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XCIV. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from p. 284.] 



January 21st, 1920.— Mr. G. W. Lamplugh, F.R.S., President, 

 in the Chair. 



Mr. Richard Dixox Oldham, F.R.S., V.P.G.S., gave a de- 

 monstration on a Model to Illustrate the Hypothesis of a 

 somewhat Rigid Crust resting on a somewhat Yielding 

 Substratum, as applied to the Problem of the Origin 

 of Mountain-Ranges. 



He remarked that geodetic measurements in the Himalayas, the 

 Pamirs, and the Andes show that in each case there are systematic 

 departures from equilibrium, in the form of parallel zones in which 

 the surface-level stands alternately above and below the level of 

 equilibrium, the differences being very considerable, and amounting 

 to the equivalent of somewhat over 2000 feet thickness of rock of 

 average density. These zones run parallel to the direction of the 

 axis of greatest elevation of the range, and are explicable by an 

 hypothesis that the elevation of the ranges was due to direct uplift 

 produced by changes in volume of the material underlying the 

 crust, if this material be supposed to possess a certain limited 

 amount of compressibility or plasticity and the crust to have a 

 certain amount of rigidity, which would offer resistance to an 

 exact adjustment of the uplift of the surface to the varying amount 

 of uplifting force developed in the material below the crust. 



The model is designed to visualize the consequences of such an 

 hypothesis. It consists of two strips of spring steel, supported at 

 regular intervals by connecting links to a series of blocks capable 

 of vertical movement. For one strip these links are of fixed 

 length, representing a condition in which surface-elevation will be 

 exactly equivalent to the magnitude of the uplifting force. For 

 the other the links have a limited possibility of variation in length, 

 representing a condition where the rigidity of the crust is given a 

 certain possibility of influence on the resulting elevation of the 

 surface. On giving differences in height to the elevating blocks, 

 to represent the varying amount of uplifting force supposed to 

 exist under the mountain-range, it is found that the two strips do 

 not run at the same level, but the second runs alternately higher 

 and lower than the first, just as geodetic measurements have shown 

 is the case in the great ranges of mountains. The model is of no 

 value as evidence in favour of the hypothesis which it was designed 

 to illustrate, but is regarded as of some interest in visualizing the 

 consequences of an hypothesis which seems worthy of closer in- 

 vestigation than it has yet received. 



Phil. Mag. S. 6. Vol. 40. No. 240. Dec. 1920. 3 H « 



