our personal night and ours alone, and, although the stars seem enmeshed in its velvet sides, it is not the night of earth that holds the stars,—its darkness merely shuts out the glare of the sun and the stars of the universe therefore become visible. This remarkable view gave to man a science set in beauty so keen that in the world's young days it was thought that it was music too sweet to be heard by any but the gods. But now we know that all Nature is like that, and there is not a cliff, rock, shell, mountain, valley, water, land or any living thing in the botanical or zoological kingdom, no matter how common or how lowly, which does not carry, even as the stars, its quota of science and song.