256 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



attaching to a great object, which, except for the surface layers, is as 

 yet unexplored. 



It is now generally accepted that the earth was created from the 

 parent sun about 2,000 million years ago through tidal disruption by 

 a passing star. The subsequent liquefaction and solidification of the 

 detached mass of glowing gas formed the juvenile earth. This notion, 

 advanced by Jeans and Jeffreys, is quite different from that involved 

 in the nebular hypothesis of Laplace, according to which the sun was 

 originally surrounded by a rarefied nebula which rotated about the 

 central mass. As the nebular material cooled it was supposed to 

 contract and increase its speed of rotation, until fmally the centrifugal 

 force was sufficient to detach a ring of material, which condensed to 

 form a planet. Although accepted for many years, the hypothesis 

 was finallj^ discarded on purely mathematical grounds. The theory 

 now in favor also differs in many hnportant details from another 

 tidal theory, the planetesimal hypothesis enunciated by Chamberlin 

 and Moulton. This was the first to account satisfactorily for many 

 of the major features of the solar system, and involved the formation, 

 from the tidal protuberances, of swarms of solid fragments (plano- 

 tesimals), which coalesced around various nuclei and thus produced 

 the planets. The significance of the modern theory, for the purposes 

 of this discussion, lies in the supposition that the earth for a briej time 

 after its creation was entirely molten, well stirred by convection, and, 

 to the extent that the component substances were miscible in the 

 liquid state, quite homogeneous in composition. 



NATURE OF THE PROBLEM 



The composition and state of the earth's interior has long remained 

 a problem of great difficulty. It is at the same time a subject of 

 lasting mterest, alike for the scientist and for the layman. There is 

 always a certain fascination in the mysterious and unknown, especially 

 when it appears impossible to solve the problems that are presented. 

 If it should seem a hopeless task to learn anything about the interior 

 of the earth, we might profit by adopting that philosophical attitude 

 toward the origin of the earth and the nature of its interior which 

 was expressed by Barrell ^ as follows: 



The history of the earth is read in the rocks which have been thrust up by 

 internal forces aud beveled across by erosion. The nearer events are clearly 

 recorded in the sequence aud 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 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. 



• Barrel!, Joseph, ch. I in The Evolution of the Earth, by R. S. Lull and others. New Haven, 1919. 



