ORIGIN OF MO UNTAIN RANGES. 5 7 1 



the sides must lose as much as the sediments gain, and therefore 

 must contract and make room for the lateral expansion, and 

 therefore there would be no folding and no elevation. I do not 

 see any escape from this objection. 



Thus it seems that Reade's theory cannot be accepted as a 

 substitute. Is there any other ? 



ii. dutton's isostatic theory. 1 



Dutton's discussion of isostasy is admirable, but his applica- 

 tion of it to the origin of mountains is weak. The outline is as 

 follows : 



Suppose a bold coast line, powerful erosion and abundant 

 sedimentation. The coast rises by unloading and the marginal 

 sea-bottom sinks by loading. Now if isostasy is perfect, there 

 will be no tendency to mountain formation. But suppose a pil- 

 ing up of sediments, but — on account of earth rigidity — without 

 immediate compensatory sinking, and a cutting down of coast 

 land without compensatory rising. Then 'there would be an isostatic 

 slope toward the laud. And the accumulated and softened sedi- 

 ments would slide landzvard, crumpling the strata and swelling them 

 up into a mountain range. 



The fatal objection to this view is that complete isostasy is 

 necessary to renew the conditions of continued sedimentation and 

 therefore to make thick sediments, otherwise the sediments 

 quickly rise to sea-level and stop the process of sedimentation at 

 that place. But it is precisely a want of complete isostasy which 

 is necessary to make an isostatic slope landward. Dutton refers 

 to Herschel as having suggested a similar cause of strata crump- 

 ling and slaty cleavage 2 ; but the principles involved in the two 

 cases are almost exactly opposite. Herschel supposes sediments 

 to slide down steep natiiral slopes of sea-bottoms and therefore 

 seaward. Dutton supposed sediments to slide up natural, though 

 down isostatic slopes, landward. Herschel's is a theory of strata- 



1 Phil. Soc. of Washington, Bull. Vol. XI, pp. 51-64, 1889. 



2 Phil. Mag., Vol. 12, 197, 1856. 



