relation of isostasy to mountain formation 335 



Geological Structure of Himalayas 



If we examine the geological sections drawn across the Himalayas, the 

 granite appears to be the main factor ; s in large-scale sections of sub- 

 sidiary features much folding has been shown, but these folds are super- 

 ficial and local; 9 their span varies from a furlong up to a mile or two. 

 while the base of the Great Himalayan Range is -±0 or 50 miles wide; by 

 no stretch of imagination can the Great Himalayan Range be described 

 as a fold of the earth's crust. 



Moreover, even among the sedimentary rocks their folding is not so 

 much in evidence as their tilting and faulting; the strata are neither 

 arched nor curved to the same extent as they are tilted and fractured. 

 On the flank of the Himalayas the inward dip of the strata toward the 

 main range without curvature is a remarkable and impressive phenomena. 

 Bowie's explanation of the Appalachian system appears to me to fit the 

 observed facts of the sub-Himalayan zone ; I refer to the following 

 passage : 



"When uplift begins it is probable that it takes place in different sections 

 of the area affected at different rates and at different times. There would be 

 more resistance to the uplift in some parts of the area than in others. . . . 

 It is conceivable that in the process of uplift in a large area we should get 

 distortions such as are found in most areas of mountain uplift. 



"We have evidence that much material of the earth has been uplifted with- 

 out distortion. The great platea*us of the West have their strata practically 

 horizontal, and in some cases they extend for miles." lu 



Eelation of Isostasy to Mountain Formation 



No discussion of the origin of mountains is complete without full con- 

 sideration of the arguments from isostasy ; it is the theory of isostasy that 

 has shown us the expansions and contractions of rock masses in the crust. 

 So strongly do I realize our great debt to this theory that I wish to give 

 a warning against the risks of misapplying it. Any discovery that a 

 theory is being misapplied is liable to bring it into unfair discredit with 

 laymen and to postpone its acceptance. The lesson taught by the theory 

 of isostasy and founded on researches initiated by Hay ford is that the 

 rock of the crust undergoes changes of density which not only produce 

 changes of surface elevation, but which also maintain the equilibrium of 



s Sketch Himalayan geography and geology. Hayden's sections, plate xxxviii. Hayden's 

 section of Kinchinjunga. 



9 Middlemiss\s sections, Geological Survey of India, vol. xxiv : Hayden's sections. (Geo- 

 logical Survey of India, vol. xxxvi. 



10 American Journal of Science, vol. ii, July, 1921, p. IT. 



