THE NATURE AND INTERPRET AT ION OF GEODETIC EVIDENCE. 31 



a dynamic process, 1 which could only take place in a medium pos- 

 sessing a considerable degree of plasticity under the stresses to 

 which it is exposed. 



The various hypotheses which have been proposed, to account 

 for the elevation of mountain ranges, excluding those which do not 

 provide for compensation, may be divided into two categories, 

 those which regard the elevations of the earth's surface as being sup- 

 ported by some form of tumefaction, and those which regard them 

 as supported by some form of flotation. The earliest suggestion, 

 that of Sir G. B. Airy, 2 was of the latter class ; adopting the 

 notion, still prevalent when he wrote, that the earth consisted of 

 a comparatively thin solid crust floating on a fluid core, he showed 

 that the crust would not be able to support the stresses set up by 

 the weight of a great mountain range, which would break through 

 the crust, and sink into the denser magma, till the buoyancy of 

 this depressed portion was sufficient to support the weight of the 

 range, and the difference in weight, between the depressed portion 

 of the crust and the denser magma displaced by it, while support- 

 ing the range, would also produce that compensation which the 

 observations indicated. 



This hypothesis was afterwards adopted and developed by Rev. 

 0. Fisher 3 who, taking the elevation of mountain ranges as due 

 to compression, and consequent thickening, of the earth's crust, 

 recognised that the additional weight thereby imposed on the 

 mountain region would cause a depression of the crust into the 

 underlying denser magma and give rise to a protuberance on the 

 underside of the crust corresponding to the mountains on the upper. 



Though both of these investigators based their explanation 

 on the notion of a comparatively thin crust, floating on a fluid 

 earth of greater density, it must be remarked that the latter condi- 

 tion is by no means essential, for the whole of the processes concerned 

 would take place within the outermost 60 miles from the surface 

 of the earth, leaving the odd 3,900 miles of the radius unaffected, 

 so that, provided there was a fluid or even plastic layer, of greater 

 density than the overlying crust, in that region which lies above 

 a depth of 60 miles from the surface, all the requirements of the 

 hypothesis would be fulfilled, and the constitution of the more 



1 Major Dutton reoognised thai the term was inappropriate, but the word which 

 would have expressed his intention was preoccupied in a different sense. 



2 PHI. Tram., OXLV, 101-104 (185;-)). 



3 Physic* of the Earth's Cruel ; 1st ed. 1881 ; 2nd cd. 1889. 



[ 179 J 



d2 



