256 LEADING AMERICAN MEN OF SCIENCE 



becomes so weak that the ever persistent tangential pressure 

 crushes it together. The strata are thrown into alternating anti- 

 clines and synclines, or one part forced over another in great 

 thrust faults, while slaty cleavage and more decided forms of 

 metamorphism may be produced. A mountain range thus pro- 

 duced Dana calls a synclinorium. In such a process, the final 

 crushing of the geosyncline is a movement relatively rapid in com- 

 parison with the age-long accumulation of sediment in the sub- 

 siding trough. Hence, in the history of any region, there are long 

 ages of comparative tranquillity alternating with brief epochs of 

 rapid geographical change "revolutions," as Dana has somewhat 

 poetically called them. These revolutions are the time boundaries 

 of geological history, delimiting the eons and eras into which 

 geological time is divided. Thus an evolutionary geology recog- 

 nizes the truths in the .two systems of catastrophism and uniformi- 

 tarianism which it has displaced. 



While in most mountain regions the thick masses of strongly 

 folded, thrust, and metamorphosed strata bear witness to the 

 crushing of a geosyncline, Dana also recognized that a geanti- 

 cline may result in a permanent elevation. Such a mountain 

 range he proposed to call an anticlinorium. There is reason to 

 believe that the actual history of most mountain ranges is complex. 

 Thus, the Appalachian range was formed by the crushing of a 

 geosyncline at the close of Paleozoic time, degraded nearly to 

 base-level by atmospheric and aqueous action in Mesozoic time, 

 and again elevated by a broad geanticlinal movement early in 

 Cenozoic time. In these two phases of elevation it may serve to 

 illustrate Dana's two types of mountain range the synclinorium 

 and the anticlinorium. Its present topography of ridge and valley 

 is due to erosion subsequent to the Cenozoic elevation. 



There are unquestionably weak points in the theory of mountain- 

 making as thus developed; and, in our ignorance of the conditions 

 in the interior of the earth and of the forces there in action, it ill 

 becomes us to be dogmatic. But the theory is certainly a beautiful 

 one, and is worthy of a provisional acceptance as the most plausible 

 explanation of orogenic processes yet suggested. 



