FACTS AND INFERENCES 5 



proved illusory. Thus, when we find a rock composed of cemented 

 sand-grains, arranged in regular beds or layers, we infer that it 

 was laid down under water, because of its exact resemblance to 

 accumulations of sand which are forming under water to-day. If 

 the sandstone be 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 prac- 

 tically 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, were abandoned long ago. 



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 pro- 

 posed, 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 facts. 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. 



While, in its most comprehensive sense, geology consists in the 

 application of nearly all the physical and natural sciences to the 

 elucidation of the earth's history, the geologist has his own special 

 field of investigation. This he finds in the rocks, and every ex- 



