126 THE EARTH AS MAN'S ABODE cooling of a formerly much hotter body, but modern knowledge suggests that it is a result of tremendous pressures in the deep interior. These very slow gradational and mountain-building changes brought valuable minerals to the surface, deposited layers of salts, rock, and coal, and finally produced the soil required for our crops.
UNIT III SECTION 1 THE ORIGIN AND AGE OF THE EARTH ARE SUBJECTS FOR MUCH SCIENTIFIC SPECULATION Introduction. Modern exploration and study of changes now taking place in earth's crust give valuable information concerning the probable history of the earth. This section presents a brief resume of the conclusions concerning the origin, age, and development of the earth which the geologist and his fellow-scientists have pieced together. The picture presented lacks many details, because geology, the study of the earth, is a comparatively new science. Three Scientific Hypotheses to Account for the Earth's Form Have Emerged. The Nebular Hypothesis. This hypothesis, sometimes called the Laplacian hypothesis in honor of Simon Laplace, who formulated it in 1796, assumes that the solar system was formed by the condensation of a vast cloud of hot gas. As it condensed, large portions of material separated as rings around the center, which remained as the sun. the same way these portions of the nebula formed miniature solar systems of their own, thus forming the planets with their moons. Anyone who doubted this satisfactory theory was invited to have a look at Saturn and its rings, which were supposed to represent the early stage in the formations of moons. Many valid objections to this theory were advanced as more detailed knowledge became available. One of the objections was based on the observation that the two outer moons of Jupiter and the outer moon of Saturn revolve in orbits opposite to the direction of the revolution of the planets themselves. This change in direction could not have taken place if these moons were formed from the body of the planet, according to Newton's laws. the Again, according to this theory, satellites should rotate slower than the parent planet, but they actually rotate faster in some cases. 127 In