A CENTURY OF GE0L0(3tY. 269 



onward toward the present condition and toward man as its goal. The 

 recognition 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 haA^e changed at every step, and absolute uniform- 

 ity is impossible. Elxtreme uniformitarianism is therefore untenable. 

 Catastrophism and uniformitarianism are opposite extremes which 

 must l)e com))ined and reconciled. This reconciliation is only now 

 being completed, and we therefore put oil' its discussion for the pres- 

 ent. Suffice it to say now that geologic thought in this regard has 

 passed through three stages — catastrophism, uniformitarianism, and 

 evolutionism. And this latter is the iinal stage, because (1) it is a 

 complete reconciliation between the other two, and (2) because it is 

 plastic and indefinitely modifiable and progressive, wliile the other 

 two are equally rigid and unchangeable b}- their mutual antagonism. 



With these fundamental principles in mind, we proceed to touch 

 briefly the most im]X)rtant advances during the century. 



EVOLUTION OF P^.ARTH FORMS. 



The idea of tlie 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 geolog}^ 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 

 W'as an account of the succession of formations and their fossil con- 

 tents. 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 incom- 

 plete. 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 l)elief in the supernatural origin and the 

 unchangeableness of organic forms. He lived to make his conception 

 of evolution as a natural process, ])oth of the earth and of organic 

 forms, complete. 



Ocean hasiwi and conthentH. — If we divide geological causes and 

 processes into two general kinds as to their origin — viz, internal or 

 earth-derived, and external or sun-derived— evidently the former is 

 the original and fundamental kind. These determine earth forms, 

 while the other only modify them; these determine the great features, 

 the other only the lesser features: the former rough-hews the earth 

 features, the latter shapes them. It is the effects of these interior 



