16 W. Bowie — Relation of Isostasy to 



tain mass will not be an extra load on the column. It is 

 evident that this is the case, for the area of sedimentation 

 must have been in isostatic adjustment or at least not 

 lighter than normal, and we find that the mountain mass 

 is not an extra load judging from the results of the inves- 

 tigations of the deflections of the vertical and the gravity 

 observations. 



If this mountain mass is not an extra load, then it 

 could not have been brought from some other area to the 

 one it now occupies. If it is not an extra load, it could not 

 be due to horizontal thrusts operating in the earth's isos- 

 tatic shell and extending far beyond the mountain areas. 

 We are led to the conclusion that the cause of the moun- 

 tain formation is a local one, and the only local cause 

 seems to be a change in density in the column. This 

 change must have been the result of a local expansion in 

 the isostatic shell under the sedimentary material which 

 was thrown up to form the mountain mass. In most 

 cases the mountains are formed in areas where heavy 

 sedimentation previously existed. 



The mountain mass is not a permanent feature on the 

 face of the earth as is shown by the fact that areas that 

 are now high were once at or below sea-level and other 

 areas, which at one time were high, are now depressed. 

 The mere fact that we have had this oscillation of the 

 crust and that all types of topography in areas where 

 geodetic investigations have been made are in equilibrium 

 leads us to conclude that there has been, in the past, 

 changes in density in a column. At one time the density 

 would increase, at another decrease. 



It is not known just how these changes have been 

 brought about but, in an area of sedimentation, the base 

 of the sedimentary material may have been depressed as 

 much as ^iyq miles. This would lead us to believe that all 

 of the material below the base of the sediments down to 

 the zone of isostatic flow had been depressed an equal 

 aniount and that the temperature of the material had been 

 raised several hundred degrees Fahrenheit over what it 

 was in its original position. 



The isogeotherms may be depressed with the sinking 

 of the column and, after the sinking has ceased, the lines 

 of equal temperature may rise to their normal positions. 

 In any event, the material of the column has been 

 depressed to hotter zones and probably chemical and 



