"Physics and chemistry provide the natural laws which also hold in geology. Additional to those, however, the geologist works with some general concepts which are the more specific rules of the game for his scientific investigations. These concepts, such as the principle of "uniformitarianism," are his guide in the interpretation of the available facts. Because of the higher complexity of the evolution of the earth, our present basic concepts need still more footing and development than those of physics and chemistry. A repeated to and fro between induction and deduction is necessary."