[In geology,] As in history, the material in hand remains silent if no questions are asked. The nature of these questions depends on the "school" to which the geologist belongs and on the objectivity of his investigations. Hans Cloos called this way of interrogation "the dialogue with the earth," "das Gesprach mit der Erde."

As geology is essentially a historical science, the working method of the geologist resembles that of the historian. This makes the personality of the geologist of essential importance in the way he analyzes the past.

The subjective element in geological studies accounts for two characteristic types that can be distinguished among geologists. One considering geology as a creative art, the other regarding geology as an exact science.

[In geology,] As in history, the material in hand remains silent if no questions are asked. The nature of these questions depends on the "school" to which the geologist belongs and on the objectivity of his investigations. Hans Cloos called this way of interrogation "the dialogue with the earth," "das Gesprach mit der Erde."

Geology differs from physics, chemistry, and biology in that the possibilities for experiment are limited.

There is, however, no universal recipe for scientific advance. It is a matter of groping forward into terra incognita of the outer world by means of methods which should be adapted to the circumstances.

Experiments in geology are far more difficult than in physics and chemistry because of the greater size of the objects, commonly outside our laboratories, up to the earth itself, and also because of the fact that the geologic time scale exceeds the human time scale by a million and more times. This difference in time allows only direct observations of the actual geologic processes, the mind having to imagine what could possibly have happened in the past.