state, shift into positions which ease the strain, and again enter into a crystalline solid condition. The proof that such a process exists in the earth is based on several lines of evidence. First, evidence of a broad isostatic equilibrium notwithstanding the agencies of mountain folding, of erosion, and of sedimentation, all of which work through geologic time tending to destroy those relations of elevation which are needed to maintain isostasy, giving equal pressures by broad crustal areas of unlike density upon the yielding zone below. Second, the evidence of increasing temperature with depth, giving temperatures close to those of fusion at depths below 40 to 50 miles. Third, the evidence from tides and earthquakes that the earth as a whole is more rigid than steel and cannot possess a fluid shell beneath the crust. Fourth, the physical principle that at temperatures close to fusion a crystalline substance is incapable of supporting permanent shearing stresses, but yields slowly by recrystallization, notwithstanding the fact that under short stresses the same substance may be as rigid as steel. The conclusion to which this argument leads is that an outer crust or lithosphere, the rock sphere, 50 to 75 miles thick and very strong, is marked by broad variations in density amounting to as much as 5 per cent, and more local variations up to 10 per cent, which correspond to the broader relief of the earth's surface. Below this lies a thick, hot, basic, rigid yet weak shell which the writer has named the asthenosphere," the sphere of weakness. The problem of the origin of the ocean basins and of continental platforms resolves itself consequently into the origin of the density differences in the lithosphere and the maintenance of the heated and weak condition in the asthenosphere. 7 Barrell, Joseph, "The Strength of the Earth's Crust." Jour. Geology, vols. 22, 23, 1914, 1915. Rise of basic magmas from the asthenosphere. The series of radioactive elements slowly break down into elements of lower atomic weight and give off in the process enormous quantities of energy. Uranium, in degenerating through radium to the stable element lead, develops more than a million times the heat given by the combustion of an equal weight of coal, but the disintegration of the element and the liberation of its heat are so slow that the whole duration of geologic time has not sufficed to eliminate uranium from the crust of the earth. Therefore it has acted as a permanent generator of heat in the rocks which contain it. Uranium and thorium, the parents of the radioactive series, are widely though sparsely diffused through the lithosphere. It has been calculated that, if they extend in their surface amount to a depth of 40 miles, they must supply heat to the surface as fast as it is lost by radiation into space. The earth therefore appears not to be growing colder, though losing heat. The small content of radioactive elements in the basaltic shell below the granitic crust of the continents would then supply that slow increment of heat which is necessary to generate new molten rocks. The granitic shell above, though somewhat richer in radioactive elements, is sufficiently near the surface to lose its excess heat by conduction. The excess heat generated in the asthenosphere is, on the contrary, so deeply buried that it cannot escape in that manner but must slowly transform some of the solid rock into liquid form. Reservoirs gather, until their mass, combined with their decreased density in the fluid form, enables them to work their way through the crust above and demonstrate their existence in igneous activity at the surface of the earth. The magma which thus comes from the greatest depth and in greatest volume would, because of the initial density stratification, produce a notable increase in the density of the outer crust. In order to reëstablish isostatic equilibrium such a region must subside. Most of the igneous rock of later geologic ages which has been intruded into the outer continental crust clearly has not increased the density sufficiently to produce a foundering and would appear therefore either to have come from somewhat higher levels or to have risen in lesser quantity. In some regions, however, as in that of the Lake Superior basin, large masses of basic magma do seem to have overweighted the crust in an early geologic period and produced a tendency to settle as a basin. The same effect may have taken place to even a larger degree in some regions of notable subsidence, as in the Mediterranean basins. In the earliest times, following the solidification of the earth, the forms and relations of the ocean basins suggest that dense molten matter from the depths of the earth broke into or through the outer crust, on a gigantic scale, eruption following eruption until the widespread floods had weighted down broad areas and caused their subsidence into ocean basins. The As seen in the lava plains of the moon, such an action, once started at a certain point, is conceived to have gone forward with widening radius, leading to the origin of the many rudely circular outlines characteristic of the ocean basins. process left great angular segments of the original lighter crust as continental platforms standing in relief between the coalescent basins. The waters gathered into the basins and the continents emerged. THE REIGN OF SURFACE PROCESSES AND BEGINNING OF THE ARCHEAN It is possible that shallow ocean basins began to form nearly as fast as the waters gathered, tending to maintain some land areas above the level of the primordial sea. Or the lands may have emerged later, as the ocean basins spread and deep ened. With the separation of the lands from the seas, erosion began, carbon dioxide was abstracted from the atmosphere to make carbonates, and a further cause of atmospheric depletion was initiated. Thinner, rarer, and colder grew the gaseous envelope, until an oscillating balance was established between the supplies of new gases from the uprising molten rocks and the losses involved in the weathering of their solid forms. Nitrogen was at first relatively small in quantity and oxygen not present in more than a trace. An evolution in atmospheric composition had still to go forward through following ages to transform it into a gaseous medium for the support of the higher life. But even in the early periods. following the gathering of the oceans and the emergence of the lands, the sun warmed the atmosphere and earth. An environment suitable for the lowest organisms had arisen and the earliest forms of life may not have been long in coming into existence. The reign of the surface processes had begun, but, at age-long intervals, the still youthful energies of the interior broke forth. Magmas in great volume ascended, now seen as the most ancient granite-gneisses. In the great crustal overturning of these earliest revolutions the foundation rocks appear to have been everywhere destroyed. The oldest rocks preserved are mashed and crystallized sediments and lava sheets resting as fragments of a cover on the reservoirs of younger magma. Such sediments, altered and intruded, are the oldest Archean rocks. It is not known how close they stand in point of time to the formative processes whose description has been attempted. With these oldest rocks, the dimly known, heroic and mythical eon of the earth is closed and the first historic eon opens as the remote and long enduring Archean division of geologic time. CHAPTER II THE EARTH'S CHANGING SURFACE AND CLIMATE DURING GEOLOGIC TIME CHARLES SCHUCHERT PROFESSOR OF PALEONTOLOGY IN YALE UNIVERSITY UNIFORMITY OF NATURE. The previous lecturer in this course had to seek for the probable origin of the earth in far-off space among the stars, examining them through the great telescopes of our time, through the light-sensing chemical plate of the photographic camera, and through the even more wonderful spectroscope. With this knowledge in hand, postulate upon postulate has been tried out through that talismanic study, mathematics, and so through astronomy and the science of numbers there is revealed an earth evolution still hazy, to be further established through geodesy, mechanics, and chemistry before the geologist comes to be its interpreter. Then, hand in hand with the geologist, the paleontologist, or student of ancient life, reveals the organic hordes that have gone on and through whose fossil remains is unraveled the history of the earth. In all of this we see the brotherhood of the sciences and the fundamental postulate of the uniformity of nature. The geological time-table. As some geologic terms must be used in this lecture, it is desirable to define them here. The geologic history of the earth is now divided as follows: Present time. Psychozoic era. Age of man or Age of reason. Includes the present or "Recent time," and the time during which man attained his highest civilization, estimated to be probably less than 30,000 years. |