MECHANICS OF FORMATION OF ARCUATE MOUNTAINS 85 



the breadth of the fringes.' The narrow Liu Kiu and Kurile 

 island arcs — mere hnes of volcanic summits with no visible Cor- 

 dillera — show evidence of relatively slight elevation only, and these 

 lie opposite the great sinking deltas of the Hoang and Yangtse 

 in the one case and the Amur in the other. Still farther out 

 toward the center of the Pacific, the Bonin group indicates some 

 islands rising, others sinking, and still others rising after a depres- 

 sion — the group as a whole without as yet very strong indications 

 of general uplift. 



There is still the further argument that throughout the area 

 of the coral seas, as was first pointed out by Charles Darwin and 

 James D. Dana, working independently, the ocean floor has long 

 been sinking, since in no other way can atolls be adequately 

 accounted for.^ 



Any general movement of the ocean floors in the direction of 

 the earth's center which is in excess of movement in the same 

 direction in the continental areas must be accompanied by an 

 out-thrusting against the continental margins such as would be 

 required to explain the formation of mountain arcs uniformly 

 facing the sea. That such thrusts are in reality carried out to 

 the continents from the oceanic areas and cause landward migra- 

 tion of strata is also necessary to account for the fact that the 

 elevation of mountains is not accompanied by tensional phenomena 

 such as the gaping of fissures, etc. Mountain growth accompanied 

 by strong earthquakes reveals in the behavior of rails, pipes, bridges, 

 etc., the fact that not expansion but contraction of the surface has 

 resulted from the movement.^ The so-called block or Schollen 

 theory of formation of mountains, amply demonstrated by observa- 

 tion in many regions, has had to contend with the supposed theo- 

 retical difficulty that opening of fissures by tension should result. "* 



' Bull. Geol. Soc. Am., XVIII, (1907) 233-50, PI. 5. 



^ The ingenious rival theories of Sir John Murray and other biologists fail utterly 

 to explain certain necessary geological consequences. Professor Davis has utilized the 

 Dana centennial to recall Dana's decisive discussion upon this point and to strengthen 

 it by his own arguments. 



3 "A Study of the Damage to Bridges during Earthquakes," Jour. Geol., XVI 

 (1908), 636-53. 



"■Proc. Am. Phil. Soc, XLVII (1909), 27-29. 



