168 THE EARTH AS MAN'S ABODE study of rock strata. It is the purpose of this Section to give a brief r^sum6 of what the geologist has learned about the history of the earth. Knowledge of the Earth's History Is Quite Recent. It is only natural that knowledge of the forces and the changes which these forces have combined to produce on the earth had to wait until scientific exploration developed. At the time of Columbus' discovery of America large sections of the earth remained unknown. Nearly three hundred years elapsed before man began to penetrate the wilderness of the Americas. Baron von Humboldt (1769-1859) spent five years exploring South America and the Mexican Gulf. The results of his numerous explorations were published in four volumes during the period 1845-1858. Other famous explorations were conducted during the nineteenth century, among them the voyage of the Beagle (1831) to Patagonia, Chile, Peru, and other points. voyage. Charles Darwin was the naturalist on this Joseph Hooker (1817-1911) and Sir James Ross led another expedition in 1839 to explore the Antarctic. In 1872, the Challenger set out on a scientific cruise in the Atlantic and Pacific oceans that lasted for several years. James Hutton (1726-1797) was the first man to observe and demonstrate that the past history of the earth might be understood through the study of processes that are taking place today. Tipped Strata Make It Possible to Read the History of Geological Events Long Past Which Have Been Written in Stone. The thickness of sedimentary rocks which were formed by deposition in river beds, lakes, and oceans may reach several miles — oil has been obtained from oil-bearing strata at Earth-8 Surface ^ depths of 15,000 feet. The study of the cores obtained by oil-well drillers has aided the geologist immensely because it is not feasible „ ^^ „. , to dig shafts 15,000 feet deep or , , riG. 50. 1 ipped strata of rock may „ i r i i '^^'^- Fortunately for the geoloexposed. gist, there are many places on be worn away b>- erosion and left the earth's surface where the strata have been tipped on edge, so that they can be examined on the surface of the earth. Again, nature has dug tremendous holes, such as the Grand Canyon of the Colorado River, which expose many thousands of feet of stratified rock.
THE HISTORY OF THE EARTH 169 Unconformities Are Nature's Method of Recording Lost Intervals of Time. Occasionally railroad and highway cuts expose stratified deposits in which horizontal strata lie on strata which have been tipped and worn off by erosion, thus leaving a line of demarcation where the strata are not parallel or do not conform to the strata below. Such an angular uncomformity may separate two periods or even eras which are typified by great uplifts in the earth's surface. Fig. 51. Earth's Surface An unconformity. Fossils Are Geological Records of Prehistoric Life. Traces of plants and animals in the rocks reveal the succession of plant and animal life of the past and its orderly progressive changes to present forms. These indelible traces are called fossils, regardless of their nature. Relatively few of the countless living organisms have been in a position to form fossils. Living organisms must be buried at death in such media as clay, sand, silt, dust, volcanic ash, or asphalt in order to form Fig. 52. Scattered fragments of petrified Aruacarioxylon Arizonicuni in Petrified Forest, Arizona. (Courtesy of the U. S. Forest Service.)