274 HOBBS— THE EVOLUTION AND THE [April 24. 



that both earthquakes and volcanic activity are different indica- 

 tions of the operation of a more fundamental geological process — 

 mountain formation, with its concomitant manifestation in changes 

 of level. 



Going back in the direction of the ultimate cause of mountain 

 building, we are probably correct in assuming that it is a conse- 

 quence of the contraction of volume of the planet and the wrinkling 

 of the outer shell, as that shell adjusts itself over the diminished 

 volume of the core beneath. In the past much confusion has arisen 

 from assuming that flexuring has taken place within the outermost 

 shell of the earth, and that the faults discovered are an incident to 

 the folding process within one and the same set of beds. Thus we 

 have come to speak of " dip faults " and " strike faults," " longi- 

 tudinal faults " and " cross faults." Later studies have shown that 

 the processes of folding and of faulting within rocks take place 

 under different conditions of load corresponding to different depths 

 below the surface; and that, therefore, the folding which accom- 

 panies the rise of a mountain range is so deeply buried beneath the 

 roots of the range that it can be laid open for study only after a 

 blanketing layer of rock some miles in thickness has been removed. 

 Those mountains which are growing to-day — such, for example, as 

 the Sierra Nevadas of the Pacific border of our own country — are 

 being pushed up in blocks which are outlined by steep faults. The 

 elevation goes on spasmodically, and each successive uplift causes 

 a jolt which is manifested as an earthquake more or less destructive, 

 according as the movement is of large or of small amplitude. Deep 

 below the surface, the rising blocks of the crust rest upon arches of 

 folds which a future generation of geologists may be privileged to 

 study after a layer of the present surface some miles in thickness 

 has been carried away. Those parts of the earth's crust which are 

 not shaken by earthquakes are, in the language of de Montessus, no 

 longer living — they are dead. 



Not only are earthquakes the indication of changes in level such 

 as accompany the process of mountain growth, but active vol- 

 canoes are now recognized to afford evidence of the same move- 

 ments. Wherever mountain ranges are now rapidly growing, there 

 active volcanoes are to be found. The full significance of this fact 



