THE 



JOURNAL OF GEOLOGY 



SEPTEMBER-OCTOBER, 1893. 



THEORIES OF THE ORIGIN OF MOUNTAIN RANGES. 



Mountains are the focal points of geological interest. In 

 their complex structure are contained all kinds of rocks, sedi- 

 mentary, eruptive and metamorphic ; and in their formation are 

 engaged all geological forces in their greatest intensity. They 

 are the culminating points, the theatres of greatest activity of all 

 geological agencies; igneous agencies in their formation, aqueous 

 agencies by sedimentation in their preparation, and by erosion in 

 their subsequent sculpture. Their discussion, therefore, is a 

 summation of all the principles of structural and dynamical 

 geology. But they are equally important in historical geology, 

 for the birth of mountains marks the times of great revolutions 

 in the history of the earth, and therefore determine the primary 

 divisions of geological time. Evidently therefore the theory of 

 mountains lies at the very basis of theoretic geology, and a true 

 theory must throw abundant light on many of the most difficult 

 problems of our science. 



But if this is the most important, it is also the most difficult 

 of all geological questions. My object now is to give, as briefly 

 as possible, the present condition of science on this subject. But 

 in all complex subjects there is a region of comparative certainty 

 and a region of uncertainty ; a region of light and a region of 

 twilight. My farther object, therefore, will be to separate 

 sharply these two regions from one another, and thus to clear 

 the ground, narrow the field of discussion and direct the course 

 of profitable investigation. 



Vol. I., No. 5. 543 



