CHAPTER I

THE ORIGIN OF THE EARTH1

JOSEPH BARRELL

PROFESSOR OF STRUCTURAL GEOLOGY IN YALE COLLEGE

INTRODUCTION

THE logic of all branches of science points to the existence of some system of evolution of the universe, its complete nature hidden in the vastnesses of time and space, but nevertheless developed in accordance with Nature's laws. The earth is one of the celestial host, its beginnings are bound up with that of other bodies. In the history written in the structure of the earth and in the relations of the earth to the planets, stars, and nebulæ lies concealed the story of its genesis. Two chief methods of approach, the geologic and astronomic, lead toward the solution of this fundamental problem.

The history of the earth is read in the rocks which have been thrust up by internal forces and beveled across by erosion. The nearer events are clearly recorded in the sequence and nature of the sedimentary rocks and their fossils. But the oldest formations have been folded, mashed, and crystallized out of all resemblance to their original nature, and intruded by molten masses now solidified into granite and other 1 Also presented before the Geological Society of Boston, January 19, 1917. Some pages of the following article have been drawn from one by the writer entitled "Origin of the Solar System Under the Planetesimal Hypothesis," published as Chapter XXV in Pirsson and Schuchert's "Text-book of Geology," 1915. For permission to use this material grateful acknowledgment is made to the authors and publishers of that work.

igneous rocks. Fossils, the time markers of geology, if once existent, have been destroyed, and, as in the dawn of human history, vast periods of time are dimly sensed through the disordered and illegible record. This crystallized and intricately distorted series of the oldest terrestrial rocks tells of an earth surface on which air and water played their parts, much as now. But it was a surface repeatedly overwhelmed by outpourings of basaltic lava on a vaster scale than those of later ages, and the crust was recurrently broken up and engulfed in the floods of rising granitic magmas. Here the geologic record begins, but the nature of its beginning points clearly to the existence of a prehistoric eon. At the farther bounds of this unrecorded time, forever hidden from direct observation, lies the origin of the earth.

But the mind of man will not be baffled. Since he may not see directly he will see by inference. Convergent lines of evidence derived from various fields of knowledge may be followed part way toward this goal, like those rays perceived through the telescope on the full moon near the margin of its visible hemisphere, which converge toward craters on the side of the moon that no eye shall ever see.

Leaving the geologic field of evidence, the problem of the origin of the earth may be approached from the astronomic side. The relationships of the earth to the stars and the planets are displayed in the depths of the heavens, and vestiges must there exist of the cosmic conditions which gave birth to our world and the other planets of our system. The forces of nature are found to obey the same laws as far as the telescope can penetrate. The spectroscope detects the familiar chemical elements in distant stars. These instruments give assurance of the unity of the cosmos, but the diversity of objects indicates various stages and various types of evolution. Which approaches nearest to that of our solar system? We must be content to study very much larger and therefore unlike

systems, since from the distances of even the nearer stars the earth and her sister planets would be hopelessly invisible in the most powerful telescope. We cannot, then, follow into the planetary stage the evolution of other systems comparable to our own. Yet in nebulæ, in stars, and in the inherited motions and configurations of our planetary system are clues which pieced together lead up toward the origin of the earth.

The problem of the origin of the earth is within the domain of scientific investigation, but as yet the pictures which may be drawn are varied. The vague outlines shift and change but become clearer with the growth of knowledge. Where the solution of a problem is not yet definitive and certain, the method of multiple working hypotheses should be used. All facts and theories should be matched to these several hypotheses to determine which one of them shall be selected and modified, and which shall meet the fate of the unfit. At the present stage of investigation any one view should not be regarded as established beyond question, even though the assembled evidence seems strongly to support it. In a single presentation, however, all hypotheses cannot be equally treated and each investigator, while recognizing the existence of other hypotheses, may properly emphasize that one which seems to him most in accordance with the various categories of facts and more firmly established inferences.

The hypotheses of earth origin begin more especially in the astronomic field in the search for initial causes; they end in the geologic field where they dovetail into the known relations. The surviving hypothesis must give a sound explanation of those broader terrestrial conditions of atmosphere, hydrosphere, and lithosphere, of ocean basins and continental platforms, which had become established by the beginning of the geologic record. But, although much has been learned, it is still unsettled among geologists as to how far those fundamental conditions in early geologic times were different from

those of the present. On the whole, the problem of the genesis of the earth appears to lie somewhat more in the field of the geologist than in that of the astronomer.

THE PLACE OF THE EARTH IN THE UNIVERSE

The earth, a member of the solar system. The earth is but one among the planets which together with the sun constitute the solar system. It is neither the largest nor the smallest, neither nearest to nor most remote from the sun.

The sun is a star and is but one among the millions of stars, and, though apparently so great as seen from the earth, is mediocre among them in size and brightness. The origin of the earth is obviously bound up with the origin of the other planets and all in the history of the sun. A presentation of the significant facts of magnitudes, motions, and distribution of these bodies, familiar though they are to most readers, should therefore precede the consideration of the genesis of the earth.

The planets visible to the unassisted eye are, besides the earth, five in number, distinguished by the ancients from the stars by their steady light and by their wanderings through the zodiacal path in the sky, wanderings produced as a result of the combined effect of their motions and that of the earth in nearly circular paths about the sun, their common center. The telescope has added to the number of planets two large ones, Uranus and Neptune, invisible to the naked eye because of their distance from sun and earth, and, in one zone of intermediate distance, a swarm of smaller bodies, the asteroids, better called planetoids. In size the planets sink to vanishing insignificance in comparison with the sun or any other star. Their distances from the sun and from each other are also almost infinitesimal in comparison with the distances which separate the stars. They shine by light reflected from the