490 Mr. T. M. Reade on the Origin of Mountain- Ranges 



mountain-range. There can be no deposition if there is not 

 land-area enough either in the shape of continents, islands, or 

 active volcanic orifices, or all combined or successive, to yield 

 the necessary sediment. This furthermore implies consider- 

 able stability of conditions over lengthened periods of time 

 combined with local mutations and changes of level, and, as 

 I have indicated, we have the history of these mutations 

 within the rocks of a range. The distribution of sediments is 

 dependent upon the depth of the w r ater surrounding the land 

 and the currents of the sea (when they are not laid down 

 in lakes or subaerially by rivers) ; but, whatever the con- 

 formation of the coast and sea-bottom, a continuous discharge 

 of sediment upon it must in time load it, and, as proved by the 

 enormous thickness of rocks composing great mountain- 

 ranges, bend the crust below the maximum depth of any 

 oceanic depression. 



This necessary subsidence again insures the establishment 

 of the basin of deposition and its continuous existence. 



Displacement of Matter in the Shell of Greatest Mobility. 



If the matter in a shell of the Earth between the nucleus 

 and crust is in the condition I have postulated, it is evident 

 that a lateral displacement of the matter of the shell must take 

 place to some extent through weighting by sediment, and this 

 will have its effect in raising the levels of the Earth's crust 

 surrounding the basin of deposition ; but will not be an agent 

 in mountain-building. 



Movement of the Isogeotherms. 



It is evident, from the variations in the rate of increase of 

 temperature that exist in various localities as the crust is 

 penetrated, that the lines of equal temperature (isogeotherms) 

 in the Earth's crust are subject to change, for it is not to be 

 supposed that the temperature gradients have remained in 

 their existing relations for all time. 



It is also evident, as first shown by Babbage and Herschel, 

 that the covering of any particular area of the Earth with 

 sediment will necessarily raise the temperature of the crust 

 below^ *. If, therefore, we assume a thickness of 10 miles of 

 sediment to be laid down in a basin' of deposition or earth- 

 trough, and the rate of increase of temperature to be 1 in 

 50, what w r ere originally surface-rocks possessing a surface- 

 temperature determined by the climate of the locality will be 



* This is well explained "both by Babbage and Herschel in the 9th 

 Bridgewater Treatise. 



