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SCIENCE 



[N. S. Vol. XXV. No. 631 



side, the oldest strata lean in a nearly ver- 

 tical position; farther from the center, 

 strata of less antiquity lean with more 

 gentle dips against the steeper and older 

 strata; and finally, on the flanks of the 

 mountain range, still newer strata rest 

 nearly horizontally. The diagram repre- 

 sents a dislocation of the strata by repeated 

 vertical upthrusts of the crystalline core. 

 Of course, it was seen at once that there 

 was nothing common to these ideal sections 

 and the real sections of the Appalachians 

 revealed by the Pennsylvania Survey. The 

 billowy form of the Appalachian folds 

 meant something, and something very dif- 

 ferent from the prevalent conception of 

 mountain ranges. 



The true interpretation of the Appa- 

 lachian waves is probably to be found in 

 the contractional theory of mountain ele- 

 vation, of which Dana was the leading ex- 

 pounder. The views of Le Conte on the 

 subject of mountain-making were in most 

 respects similar to those of Dana. But, 

 while Le Conte 's discussions were of great 

 value, the priority in the general develop- 

 ment of the theory belongs to Dana. "To 

 the North American geologists," says von 

 Zittel, "undoubtedly belongs the credit of 

 founding the theory of horizontally acting 

 forces and rock-folding upon an ample 

 basis of observation." 



That the main cause of mountain eleva- 

 tion is tangential pressure in the crust re- 

 sulting from internal contraction, is now 

 generally acknowledged, though there may 

 be doubt whether the main cause of con- 

 traction is the cooling of the earth from an 

 incandescent condition, as assumed in the 

 commonly accepted form of the nebular 

 theory, or the gravitational adjustment of 

 an incoherent mass of meteors, as assumed 

 in the more recent planetesimal hypothesis 

 of Chamberlin and Moulton. 



The idea of the contractional origin of 

 mountains was not, indeed, original with 



Dana. There was a glimmer of the idea 

 in the writings of Leibnitz, and Constant 

 Prevost dev^oped the idea into a definite 

 scientific theory; but the elaboration of 

 the theory into its present form we owe 

 chiefiy to Dana. His discussion of the 

 subject began in the Journal of Science, 

 in 1847. In later years he returned to the 

 subject again and again; and the theory, 

 as shaped by his maturest thought, appears 

 in the last edition of the 'Manual.' In his 

 earlier writings his views of the origin of 

 continents and mountains were developed 

 on the assumption of a liquid globe. In 

 later years he abandoned that view, and 

 adjusted his theories to the more probable 

 doctrine of a globe substantially solid. 



The conception of the subsidence of the 

 suboceanic crust, which led Dana to his 

 views of the permanence of continent and 

 ocean, is an important element in his the- 

 ory of mountain-making. In the contrac- 

 tion of the earth's interior, the suboceanic 

 crust necessarily flattens in its subsidence, 

 so that its section continually approaches 

 the chord of the arc, thus exerting a tan- 

 gential thrust toward the continental areas. 

 The rather abrupt change in the radius of 

 curvature in passing from the oceanic to 

 the continental areas, makes the continental 

 borders lines of weakness which determine 

 in general the location of the great moun- 

 tain wrinkles. In the two continents of 

 North and South America, with their 

 mountain borders on the east and west and 

 their vast interior plains, Dana found ex- 

 emplified the typical continent. Their iso- 

 lated situation seemed to allow a more 

 typical development than was possible in 

 the Old World, where continents are 

 massed together; as the laws of crystalline 

 form can exhibit themselves in perfection 

 only where a single crystal in a solution or 

 magma is allowed to grow without inter- 

 ference of other growing crystals. 



According to the views of Dana and 



