ISOSTASY AND MOUNTAIN RANGES. 



By harry fielding REID. 



(Read April 21, 19 n.) 



The cause of the elevation of mountains has ahvays been a most 

 fascinating subject of study, and we find the earher geologists giv- 

 ing much attention to it. In the first half of the nineteenth century 

 the prevailing idea was that mountain ranges were due to the upward 

 pressure of liquid lava and that their elevation was closely related 

 to the volcanic forces. As late as the middle of the century Elie de 

 Beaumont upheld this idea with all the prestige of his great authority. 



But a more detailed study of the structure of the rocks wdiich 

 make up the mountains led to dift'erent conceptions. It was found 

 that the whole mass had been subjected to tremendous compressional 

 forces in a line at right angles to the mountain range. This was 

 shown by the immense folding of the rocks, the existence of thrust- 

 faults and of cleavage and the evident flattening out of fossils; so 

 that the existence of these tangential forces was thoroughly proven. 

 This led then to the idea that mountains owe their origin not to 

 vertical forces, but to the great tangential forces which folded the 

 rock and squeezed it upwards. Professors Heim and Suess in 

 Europe, and Dana, Hall and Le Conte in America, were all very 

 active in developing this point of view, though Dana realized that 

 vertical forces also played some part in the elevation of mountains ; 

 but the dominant influence of the tangential forces was recognized 

 in the udLine orogenic, or mountain-making {orcts,\\\-\\c\-\\\2iS reserved 

 entirely for them. Without doubt, confidence in the efficiency of 

 tangential forces was greatly strengthened by the fact that these 

 forces could be satisfactorily accounted for by the cooling of the 

 earth ; for the cooling is greatest at a short distance below the sur- 

 face and the exterior layers are subjected to tangential crushing to 

 accommodate themselves to the shrinking interior. 



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