The treatment of geological problems from the viewpoint of present causes was begun after the publication of Lyell's "Principles of Geology" (1830-1833). Earlier geologists were aware of the fact that many of the rock formations had been derived from other consolidation of sand and mud beds and by other actions which may be studied in operation to-day. But the systematic manner in which Lyell treated the whole field of geology made such an impression upon geologists that the publication of his great work marked a new era in the science. De la Beche, Buckland, Geikie, Bonney, and other geologists in England; Dana, and a number of scientists in the United States Geological Survey, in America; Vogt and Naumann, in Germany; Studer in Switzerland; Stopanni, in Italy, and many specialists in other countries took up the work of Lyell, and at present practically every important geological factor is known and the effects of its operations have been described. The succession of life in geological periods is studied under paleontology. This science developed at the same time as systematic and descriptive geology. Many great naturalists have contributed to it. Agassiz, Hall, Dawson, Walcott, Marsh, and others in the United States and Canada; Owen, Prestwich, and others in England; and numerous writers in Europe have published valuable monographs on various phases of fossil and strata-graphical geology. Paleontology, by fixing the succession of animal and vegetable eras, has served as a basis for measuring time, revealing the antiquity of man and of the principal mammals, as well as showing changes in climate, and in land and sea areas. The application of geology to many industries called forth another branch of the science known as economic geology. This deals with the origin and geographical distribution of the useful minerals, the derivation of underground waters and petroleum, and the changes undergone by soils. The first important impetus to economic geology was given by the publication of Whitney's "Metallic Wealth of the United States" in 1854, Von Cotta's work on ore deposits in 1859, and the economic references in the textbooks of the leading European and American geologists. The recent work of Bonney, Groddeck, De Launay, Phillips, Prosepny, Van Hise, Emmons, Le Conte, Lindgren, and others has greatly advanced the interest and usefulness of the science.