A CENTURY OF GEOLOGY. 



435 



higher and higher conditions, as a gradual movement onward to- 

 ward the present condition and toward man as its goah The rec- 

 ognition of this is only now approaching clearness. If geology 

 is the history of the evolution of the earth from primal chaos until 

 now, then the conditions have changed at every step, and absolute 

 uniformity is impossible. Extreme uniformitarianism is therefore 

 untenable. Catastrophism and uniformitarianism are opposite ex- 

 tremes which must be combined and reconciled. This reconcilia- 

 tion is only now being completed, and we therefore put off its dis- 

 cussion for the present. Suffice it to say now that geologic thought 

 in this regard has passed through three stages — catastrophism, uni- 

 formitarianism, and evolutionism. And this latter is the final 

 stage, because (1) it is a complete reconciliation between the other 

 two, and (2) because it is plastic and indefinitely modifiable and 

 progressive, while the other two are equally rigid and unchange- 

 able by their mutual antagonism. 



With these fundamental principles in mind, we proceed to 

 touch briefly the most important advances during the century. 



EVOLUTION OF EARTH FORMS. 



The idea of the progressive development of the earth in its 

 greater features throughout all geologic time by the action of 

 forces resident in the earth itself preceded the acceptance of the 

 evolution of organic forms. We have said that the fundamental 

 idea of geology is that of the evolution of the earth through all 

 time. Now, it was Dana who first studied geology wholly from 

 this point of view. For him geology was the development of 

 the earth as a unit. Before him, doubtless, geology was a kind 

 of history — i. e., a chronicle of thrilling events — but Dana first 

 made it a philosophic history. Before Dana, geology was an 

 account of the succession of formations and their fossil contents. 

 Dana made it an account of the evolution of earth forms and 

 the concomitant and resulting evolution of organic forms. It is 

 true that first and for a long time his evolutional conception was 

 incomplete. It is true that while he attributed the evolution of 

 earth forms to natural causes and processes, he still shrank from 

 applying similar causes to the changes in life forms, but this was 

 the almost necessary result of the then universal belief in the 

 supernatural origin and the unchangeableness of organic forms. 

 He lived to make his conception of evolution as a natural process, 

 both of the earth and of organic forms, complete. 



Ocean Basins and Continents. — If we divide geological causes 

 and processes into two general kinds as to their origin — viz., in- 

 ternal, or earth-derived, and external, or sun-derived — evidently 



