Geology is the study of the structure, history, and development of the earth and its inhabitants, as revealed in the rocks.

From this definition it is apparent that the central problem in geology is the deciphering of the earth's history, and that the historical standpoint is dominant throughout. For this purpose it is necessary to apply the results and principles of all those sciences which can aid us in interpreting the record contained in the rocks. Astronomy, physics, chemistry, mineralogy, physical geography, botany, and zoology are all needed in the task, and geology as a true science did not become possible until the other sciences were sufficiently advanced to afford a solid foundation for it.

The history of the earth involves vast periods of time, to be measured only in millions of years, - no one can say how many, - so that all our familiar conceptions of " ancient "and " modern," derived from the history of our own race, must be greatly changed before they can be applied to geological time. In reaching its present condition, the earth has passed through many stages of change in its geographical, climatic, and biological relations, most of these stages leaving behind them records which are preserved in the successive layers of rock.

In order to read the record contained in the rocks, it is first of all necessary to learn the language in which it is written. This can be done only through an intimate acquaintance with all the methods in which rocks are made, and with the changes which the rocks undergo. This, in turn, implies a knowledge of all those processes which are now at work in modifying and changing the globe internally and on the surface. Just because our knowledge of these methods and processes is often incomplete and vague, do we so frequently find the geological record ambiguous, open to several interpretations, or even quite unintelligible. Again, many changes go on under conditions which render direct observation impossible, either because they are confined to the deep interior of the earth and are thus beyond our reach, or because their operation is so slow that a lifetime is all too short for their detection. In such cases we must deduce the invisible cause from the visible effect, but it is often extremely difficult, or even impracticable, from many possible causes, to select the real and rightful one.

Hence come the wide differences of opinion which the interpretation so often calls forth.

As a living and growing science, geology is subject to continual change, a change which is by no means a simple advance from one point to another, but an unending revision of opinions, a perpetual tearing down and rebuilding.

To many intelligent people this continual modification of scientific opinion, which is a necessary consequence of advancing knowledge, is a source of annoyance. This attitude of mind comes from a failure to discriminate between fact on the one hand, and inference, or hypothesis, on the other. Accurately observed facts may be added to, but they remain trustworthy; the changeable element is the inference which is drawn from the facts. These inferences are of very different degrees of certainty. Some such deductions which were made centuries ago remain unshaken to-day, while others of far more recent date have proved illusory. Thus, when we find a rock composed of cemented sand-grains, arranged in regular beds or layers, and full of marine shells, we infer that it was formed under the sea, and further that the land where the rock is now found was once covered by the sea. Such inferences are practically certain, because they explain all the known facts and are in conflict with none. On the other hand, the hypotheses of Cuvier and others as to the character of the earth's development, and the manner in which the successive assemblages of animals , and plants were called into being, have been long abandoned.

In the process of reasoning from the known to the unknown, the inferences become the more uncertain, the farther we recede from demonstrable facts. Hypotheses are assumptions which we make to explain and coordinate large numbers of facts, and so long as their true nature is understood, they are useful, indeed indispensable, means of reaching the truth. The objection is that they are too often taught as though they were established beyond dispute. A true hypothesis will prove to be in harmony with newly discovered facts, which will take their place under it simply and naturally. A false hypothesis, on the other hand, may be in accordance with all the facts known at the time when it was proposed, but the progress of discovery will bring to light facts which are inconsistent with the hypothesis, until it is plainly seen to be inadequate and misleading. Yet even a false hypothesis may serve a useful purpose, for it puts before us a definite problem, instead of a mere catalogue of uncorrelated observations. The pathway of every science is strewn with wrecks of hypotheses which have been used, worn out, and thrown aside.

In all our thinking and reasoning the distinction between hypothesis and fact must be steadily held in view.